CLEANING AND CORROSION CONTROL VOLUME I ... material/NAVAIR 01-1A-509-1.pdfNAVAIR 01-1A-509-1 TM 1-1500-344-23-1 TO 1-1-689-1 TECHNICAL MANUAL CLEANING AND CORROSION CONTROL VOLUME

NAVAIR 01-1A-509-1TM 1-1500-344-23-1

TO 1-1-689-1

TECHNICAL MANUAL

CLEANING AND CORROSION CONTROL

VOLUME ICORROSION PROGRAM AND

CORROSION THEORY

01 MARCH 2005

This publication supersedes NAVAIR 01-1A-509/TM 1-1500-344-23, dated 1 May 2001and NAVAIR 16-1-540/TM 1-1500-343-23/TO 1-1-689, dated 1 Sep 2000.

DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited.

DESTRUCTION NOTICE - For unclassified, limited documents, destroy by any method that will preventdisclosure of contents or reconstruction of the document.

PUBLISHED BY DIRECTION OF COMMANDER, NAVAL AIR SYSTEMS COMMAND

0801LP1043459

NATEC ELECTRONIC MANUAL

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LIST OF EFFECTIVE PAGESDates of issue for original and changed pages are:

Original ........................ 0 ......................... 01 Mar 2005Change ....................... x ........................ xx XXX 199X

Insert latest changed pages; dispose of superseded pages in accordance with applicable regulations.

NOTE: On a changed page, the portion of the text affected by the latest change is indicated be a vertical line, orother change symbol in the outer margin of the page. Change in illustrations are indicated by miniature pointinghands. Changes to wiring diagrams are indicated by shaded areas.

Total number of pages in this manual is 52, consisting of the following:

Page *Change Page *Change Page *ChangeNo. No. No. No. No. No.

Change ....................... 0 ......................... 15 Sep 1993Change ....................... x ........................ xx XXX 199X

A Change X

*Zero in this column indicates an original page.

Title ........................................ 0A ............................................. 0i-ii ........................................... 0TPDR-1 .................................. 0TPDR-2 Blank ........................ 01-1 - 1-3 ................................. 01-4 Blank ................................ 02-1 - 2-4 ................................. 03-1 - 3-25 ............................... 03-26 Blank .............................. 0Glossary-1 - Glossary-8 ......... 0Index-1 - Index-3 .................... 0Index-4 Blank ......................... 0

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LIST OF ILLUSTRATIONS ......................................... iiLIST OF TABLES ........................................................ iiLIST OF TECHNICAL PUBLICATIONS

DEFICIANCE REPORTS (TPDR)INCORPORATED ............................TPDR-1

1 INTRODUCTION

1-1. Overview ......................................... 1-11-2. Purpose ........................................... 1-11-3. Scope .............................................. 1-11-4. Arrangement of Manual .................. 1-11-5. Related Publications ....................... 1-21-6. Usage and Coflicts .......................... 1-21-7 Reporting Errors and Improvement

Recommendations ........................ 1-21-8. Manual Change Procedures ........... 1-31-9. Requisitioning and Automatic

Distribution ................................... 1-3

2 PREVENTATIVE MAINTENANCEPROGRAM

2-1. Overview ......................................... 2-12-2. Corrosion Prevention Philosophy ... 2-12-3. Preventive Maintenance ................ 2-12-4. Aircraft Preventive

Maintenance Program .................. 2-12-5. Avionics Preventive

Maintenance Program .................. 2-2

2-6. Corrosion Control Program ............. 2-22-7. Corrosion-Related Failure

Data Feedback ............................. 2-32-8. Safety .............................................. 2-42-9. Materials ......................................... 2-4

3 CORROSION THEORY

3-1. Overview ......................................... 3-13-2. Purpose ........................................... 3-13-3. Scope .............................................. 3-13-4. Definition of Corrosion .................... 3-13-5. Chemical Definitions ....................... 3-13-6. Theory of Corrosion ........................ 3-13-7. Development of Corrosion .............. 3-23-8. Factors Influencing Corrosion ......... 3-33-9. Types of Corrosion .......................... 3-63-10. Metals Affected by Corrosion ........ 3-113-11. Degradation of Non-Metals ........... 3-163-12. Effects of Environment on

Corrosion .................................... 3-183-13. Natural Environment ..................... 3-193-14. Biological Corrosion ...................... 3-213-15. Man-Made Environments .............. 3-23

GLOSSARY ................................................ Glossary-1ALPHABETICAL INDEX .................................. Index-1

TABLE OF CONTENTS

Chapter Page Chapter Page

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LIST OF TABLES

Table Title Page Table Title Page

1-1. Outline of Manual - All Volumes .................. 1-11-2. Outline of Volume I ....................................... 1-2

3-1. Effects of Corrosion on Metals .................... 3-133-2. Effects of Deterioration on NonMetals ........ 3-17

Figure Title Page Figure Title Page

LIST OF ILLUSTRATIONS

2-1. Corrosion Prevention Program ..................... 2-12-2. Basic Maintenance Functions ....................... 2-3

3-1. Simplified Corrosion Cell ............................... 3-23-2. Elimination of Corrosion by Application

of an Organic Film to a Metal Surface ....... 3-23-3. Effect of Sea Water on

Galvanic Corrosion .................................... 3-23-4. Galvanic Series of Metals and

Alloys in Sea Water ................................... 3-43-5. Galvanic Corrosion in a Flashlight Battery ... 3-53-6. Effect of Area Relationship in

Dissimilar Metal Contacts .......................... 3-53-7. Surface Corrosion on Frequency Test Set ... 3-63-8. Galvanic Corrosion of Magnesium

Adjacent to a Steel Fastener ..................... 3-63-9. Variations in the Cross-Sectional

Shape of Corrosion Pits ............................ 3-63-10. Pitting of an Aluminum Wing Assembly ........ 3-73-11. Cross-Section of 7075-T6 Aluminum Alloy ... 3-73-12. Scanning Electron Micrograph of a

Corroding Aluminum Surface .................... 3-7

3-13. Intergranular Corrosion of7075-T6 Aluminum Adjacent toSteel Fastener ........................................... 3-8

3-14. Extreme Example of Exfoliation atEdge of Sheet ............................................ 3-8

3-15. Exfoliation Adjacent to Fasteners ................. 3-83-16. Crevice Corrosion Mechanisms .................... 3-93-17. Filiform Corrosion Found Under Paint

Coating on a Magnesium Panel ................ 3-93-18. Schematic of the Development of Filiform

Corrosion on an Aluminum Alloy ............... 3-93-19. Cracking (Typical of Stress Corrosion or

Corrosion Fatigue) ................................... 3-103-20. Fretting Corrosion ....................................... 3-113-21. Hot Corrosion on Fasteners ........................ 3-123-22. Hot Corrosion on Engine Components ....... 3-123-23. Aluminum Surface Corrosion Products ....... 3-133-24. Magnesium Corrosion Products .................. 3-133-25. Steel Corrosion Products ............................ 3-143-26. Color Changes in Titanium Due to

Heating .................................................... 3-143-27. Cadmium Plated Surface Conditions .......... 3-153-28. Failed Chromium Plate ............................... 3-153-29. Corroded Circuit Card ................................. 3-163-30. Biological Growth on Helicopter Wall .......... 3-21

3-3. Effects of Moisture and Fungi on VariousMaterials .................................................. 3-22

3-4. Effects of Airframe Flluid Intrusion .............. 3-24

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Report Control Number (RCN) Location

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LIST OF TECHNICAL PUBLICATIONS DEFICIENCY REPORTS INCORPORATED

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CHAPTER 1INTRODUCTION

1-1. OVERVIEW. Prevention and repair of corrosiondamage to aircraft and avionic systems continues to bean ever increasing cost and safety burden for militaryaircraft. Equipment is routinely exposed to changes intemperature and pressure, varying humidity levels,dust, dirt, ultraviolet light, aircraft fluids, and environmentsthat promote corrosion. Increasing environmental andsafety restrictions, which limit traditional corrosion controlmaterials, are also a significant factor in the safe andeconomic operation of aircraft and avionics.

1-1.1. The Cleaning and Corrosion Control manualwas established jointly by the Navy, Air Force, and Armyas a combined effort to consolidate and coordinatecorrosion control best practices for aircraft and avionics.

1-1.2. This volumized set of corrosion manualscombines and replaces the former Aircraft WeaponsSystems Cleaning and Corrosion Control (NAVAIR 01-1A-509/TM 1-1500-344-23) and Avionics Cleaning andCorrosion Prevention/Control (NAVAIR 16-1-540/TO1-1-689/TM 1-1500-343-23) manuals.

1-2. PURPOSE. The purpose of this manual is toprovide information on materials and procedures to

prevent, control, and repair corrosion damage to aircraftand avionics on land or at sea.

1-3. SCOPE. The material in this manual containsbasic corrosion prevention and corrective maintenanceinformation to be used at Organizational, Intermediate,and Depot levels.

1-4. ARRANGEMENT OF MANUAL.

1-4.1. OVERVIEW OF ALL VOLUMES. The NAVAIR01-1A-509/TM 1-1500-343-23/TO 1-1-689 series ofmanuals is arranged as shown in Table 1-1.

1-4.1.1. A complete set of manuals to perform aircraftcleaning and corrosion control functions consists ofVolumes I, II, and IV (replaces NAVAIR 01-1A-509/TM 1-1500-344-23).

1-4.1.2. A complete set of manuals to perform avionicsand electronics cleaning and corrosion control functionsconsists of Volumes I, III, and IV (Navy and Army) orVolumes I, II I , and V (Air Force) (replacesNAVAIR 16-1-540/TM 1-1500-343-23/TO 1-1-689).

This volume was prepared under the technical cognizance of theMaterials Engineering Division, NAVAIR North Island, San Diego, California.

Table 1-1. Outline of Manual - All Volumes

EMULOV ELTIT NOITPIRCSEDFEIRB

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.tiezingocer

II tfarcriA serudecorplortnocnoisorrocdnagninaelcsedivorpemulovsihT.tfarcriarof

III scinortcelEdnascinoivA serudecorplortnocnoisorrocdnagninaelcsedivorpemulovsihT.scinortcelednascinoivarof

VI tnempiuqEdnaslairetaMelbamusnoCscinoivAdnatfarcriArof

gninaelcroftnempiuqednaslairetamdevorppastsilemulovsihT.noitamrofnigniredrosedivorpdna,lortnocnoisorrocdna

V tnempiuqEdnaslairetaMelbamusnoCscinoivArof

stsiltI.ecroFriAehtfotseuqerehttaderaperpsawemulovsihTnoisorrocdnagninaelcroftnempiuqednaslairetamdevorppa

.ylnoscinoivarof,noitamrofnigniredrosedivorpdna,lortnoc

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1-4.2. ARRANGEMENT OF VOLUME I. Volume Iconsists of three chapters and a glossary, arranged asshown in Table 1-2.

1-5. RELATED PUBLICATIONS. A listing of relatedpublications is provided in Chapter 1 of each volume ofthis manual, as applicable.

1-6. USAGE AND CONFLICTS.

1-6.1. Supervisory and maintenance personnel shalluse this manual as a guide for all corrosion control andmaintenance efforts. Contractors who maintain andrepair corrosion for military aircraft and avionics shallalso comply with the requirements of this manual.

1-6.2. This manual shall be used in conjunction withand in support of the appropriate Army TechnicalManuals (TMs), Technical Bulletins (TBs), Departmentof the Army Pamphlets (DA PAMs), Navy MaintenanceInstruction Manuals (MIMs), Navy Structural RepairManuals (SRMs), Maintenance Requirement Cards(MRCs), or Air Force Technical Orders (TOs).

1-6.2.1. In the case of a conflict between this manualand other Navy manuals, this manual shall takeprecedence; however, maintenance activities shallcontact the appropriate Cognizant Field Activity (CFA)/Fleet Support Team (FST) for immediate resolution ofthe conflict.

1-6.2.2. The Army and Air Force specific systems/components manuals shall take precedence over thismanual.

1-6.3. WORDING. The following definitions are adheredto in preparing this manual.

1-6.3.1. Shall is used when a procedure is mandatory.

1-6.3.2. Should is used when a procedure isrecommended but not mandatory.

1-6.3.3. Will indicates future action but does not indicatea mandatory procedure.

1-6.3.4. May is used only when a procedure is optional.

1-6.4. SYMBOLS (WARNINGS, CAUTIONS andNOTES). The following definitions apply to WARNINGS,CAUTIONS and NOTES found throughout the manual.

1-6.4.1. WARNING. An operation or maintenanceprocedure, practice, condition, or statement, which ifnot strictly observed, could result in injury to or death ofpersonnel, or long term health hazards to personnel.

1-6.4.2. CAUTION. An operating or maintenanceprocedure, practice, condition, or statement, which ifnot strictly observed, could result in damage/destructionof equipment or loss of mission effectiveness.

1-6.4.3. NOTE. An operating procedure, practice, orcondition which is essential to emphasize.

1-6.5. SERVICE DESIGNATIONS. Since this is a tri-service manual, not all sections apply to all services.Information within the text that does not apply to all threeservices is designated after the paragraph number asfollows: (N) NAVY ONLY, (A) ARMY ONLY, or (AF) AIRFORCE ONLY. Large sections that are service specificare included as appendices in the appropriate volume.

1-7. REPORTING ERRORS AND IMPROVEMENTRECOMMENDATIONS.

1-7.1. GENERAL. All activities using this manual areinvited to submit recommended changes, additions, ordeletions.

Table 1-2. Outline of Volume I

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.launamdezimuloveritneehtfoweivrevonadna,launamehtfo

2 smargorPnoisorroC margorpecnanetniamevitneverpdnalortnocnoisorrocehtseniltuoretpahcsihT.noitamrofniytefaslarenegsedivorpdna,stnemeriuqer

3 yroehTnoisorroC ,ekatnactismrofsuoiraveht,sruccotiyhw,sinoisorroctahwsnialpxeretpahcsihT.tiezingocerotwohdna

yrassolG dnatfarcriagnimrofreplennosrepybdesuylnommocsmretsenifedyrassolgehT.lortnocnoisorrocdnagninaelcscinoiva

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1-7.2. SPECIFIC REPORTING REQUIREMENTS.Recommended changes, additions, or deletions shallbe reported as follows:

1-7.2.1. Navy personnel should submit recommendedchanges to the appropriate technical services facilityusing the reporting system outlined in OPNAVINST4790.2.

1-7.2.2. Air Force personnel should refer to TO 00-5-1to report changes.

1-7.2.3. Army personnel should submit completed DA2028/2028-2 forms to Commander, U.S. Army Aviationand Missile Command, ATTN: AMSAM-MMC-MA-NP,Redstone Arsenal, AL 35898-5220. Changes may alsobe submitted electronically via the Army website,https://amcom2028.redstone.army.mil, or via email to:2028@redstpme/army.mil.

1-8. MANUAL CHANGE PROCEDURES.

1-8.1. RESPONSIBILITY. This manual is a tri-servicedocument, coordinated by the Materials EngineeringDivision, Naval Air Depot North Island, Code 4.9.7,San Diego, CA. The following activities are responsiblefor maintaining this document: the Naval Air SystemsCommand, the Air Force Corrosion Program Office,and the U.S. Army Aviation and Missile Command. Asnecessary, representatives from these activities shallmeet to review proposed engineering and logisticalchanges to this manual. Changes are approved by allservices, except for service-specific information.

1-8.2. PROCEDURES. The Navy is the lead servicefor publication of this manual; therefore, the followingNavy publication change procedures apply:

1-8.2.1. Revisions. Volumes will be updated periodicallyby the issuance of a revision, which is a completereplacement of all pages with all change informationincorporated.

1-8.2.2. Routine Changes. Between revisions, routinechanges may be issued in the form of corrected pages

to a portion of the existing manual. They consist ofreplacement change pages for that section of the manualaffected by the change.

1-8.2.3. Rapid Action Changes. Changes may beissued as a formal Rapid Action Change (RAC) or anInterim Rapid Action Change (IRAC). IRACs are issuedas naval messages to expedite the release of urgentand essential operational and maintenance changeinformation. Army and Air Force program managers areresponsible for retransmittal of IRACs to the appropriateservice addressees.

1-9. REQUISITIONING AND AUTOMATICDISTRIBUTION.

1-9.1. Procedures to be used by Naval activities andother Department of Defense activities requiring NAVAIRtechnical manuals are defined in NAVAIR 00-25-100.

1-9.2. To automatically receive future changes andrevisions to NAVAIR technical manuals, an activitymust be established on the Automatic DistributionRequirements List (ADRL) maintained by the Naval AirTechnical Data and Engineering Service Command(NATEC). To become established on the ADRL, notifyyour activity central technical publications librarian. Ifyour activity does not have a library, you may establishyour automatic distribution requirements by contactingthe Commanding Officer, NATEC, Naval Air StationNorth Island, P.O. Box 357031, Bldg. 90 Distribution,San Diego, CA 92135-7031. Annual reconfirmation ofthese requirements is necessary to remain on automaticdistribution. Use your NATEC assigned account numberwhenever referring to automatic distr ibutionrequirements.

1-9.3. If additional or replacement copies of this manualare required with no attendant changes in the ADRL,they may be ordered by submitting requisitions to theCommanding Officer, Naval Supply Systems Command,Naval Logistics Library, 5801 Tabor Avenue,Philadelphia, PA 19120-5099.

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CHAPTER 2PREVENTIVE MAINTENANCE PROGRAM

2-1. OVERVIEW. Investigations during the past tenyears have identified corrosion as a major factor inelectronics failure in the field. As much as 30% to 40%of military avionic failures are due to the corrosionprocess. This is despite steady improvements in reliabilityof avionic systems fielded to date and outlines the needfor an effective preventive maintenance program.

2-2. CORROSION PREVENTION PHILOSOPHY.Corrosion and environmental conditions are naturalphenomena that adversely affect equipment in fieldservice. Although never totally eliminated, the problemsthese factors cause can be minimized so that they areless severe and better controlled. This can be achievedby understanding equipment failure mechanisms anddevelopment/utilization of corrosion control technology.

2-2.1. As a general rule, maintenance personnel shouldassume corrosion is ongoing, regardless of visiblephysical evidence. The aim of corrosion prevention is toenable systems to perform satisfactorily for a specifiedtime period. In other words, maintenance efforts shouldallow equipment to approach its maximum lifetime.

2-2.2. The general workflow diagram, in Figure 2-1,defines procedures followed to implement a corrosionpreventive maintenance program. This process isdesigned to indicate the sequence of events needed toimplement and maintain an effective corrosionprevention and control program.

2-3. PREVENTIVE MAINTENANCE. The two mostimportant factors in preventing corrosion, and the onlyones which can be controlled by field personnel, are theremoval of the electrolyte and the application of protectivecoatings. Since the extent of corrosion depends on thelength of time electrolytes are in contact with metals,aircraft corrosion can be minimized by frequent washing.If noncorrosive cleaners are used, the more frequentlya surface is cleaned in a corrosive environment the lessthe possibility of corrosive attack. In addition, bymaintaining chemical treatments and paint finishes ingood condition, corrosion can be minimized. Thedegradation of non-metallic materials can be minimizedby avoiding the use of unauthorized maintenancechemicals and procedures. In addition, when repair orreplacement of non-metallic materials is required, onlyapproved materials shall be used. Dedication to properpreventive maintenance practices maximizes equipmentreliability.

2-4. AIRCRAFT PREVENTIVE MAINTENANCEPROGRAM.

2-4.1. The prevention and control of corrosion on aircraftand related equipment is a command responsibility.Each command must place special emphasis on theimportance of the corrosion control program and lend itsfull support to ensure that corrosion prevention andcontrol receives sufficient priority to be accomplishedalong with other required maintenance.

Figure 2-1. Corrosion Prevention ProgramGeneral Workflow Diagram

Train Personnel to Detect, Identify, Clean, Preserve, Treat

& Prevent Corrosion

(Re)Emphasize the Concept of All HandsResponsibility for Corrosion Control

Conduct Required MaintenanceCorrosion Inspections

Report Any Material/Design Deficiencies

Treat Corrosion Promptly After Detection Using the Approved

Materials, Equipment & Techniques

Clean, Preserve, & LubricateEquipment at Prescribed Intervals

Maintain Accurate Maintenance Records

CorrosionDamage Present

?

NO

YES

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2-4.2. Aluminum and magnesium alloys found inaviation equipment will normally begin to corrode if saltdeposits, other corrosive soils, or electrolytes are allowedto remain. In order to prevent corrosion, a constantcycle of cleaning, inspection, operational preservation,and lubrication must be followed. Prompt detection andremoval of corrosion will limit the extent of damage toaircraft components. An effective preventivemaintenance program requires cleaning, lubricationand preservation, as well as corrosion removal, paintremoval, surface treatment, sealing, and painting. Adisciplined preventive maintenance program includes:

a. Regularly scheduled aircraft washing as specifiedby parent service organization directives;

b. Using clean water with low chloride content foraircraft washing and rinsing (chloride content should beless than 400 parts per million, approximately the samelimit as that for potable water);

c. Regularly scheduled cleaning or wipe down of allexposed unpainted surfaces, such as landing gearstruts and actuating rods of hydraulic cylinders asspecified by parent service organization directives, witha compatible fluid or lubricant;

d. Keeping low-point drains open;

e. Inspection, removal, and reapplication of corrosionpreventative compounds (CPCs) on a scheduled basis;

f. Earliest detection and repair of damagedprotective coatings; and

g. Using padded panel racks to store panels/partsfor aircraft and equipment during maintenance andusing protective measures to prevent abrasions/scratches resulting from placement of parts, tools, ortool boxes on wings, fuselage or other aircraft surfaces.

2-5. AVIONICS PREVENTIVE MAINTENANCEPROGRAM.

2-5.1. PROGRAM REQUIREMENTS. Successfulavionic cleaning and corrosion prevention/control effortsdepend on a coordinated, comprehensive preventivemaintenance program. Everyone involved inmaintenance, repair, and operation of avionic systemsmust be concerned with corrosion, cleaning, inspection,prevention, and treatment. Specifically, avionic corrosionprevention/control is everyone’s responsibility. Eachcommand must place special emphasis on the corrosion

control program and lend their full support. This ensuresthe program receives sufficient priority to beaccomplished along with other required maintenance.The goal of a preventive maintenance program is to haltcorrosion before significant decline in equipmentperformance occurs. As such, it is important to recognizethe difference between prevention of corrosion andrepair of damage caused by corrosion. A preventivemaintenance program at the Organizational/Unit andIntermediate Maintenance Activities should:

a. Reduce the maintenance time spent repairingcorrosion damage.

b. Improve avionic system reliability, durability, andservice life.

c. Make the military avionics community aware ofthe extent of the problem.

d. Report any deficiencies with materials andprocesses associated with corrosion control.

2-5.2. APPLICABLE GUIDELINES. All activitiesresponsible for the maintenance of military aircraft andavionic systems shall establish a corrosion prevention/control program. The type of program depends on theconditions or environments to which the aircraft/avionicsystems are exposed. Those aircraft and avionic systemsexposed to salt-air and tropical environments requirethe most stringent corrosion prevention and controlprograms.

2-5.3. MAINTENANCE FUNCTIONS. Experience hasshown that all activities have a corrosion problem. Thisis regardless of whether the equipment is an installedavionic system, ground support equipment, or missilesystem. Accordingly, corrosion control efforts by allhands is mandatory. This must be a day-to-dayrequirement to prevent corrosion before it starts.Figure 2-2 depicts the basic maintenance functions.

2-6. CORROSION CONTROL PROGRAM. Allactivities responsible for aircraft maintenance shallestablish corrosion control programs as required by theparent service organization. This program shall bestructured as required by OPNAVINST 4790.2 (Navy),TO 00-20-1 (Air Force), or AR 750-59 (Army) andensure that personnel receive hazardous material/wastehandlers training. The type of program depends uponthe environment to which the aircraft may be exposed.At sea, where conditions are normally the most severe,aircraft are exposed to salt spray, ship stack gases, and

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aircraft engine exhausts. Land-based aircraft may beexposed to industrial gases, salts, rain, mud, and, nearsalt water, mists containing sea salts. A comprehensivecorrosion control program shall consist of either aCorrosion Control Work Center or a Corrosion ControlTeam with personnel trained in the prevention, earlydetection, reporting, and repair of corrosion damage.Such a program requires a dedicated effort by allmaintenance personnel to prevent corrosion before itstarts. These efforts will improve the operationalreadiness of equipment and minimize costly repairs.

2-6.1. TRAINING. Personnel performing maintenanceon aircraft shall be trained in basic corrosion controlskills as established by the parent service organization.Personnel shall be fully aware of the reasons for thecorrosion control program. Without such training andunderstanding, further damage or additional problemsmay result.

2-6.2. TRAINING AND QUALIFICATION REQUIRE-MENTS. Personnel responsible for corrosion controlmaintenance and treatment shall receive the followingtraining.

a. Supervisors and corrosion control personnel shallattend basic corrosion control courses established bythe parent service organization.

b. Cleaning and repair personnel shall be trained ininspection, identification, cleaning, treatment,preservation, lubrication, hazardous material handling/hazardous waste disposal, and proper documentationreporting.

c. Supervisors shall ensure maximum use of in-service and on-the-job-training.

2-6.3. MAINTENANCE. An effective corrosion controlprogram shall include thorough cleaning, inspection,preservation, and lubrication, at specified intervals, inaccordance with Volumes II and III of this manual.Check for corrosion damage and integrity of protectivefinishes during all scheduled and unscheduledmaintenance. Early detection and repair of corrosionwill limit further damage. When corrosion is discovered,treat corrosion as prescribed in Volumes II and III assoon as possible and use only approved materials,equipment, and techniques. Only affected areas shallbe repaired. All maintenance personnel shall reportcorrosion promptly, in accordance with directivesestablished by the parent service organization.

2-7. CORROSION-RELATED FAILURE DATAFEEDBACK.

2-7.1. Since corrosion prevention and control for aircraftand avionics is a continuing concern, it is vitally importantthat corrosion problems are properly reported. Problemscan be corrected and improvements made to preventreoccurrence in future equipment design. Identificationof the specific causes and extent of corrosion problemsis essential. Improved equipment performance andmaintenance assistance (personnel, equipment,materials, and procedures) are dependent on this data.

Establish Maintenance Requirementsfor Corrosion Control

Establish Requirementsto Inspect for Corrosion Damage

Aquire Corrosion Control Equipment & Consumable Supplies

Establish Requirements for Reporting Corrosion Damage

Develop & Maintain Maintenance Instructions Outlining

Corrosion Control Program

Develop Program to Satisfy SpecificInspection, Cleaning, Treatment,

Preservation & Lubrication Requirements

Develop Cleaning & CorrosionTraining Program

Ensure Proper Training of Personnelin Corrosion Identification & Control

Conduct Required Corrosion Inspections

Ensure Cleaning, Treatment, Preservation, & Lubrication

are Completed

Figure 2-2. Basic Maintenance Functions

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2-7.2. MAINTENANCE AND READINESS DATACOLLECTION. All activities using this manual arerequired to use the current maintenance data collectionsystem(s) of the parent service organization. This willenable a record of corrosion-related failures to besubmitted to the appropriate technical services facilityfor analysis. Reporting personnel shall identify/reportcorrosion discrepancies in accordance with OPNAVINST4709.2 (Navy), TO 00-20-2 (Air Force), or DA PAM738-750/DA PAM 738-751 (Army).

2-8. SAFETY. Safety is everyone’s business andconcern.

2-8.1. RESPONSIBILITY OF SUPERVISORS.

2-8.1.1. Work center supervisors shall receive thefollowing training in accordance with parent servicedirectives:

a. The recognition and elimination of hazards;

b. Occupational safety and health;

c. The safety of the individual;

d. Accident investigation and reporting; and

e. The inspection and maintenance of personalprotective equipment (PPE).

2-8.1.2. Supervisors shall ensure that all corrosioncontrol personnel are informed of:

a. Current safety procedures;

b. Characteristics of materials to which they will beexposed; and

c. Required protective clothing to ensure safety ofpersonnel.

2-8.1.3 In addition, supervisors shall ensure that anadequate supply of safety equipment is in a ready-for-issue condition, and that the personnel under theircontrol are given, and use, appropriate protectiveequipment to prevent accidents, injuries, andoccupational illness.

2-8.2. RESPONSIBILITY OF PERSONNEL. Mainte-nance personnel shall use appropriate equipment whileexposed to hazardous conditions, and shall report tothe supervisor any protective equipment that is bro-

ken, damaged, defective, or inadequate. No one shalluse protective equipment that is not in a satisfactoryand serviceable condition. Personnel shall comply withoccupational safety and health requirements, includingmedical examinations, respirator training and fit testing,and use of protection for eyes, ears, head, skin, and feet.

2-8.3. MATERIALS HANDLING. Many of the materialsand procedures outlined in this manual are potentiallyhazardous to personnel and potentially damaging toaircraft, especially with improper use. When using anychemicals, such as paint removers, detergents,conversion coatings, and solvents, follow the correctprocedures with appropriate protective gear to preventpersonnel injury and aircraft damage. Read theappropriate warnings and cautions in this manual priorto use of any hazardous materials. Misuse of certainmaterials can damage parts or cause corrosion whichmay lead to catastrophic failure. Refer to DoD 6050.5-LR, Hazardous Materials Information System, or theappropriate parent service organization documents forthe handling, storage, and disposal of hazardousmaterials. Refer to local directives and policies pertainingto hazardous waste management. When in doubt,contact the local safety office, industrial hygienist,bioenvironmental engineer, or regional medical center.

2-9. MATERIALS.

2-9.1. Consumable materials and equipment listed inVolumes IV and V shall be used for corrosion control.These materials and equipment have been approvedonly after extensive testing to prove their ability toperform properly and effectively without damaging anyof the metallic or nonmetallic materials used in aircraft.

2-9.2. Only those materials listed in this manual shallbe used for cleaning or corrosion control of aircraftcomponents. When several methods or materials arelisted, the preferred one is listed first, with alternatesfollowing. Materials listed in other manuals shall beused only when required procedures are not covered bythis manual. When approved materials are not available,substitutions shall only be made by the appropriateAircraft Controlling Custodians (ACC) or SystemProgram Manager (SPM).

2-9.3. Materials or processes considered to be animprovement over existing ones, after local laboratoryanalysis and evaluation, shall be forwarded to theAircraft Controlling Custodians (ACC) or SystemProgram Manager (SPM) for submission to the parentservice organization for further evaluation.

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CHAPTER 3CORROSION THEORY

3-1. OVERVIEW.

3-1.1. Maintenance of military aircraft and avionicequipment requires knowledge of why metals corrodeand materials degrade. The theory lies in the definitionand description of mechanisms that cause equipmentto fail in field service. Corrosion is the chemical orelectrochemical deterioration of a material. Thisdeterioration is complex in nature because of the varioustypes of corrosion, the frequent simultaneous presenceof several types of corrosion, and the designcharacteristics and maintenance/environmental factorsthat make aircraft and avionic systems susceptible tocorrosion.

3-1.2. Corrosion can cause complete failure ofequipment or undesirable changes in electricalcharacteristics. It is a process that is active on a 24 hourbasis. Equipment does not necessarily have to beinstalled, operated, or resident in a particularly harshenvironment. Some form of corrosion will take placeeven in near ideal environments. All personnel shouldrecognize that corrosion is the natural continuing processof materials returning to their normal state. Inadequatecorrosion prevention and control will ultimately affectequipment life cycles, downtime, and overall systemreliability.

3-2. PURPOSE. The purpose of this chapter is toprovide maintenance personnel with the backgroundknowledge necessary to understand the causes ofcorrosion.

3-3. SCOPE. This chapter is an introduction to corrosiontheory: the causes of corrosion and the factors whichinfluence its development. The theory of corrosion andthe factors influencing corrosion of aircraft metals aredescribed. The types of corrosion and how to recognizethem are discussed.

3-4. DEFINITION OF CORROSION. Corrosion is theelectrochemical deterioration of a material or itsproperties due to its chemical reaction with thesurrounding environment. This reaction occurs becauseof the tendency of metals to return to their naturallyoccurring state, usually oxide or sulfide ores. Forexample, iron in the presence of moisture and air willreturn to its natural state, iron oxide or rust. Aluminumand magnesium form corrosion products that are whiteoxides or hydroxides. When a water solution containingsoluble salts is present, corrosion of many alloys can

occur easily at ambient temperatures. This type ofcorrosion can be effectively treated by maintenancepersonnel as discussed in this manual. Corrosion canalso occur in the absence of water but only at hightemperatures, such as those found in gas turbineengines. However, the most common type of corrosion(and the one that can be most effectively treated bymaintenance personnel) is electrochemical corrosion.

3-5. CHEMICAL DEFINITIONS.

3-5.1. ATOM. The smallest unit of an element, madeup of a positively charged nucleus surrounded by asystem of negatively charged electrons. There are over100 elements, including metals (such as aluminum,magnesium, gold, platinum, iron, nickel, titanium,cadmium, chromium, copper, silver, lead, beryllium,zinc), and non-metals (such as carbon, boron, sulfur,chlorine, hydrogen, oxygen, nitrogen, and helium).

3-5.2. ELECTRON. A negatively charged subatomicparticle. An electric current occurs when electrons areforced to move through metal conductors. Electronsflow through liquid solutions only in the presence ofions.

3-5.3. ION. An atom or group of atoms or moleculeswhich has acquired a net electric charge by gaining(negative ion) or losing (positive ion) electrons. Whenions are forced to move through liquid solutions, anelectric current can occur. Ions cannot move throughmetal conductors.

3-5.4. ELECTROLYTE. A liquid (usually water) solutioncontaining ions. Sea water is an electrolyte: an aqueous(water-based) solution whose major components aresodium and chloride ions. Electrochemistry is the branchof science concerned with chemical reactions at surfacesin contact with electrolytes.

3-6. THEORY OF CORROSION. All metals will corrodeto some extent in a natural environment. When a metalcorrodes, the atoms lose electrons and become positivelycharged. In solution, the positively charged metal ionscan combine with negatively charged ions to formcorrosion products, such as metallic chlorides, oxides,hydroxides, and sulfides.

3-6.1. Four conditions (illustrated in Figure 3-1) mustexist before metal corrosion can occur.

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a. A metal which has a tendency to corrode mustbe present (the corroding metal is called the anode);

b. A dissimilar conductive material (the cathode),which has less tendency to corrode than the anode,must be present (a dissimilar metal may be a differentmetal, a protected part of the same metal, or conductiveplastic);

c. A conductive liquid (electrolyte) must connect theanode and cathode (so that ions can carry electriccurrent between them); and

d. Electrical contact between the anode and cathode(usually in the form of metal-to-metal contact) mustexist so that electrons can move between the anodeand the cathode.

3-6.2. The elimination of any one of the four conditionswill slow or stop corrosion. For example, a paint film ona metal surface will prevent the electrolyte fromconnecting the anode and cathode, thereby stoppingthe electric current (see Figure 3-2). A change in theelectrolyte can also affect the rate of corrosion. Twoconnected dissimilar metal parts placed in distilledwater corrode very slowly due to a lack of ions in solutionto conduct the electric current; in sea water the corrosionreaction is accelerated by a factor of 1000 or more (seeFigure 3-3).

3-7. DEVELOPMENT OF CORROSION. All corrosiveattack begins on the surface of the metal. If allowed toprogress, corrosion can penetrate into the metal. If

Figure 3-1. Simplified Corrosion Cell

Electron Flow

Electrolyte(Fresh or Sea Water,

Acids, Gases)

AnodicArea

Metal

CathodicArea

Figure 3-3. Effect of Sea Water onGalvanic Corrosion

corrosion begins on an inside surface of a component(for example, the inner wall of a metal tube), it may goundetected until perforation occurs.

3-7.1. When corrosion products form, they oftenprecipitate onto the corroding surface as a powderydeposit. This film of corrosion products may reduce therate of corrosion, if the film acts like a barrier toelectrolytes. Some metals (such as stainless steel andtitanium), under the right conditions, produce corrosionproducts that are so tightly bound to the corroding metalthat they form an invisible oxide film (called a passivefilm) which prevents further corrosion. However, whenthe film of corrosion products is loose and porous (suchas those of aluminum and magnesium), an electrolytecan easily penetrate and continue the corrosion process,producing more extensive damage than surfaceappearance would indicate.

Figure 3-2. Elimination of Corrosion by Applicationof an Organic Film to a Metal Surface

AnodicArea

Metal

CathodicArea

UnbrokenPaint Film

No ContactBetween

Electrolyte &Anode &Cathode

Electrolyte(Continuous Liquid Path)

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3-7.2. Paint coatings can mask the initial stages ofcorrosion. Since corrosion products occupy more volumethan the original metal, the paint surfaces may becomeblistered, flaked, chipped, or appear lumpy.

3-8. FACTORS INFLUENCING CORROSION. Factorswhich influence metal corrosion and the rate of corrosionare outlined below.

3-8.1. TYPE OF MATERIAL. The best time to preventcorrosion is at the design stage. Proper material selectionis critical for the protection of equipment against harmfulenvironmental effects. Most pure metals are not suitablefor aircraft construction and are used only in combinationwith other metals, and sometimes non-metals, to formalloys. The metals most commonly used in aircraftconstruction are aluminum, steel, titanium, andmagnesium. Cadmium, nickel, chromium, and silverare sometimes used as protective platings. Metals havea wide range of corrosion resistance. The most activemetals (those which tend to lose electrons easily), suchas magnesium and aluminum, corrode easily and arelisted at the top of Figure 3-4. The most noble metals(those which do not lose electrons easily), such as goldand silver, do not corrode easily and are listed at thebottom of Figure 3-4.

3-8.2. HEAT TREATMENT AND GRAIN DIRECTION.Most alloys are made up entirely of small crystallineregions called grains. When heat treated duringmanufacturing or repair, heavy sections of metals donot cool uniformly and, as a result, tend to vary inchemical composition from one part of the metal toanother. This can cause corrosion if one area is moreactive than another. Alloys which are fabricated byrolling, extruding, forging, or pressing have propertieswhich depend highly on direction (parallel to grainelongation vs. cross grain). Corrosion can occur onsurfaces of those regions which are less resistant andalso at grain boundaries, resulting in the formation ofpits and intergranular corrosion. For example, exposedend grains corrode much more easily than flattenedelongated surfaces in sheet stock. This explains whyexfoliation occurs at the edge of aircraft skin sections ornext to countersunk fasteners.

3-8.3. DISSIMILAR METALS. When two dissimilarmetals make electrical contact in the presence of anelectrolyte, the rate at which corrosion occurs dependson the difference in their activities, that is, their positionsin Figure 3-4. The greater the difference in activity, the

faster corrosion occurs. For example, magnesiumwould corrode very quickly when coupled with gold ina humid atmosphere. But aluminum would corrodevery slowly, if at all, in contact with cadmium. Aflashlight battery is an example of galvanic corrosionput to practical use. In Figure 3-5, the zinc batterycasing steadily corrodes, supplying a steady flow ofelectrons, but only when the switch is closed. Whenthe switch is open, there is no corrosion becauseelectrons are not able to leave the zinc anode.

3-8.4. ANODE AND CATHODE SURFACE AREA.The rate of corrosion also depends on the size of theparts in contact. If the surface area of the corrodingmetal (the anode) is smaller than the surface area ofthe less active metal (the cathode), corrosion will berapid and severe. But, when the corroding metal islarger than the less active metal, corrosion will be slowand superficial. For example, an aluminum fastener incontact with a relatively inert monel structure maycorrode severely, while a monel bracket secured to alarge aluminum member would result in a relativelysuperficial attack on the aluminum sheet (seeFigure 3-6).

3-8.5. PRESENCE OF ELECTROLYTES. Electricallyconducting solutions are easily formed on metallicsurfaces when condensation, salt spray, rain, or rinsewater accumulate. Dirt, salt, acidic stack gases, andengine exhaust gases can dissolve on wet surfaces,increasing the electrical conductivity of the electrolyte,thereby increasing the rate of corrosion.

3-8.6. ELECTROLYTE CONCENTRATION. In thesame way that metals can corrode when exposed todifferent concentrations of oxygen in an electrolyte,corrosion will also occur if the concentration of theelectrolyte on the surface varies from one location toanother. This corrosive situation is known as aconcentration cell.

3-8.7. AVAILABILITY OF OXYGEN. When some of theelectrolyte on a metal surface is partially confined (suchas between faying surfaces or in a deep crevice), metalin this confined area corrodes more rapidly than othermetal surfaces of the same part outside this area. Thistype of corrosion is called an oxygen concentration cellor differential aeration cell. Corrosion occurs morerapidly than would be expected because the reducedoxygen content of the confined electrolyte causes theadjacent metal to become anodic to the metal surfacesexposed to the air.

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ANODIC - High Corrosion Potential

CATHODIC - Low Corrosion Potential

LithiumMagnesium Alloys

Zinc (plate)Beryllium

Cadmium (plate)Uranium (depleted)

Aluminum AlloysIndium

Tin (plate)Stainless Steel 430 (active)

Lead1010 Steel

Cast IronStainless Steel 410 (active)

Copper (plate)Nickel (plate)

AM 350 (active)Chromium (plate)

Stainless Steels 350, 310, 301, 304 (active)Stainless Steels 430, 410 (passive)

Stainless Steel 13-8, 17-7PH (active)Brass, yellow, Naval

Stainless Steel 316L (active)Bronze 220

Copper 110Stainless Steel 347 (active)

Copper-Nickel 715Stainless Steel 202 (active)

Monel 400Stainless Steel 201 (active)

Stainless Steels 321, 316 (active)Stainless Steels 309, 13-8, 17-7 PH (passive)

Stainless Steels 304, 301, 321 (passive)Stainless Steels 201, 316L (passive)

Stainless Steel 286 (active)AM355 (active)

Stainless Steel 202 (passive)Carpenter 20 (passive)

AM355 (passive)Titanium Alloys

AM350 (passive)Silver

PalladiumGold

RhodiumPlatinum

Carbon/Graphite

Figure 3-4 . Galvanic Series of Metals and Alloys in Sea Water

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3-8.8. TEMPERATURE. Higher temperatureenvironments tend to produce more rapid corrosion dueto accelerated chemical reactions and, in humidenvironments, higher concentration of water vapor inthe air. In addition, nightly drops in temperature cancause greater amounts of condensation, leading toincreased corrosion rates.

3-8.9. BIOLOGICAL ORGANISMS. Bacterias, molds,fungi, and other living organisms (some microscopic)can grow on damp surfaces. Once they are wellestablished, the area tends to remain damp, increasingthe possibility of corrosion. Their presence can causethe areas they occupy to have different oxygen andelectrolyte concentrations. In addition, acidic wastesare secreted, which cause corrosion.

3-8.10. MECHANICAL STRESS. Many alloys used inaircraft construction are sensitive to a form of corrosionknown as stress corrosion cracking. Manufacturingprocesses such as machining, forming, welding, or heattreatment can leave residual mechanical stresses inaircraft parts. The addition of in-service stresses toresidual stresses can cause corrosion to proceed morerapidly than would be expected in normal service.

3-8.11. LENGTH OF EXPOSURE. As time passes,metals naturally tend to corrode. In some cases, thecorrosion process occurs at the same rate, no matterhow long the metal has been exposed to the environment.In other cases, corrosion can decrease with time, due tothe barrier formed by corrosion products, or increasewith time if a barrier to corrosion is being broken down.

Relatively LittleCorrosive Attack

HeavyCorrosiveAttack

Aluminum Sheet(Large Anode)

Monel Sheet(Large Cathode)

Aluminum Rivet(Small Anode)

Monel Rivet(Small Cathode)

Figure 3-6. Effect of Area Relationship inDissimilar Metal Contacts

Figure 3-5. Galvanic Corrosion in a Flashlight Battery

e

e

e

e

e

e

e

e

Electrolyte

Electrolyte

ZincAnode

CarbonCathode

Direction ofCurrent Flow

Metal Ions Go Into SolutionFrom the Anode

+

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3-9. TYPES OF CORROSION. Corrosion iscatalogued and typed in many ways. Occasionally,different names are used for the same type of corrosion.The common types of corrosion are described below.

3-9.1. UNIFORM SURFACE CORROSION. Uniformsurface corrosion is probably the most common type ofcorrosion. It results from a direct chemical attack on ametal surface that proceeds uniformly over the entireexposed surface (see Figure 3-7). The metal graduallybecomes thinner and eventually fails. On a polishedsurface, this type of corrosion is first seen as a generaldulling or etching of the surface and, if the attack isallowed to continue, the surface becomes rough andpossibly frosted in appearance. An example is theetching of metals by acids. The discoloration or generaldulling of metal created by exposure to elevatedtemperatures is not considered to be uniform surfacecorrosion. Coating/sealing the exposed surface willprotect it from this type of attack. Also, corrosive elementsmay be removed through air movement and drain holes.

3-9.2. GALVANIC CORROSION. Galvanic corrosionoccurs when different metals are in contact with eachother and an electrolyte, such as sea water. It is usuallyrecognizable by the presence of a buildup of corrosiondeposits at the joint between the metals. For example,aluminum skin panels and stainless steel doublers,riveted together in an aircraft wing, form a galvaniccouple if moisture and contamination are present.Figure 3-8 shows galvanic corrosion of magnesiumadjacent to steel fasteners. The potential for galvaniccorrosion is greatest when the two metals are wellseparated from each other in the galvanic series (seeFigure 3-4) and are in electrical contact.

3-9.3. PITTING CORROSION. Pitting is a form ofextremely localized attack that results in holes in themetal (see Figure 3-9). Pits can be isolated, or soclose together that they look like a rough surface. Pitsare often difficult to detect because of their small sizeand because they may be covered with corrosionproducts. Pitting is usually first noticeable as a white orgray powdery deposit, similar to dust, which blotchesthe surface. When the deposit is cleaned away, tinypits or holes can be seen in the surface. Most pitsdevelop and grow downward (in the direction of gravity)from a horizontal surface. Pitting failures are commonlycaused by electrolytes containing chloride or chlorine-

Figure 3-7. Surface Corrosion on Frequency Test Set

Figure 3-9. Variations in the Cross-SectionalShape of Corrosion Pits

* Shapes determined by microstructural orientation

Horizontal* Vertical*

Narrow & Deep

Elliptical

Subsurface Undercutting

Wide and Shallow

Figure 3-8. Galvanic Corrosion of MagnesiumAdjacent to Steel Fastener

ElectrolyteCorrosion Products

Magnesium Alloy(Anode)

SteelFastener(Cathode)

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containing ions (such as seawater). Stainless steelsare most susceptible to pitting damage, althoughaluminum, magnesium, and copper are often attacked(see Figure 3-10).

3-9.4. INTERGRANULAR CORROSION.Intergranular corrosion is an attack on the grainboundaries of the metal. A highly magnified crosssection of any commercial alloy (see Figures 3-11 and3-12) shows the granular structure of the metal. Itconsists of quantities of individual grains, each havinga clearly defined boundary, which chemically differsfrom the metal within the grain. Frequently the grainboundaries are anodic (tend to corrode more easily) tothe metal within the grain. When an electrolyte ispresent, rapid selective corrosion of the grainboundaries occurs. High strength aluminum alloys,which depend on precipitated phases of alloyingelements for strength, are particularly susceptible tointergranular attack. Figure 3-13 shows howintergranular corrosion progresses in 7075-T6aluminum alloy adjacent to steel fasteners. In thisexample, the grain boundaries are anodic to both themetal grain and the steel fastener.

3-9.5. EXFOLIATION CORROSION. Exfoliation (seeFigures 3-14 and 3-15) is an advanced form ofintergranular corrosion where the surface grains of ametal are lifted up by the force of expanding corrosionproducts occurring at the grain boundaries. The liftingup or swelling is visible evidence of exfoliation corrosion.Exfoliation occurs on extruded, rolled, wrought, andforged high strength aluminum and magnesium parts.This type of corrosion most often occurs on extrudedsections of metal and is found primarily in aluminumsheet around steel fasteners. Its prevention involvesseparating the aluminum and steel by a barrier, such aszinc-chromate primer or sealant.

3-9.6. CREVICE CORROSION. Crevice corrosion isone of the most familiar types of corrosion. Fieldexperience shows that this type of corrosion may occurin any crevice where a stagnant solution has pooled.Crevices are usually located at gasket surfaces, lapjoints, and under bolt or rivet heads. Crevice corrosionoccurs because the environment of the local area isvery different from the larger environment. As a result,the metal surfaces, even though they may be the samemetal, have different activities, and corrosion occursinside the crevice. This kind of corrosion can also occurwhen a surface is covered by a foreign material. Methodsto minimize crevice corrosion include closing the crevice

Figure 3-11. Cross-Section of 7075-T6 Aluminum Alloy

Figure 3-10. Pitting of an Aluminum Wing Assembly

Figure 3-12. Scanning Electron Micrograph of aCorroding Aluminum Surface

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by welding, sealant, or soldering, and use ofnonabsorbent gaskets (such as Teflon).

3-9.6.1. Oxygen Differential Cells. Electrolyte incontact with metal surfaces will normally containdissolved oxygen. An oxygen cell can develop at anypoint where the oxygen in the air is not allowed todiffuse into the solution, thereby creating a differencein oxygen concentration between two points. Typicallocations of oxygen differential cells are under eithermetallic or non-metallic deposits (dirt) on the metalsurface and under faying surfaces such as riveted lapjoints. Oxygen cells can also develop under gaskets,

wood, rubber, plastic tape, and other materials incontact with the metal surface. Corrosion will occur atthe area of low oxygen concentration (anode) asillustrated Figure 3-16, View A. Alloys such as stainlesssteel, which owe their corrosion resistance to surfacepassivity, are particularly susceptible to this type ofcrevice corrosion.

3-9.6.2. Active/Passive Cells. Metals which dependon a tightly adhering passive film, such as the oxidefilm on corrosion resistant steel, are prone to rapidcorrosive attack by active/passive cells. The corrosiveaction usually starts with a deposit of dirt or salt which

Figure 3-14. Extreme Example of Exfoliation atEdge of Sheet

Figure 3-15. Exfoliation Adjacent to Fasteners

Figure 3-13. Intergranular Corrosion of 7075-T6Aluminum Adjacent to Steel Fasterner

Electrolyte EntersThrough Cracks in Paint Film Intergranualar

CorrosionPaint Film

7075-T6 Aluminum(Anode)

SteelFastener(Cathode)

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creates an oxygen differential cell. The passive film isthen broken in the area of the salt deposit and the moreactive metal beneath the passive film will be exposedto corrosive attack. This small anodic area will thencorrode rapidly due to the much larger area of thesurrounding cathode (passive film). The result is rapidpitting of the surface, as illustrated in Figure 3-16,View B.

3-9.7. FILIFORM CORROSION. Filiform corrosion isa form of crevice corrosion which occurs on metalsurfaces having a thin (~4 mils) organic protectivecoating. It is recognized by its characteristic wormliketrace of corrosion products beneath the coating (seeFigure 3-17). Filiform corrosion is unusual because itonly affects surface appearance, but does not weaken

or destroy the base metal. Filiform corrosion occurswhen the relative humidity of the air is between 65 and90%, and the air temperature is between 70° and 100°F.It starts at breaks in the coating system, such asscratches and cracks around fasteners and seams, andproceeds underneath the coating, due to the diffusionof water vapor and oxygen from the air through thecoating (see Figure 3-18). Filiform corrosion can attack

Figure 3-16. Crevice Corrosion Mechanisms

Passive FilmProtects ExposedSurface

Electrolyte Low Oxygen Concentration(Corrosion Site)

High Oxygen Concentration

Pitting Corrosion

Foreign Material Creates Low OxygenRegion Which Preventsthe Re-Formationof Passive Film

Active Metal

A. Oxygen Differential Cell

B. Active Passive Cell

Figure 3-17. Filiform Corrosion Found UnderPaint Coating on a Magnesium Panel

Figure 3-18. Schematic of the Development of Filiform Corrosion on an Aluminum Alloy

CathodeOrganicCoating

Break inCoating

Base Metal

Anode

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steel, magnesium, and aluminum surfaces and maylead to more serious corrosion in some locations.Filiform corrosion can be prevented by storingequipment and aircraft in an environment with a relativehumidity below 65%, by using coating systems havinga low rate of diffusion for oxygen and water vapors, bymaintaining coatings in good condition (prompt touchuparound fasteners), and applying corrosion preventivecompounds (CPCs) when the coating is damaged.

3-9.8. EROSION CORROSION. Erosion corrosion isthe increase in the rate of attack on a metal due to theaction of a corrosive fluid against the metal surface.Generally the movement is rapid, and wear or abrasionoccurs with the corrosion. Erosion corrosion ischaracterized by grooves, gullies, waves, rounded holesand/or valleys in the metal surface. Metals that are soft(copper, lead) or metals that depend upon thedevelopment of a protective surface film (aluminum,stainless steel) are susceptible to erosion corrosiondamage. Equipment exposed to moving fluids (e.g.heat exchanger tubing, pumps, propellers, impellers)are also susceptible.

3-9.9. STRESS CORROSION. Also called stresscorrosion cracking (SCC). Stress corrosion(Figure 3-19) is the intergranular or transgranularcracking of a metal caused by the combined effects ofconstant tensile stress (internal or applied) and corrosion.Internal or residual stresses may be produced by welding,cold working, forming, and heat treatment operationsduring the manufacture of a part. Stresses remainconcealed in the part unless stress relief operationsare used. Other hidden stresses are induced in partswhen press or shrink fits are used and when slightlymismatched parts are clamped together with rivets andbolts. All these stresses add to those caused by applyingnormal loads to parts in operation. Stress corrosion isnormally localized and appears in the form of cracks.During SCC, the metal is unattacked over most of itssurface, while fine cracks progress through the interiorof the part. Cracking is generally perpendicular to theapplied stress. Metals have threshold stresses belowwhich stress corrosion cracking will not occur. Thisthreshold stress varies from metal to metal, is differentfor different tempers of the same metal, and is differentfor each of the three directions in which stress can beapplied. In aircraft, high strength steel parts (e. g.landing gear) and high strength aluminum parts areparticularly susceptible to stress corrosion.

3-9.9.1. Associated Hazards. Stress corrosion crackingis an extremely dangerous type of failure because it can

occur at stress levels far below the rated strength of ametal, starting from what appears to be a very minorcorrosion pit. This type of failure can be catastrophicand occur without warning. Parts can completely severin a split second or they can crack slowly. The rate ofcracking and the stress limit is very unpredictable inoperating service. For example, 7075-T6 aluminumalloy can fail by stress corrosion cracking when subjectedto a stress which is only 10% of its rated strength.

3-9.9.2. Causes. Specific environments have beenidentified which cause stress corrosion cracking ofcertain alloys. Salt solutions, sea water, and moist saltladen air may cause stress corrosion cracking of heattreatable aluminum alloys, stainless steels, and sometitanium alloys. Magnesium alloys may stress corrodein moist air. Stress corrosion can be prevented byplacing an insulating barrier between the metal and thecorrosive environment, or by applying protective coatingsand/or water displacing corrosion preventivecompounds. Stress relief operations during fabrication

Figure 3-19. Cracking (Typical of Stress Corrosion orCorrosion Fatigue)

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of parts will help, because it lowers the internal stresslevel of the part. Shot peening a metal increasesresistance to stress corrosion cracking by creatingcompressive stresses on the surface, which must beovercome by an applied tensile stress before thesurface sees any tension load. Changing the alloy (forexample, replacing an aluminum 7075-T6 part withone made from 7075-T73 or –T76 alloy) can greatlyincrease resistance to SCC.

3-9.10. CORROSION FATIGUE. Corrosion fatigue(see Figure 3-19) is the cracking of metals caused bythe combined effects of cyclic stress and corrosion. Nometal is immune to reduction in its resistance to cyclicstress if the metal is in a corrosive environment. Corrosiondamages the metal by pitting, followed by crack formationin the pitted area due to cyclic stress. The crack ispropagated predominantly in the fatigue mode, in whichthe rate of cracking is controlled by the stressconcentration in the main cross section, the physicalproperties of the metal, and the presence of corrosionproducts on the crack face. Fracture of a metal part dueto corrosion fatigue occurs when the remaining cross-sectional area is unable to carry the applied loads. Likestress corrosion, corrosion fatigue is normally localizedand appears in the form of cracks. The metal is generallyunattacked over most of its surface, while the crackprogresses through the part. Cracking is generallyperpendicular to the applied stress. Protection of allparts subject to cyclic stress is particularly important,even in environments that are only mildly corrosive.Preventive measures include reducing the stress on thepart, using corrosion inhibitors, and applying a metalliccoating (e.g. chromium, cadmium, or ion vapor deposition(IVD) aluminum) to the part.

3-9.11. FRETTING CORROSION. Fretting corrosionoccurs at contact areas between materials under loadsubject to repeated vibration. The relative motion neededto produce fretting is extremely small (sometimes aslittle as 10-8 cm). The corrosion products increase thewear of the surface, and the wear exposes more baremetal surface to be corroded. The overall effect isgreater than the single effects of corrosion and wearadded together. Fretting has the general appearance ofgalling, in which chunks of metal are torn from thesurface with corrosion at the torn areas or ragged pits(see Figure 3-20). Although fretting corrosion can takeplace on any metal, aluminum, stainless steel, andtitanium alloys are most susceptible. These metalsdepend on an oxide surface film to inhibit furthercorrosion. With rapid movement under pressure at theinterface, the oxides are removed and rapid oxidation

occurs. Moisture does not appear to increase thecorrosion; in fact, it tends to slow down the reaction.Fretting corrosion is normally encountered in heavilyloaded static joints which are subject to vibration, suchas landing gear component attachment areas havinglug holes with slight press fits, slip fit bushings withvery close tolerance bolts passing through the bushings,wing root access panels or wing-to-body fairings, andengine blade roots. Practical means of reducing frettingcorrosion include reducing the amount of relative motionat the surface, adding a lubricant at the interface toreduce friction and seal out oxygen, increasing thesurface hardness of the part, and increasing the overallhardness of one or both contacting metals.

3-9.12. HOT CORROSION. Also called hightemperature oxidation. Corrosion in the absence ofwater can occur at high temperatures, such as thosefound in turbine engine combustors, turbine sections,and afterburners. When hot enough, metals can reactdirectly with the surrounding gases, producing oxidescale (see Figures 3-21 and 3-22). Contaminants,such as chlorides and sulfates, can accelerate the hotcorrosion reaction by reducing the melting point of themetallic oxide and promoting its vaporization. Hightemperature ceramic coatings can reduce this type ofcorrosion but are usually applied only by themanufacturer due to the highly specialized equipmentrequired for application.

3-10. METALS AFFECTED BY CORROSION. Nometal has perfect environmental integrity and is totallyresistant to corrosion. As a result, all metals will corrodesooner or later. The characteristics of corrosion onaircraft metals are summarized in Table 3-1. The

Figure 3-20. Fretting Corrosion

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following paragraphs discuss the corrosioncharacteristics of commonly used aircraft metals.

3-10.1. ALUMINUM. Aluminum and aluminum alloysare the most widely used materials for aircraftconstruction. In addition to its uses in aircraft structure,aluminum and aluminum alloys are widely used inequipment housings, chassis, mounting racks,supports, frames and electrical connector shells.Aluminum is highly anodic, as evidenced by its positionin the galvanic series table. It is anodic to most othermetals, and, when in contact with them, galvaniccorrosion of the aluminum will occur. Aluminum alloysare subject to pitting, intergranular corrosion, andstress corrosion cracking. In some cases, the corrosionproducts of a metal in contact with aluminum arecorrosive to aluminum. However, the formation of atightly adhering oxide film offers increased resistanceunder mild corrosive conditions. The corrosion productof aluminum is a white to gray powdery material(aluminum oxide or hydroxide) which can be removedby mechanical polishing or brushing with abrasives(Figure 3-23). Therefore, it is necessary to clean andprotect aluminum and its alloys against corrosion.Since pure aluminum is more corrosion resistant thanmost alloys, aluminum sheet stock is often coveredwith a thin layer of nearly pure aluminum called claddingor alclad. However, in a marine environment, allaluminum surfaces require protection. Cladding iseasily removed by harsh treatment with abrasives andtooling, exposing the more corrodible alloy surface.Chemical conversion coating, paints, and corrosionpreventive compounds are the main methods ofprotection.

3-10.2. ANODIZED ALUMINUM. Some aluminumparts are protected with an electrochemically appliedoxide coating called anodize. Aluminum oxide film onaluminum is a naturally occurring protective film, andanodizing merely increases the thickness of the oxidefilm. When this coating is damaged in service, it can beonly partially restored by chemical surface treatment.Unnecessary destruction (e.g. nicks and scratches) ofthe anodized surface must be avoided.

3-10.3. MAGNESIUM. Magnesium alloys are thelightest structural metals used for aircraft and missileairframes. Magnesium alloys are used extensivelythroughout avionic systems as antennas, structures,chassis, supports, and frames (radar). Magnesium isalso used extensively for transmission and gearboxhousings. These alloys are highly susceptible tocorrosion when the metal surface is exposed to theenvironment without a protective finish. The corrosionproducts are white powdery snow-like mounds (seeFigure 3-24). The deposits have a tendency to raiseslightly and the corrosion spreads rapidly. When thewhite puffy areas are discovered, it requires prompttreatment or the corrosion will penetrate entirely throughthe structure. The natural oxide-carbonate film formedon magnesium alloys does not provide sufficientcorrosion protection even in the mildest environment.The rate of corrosion of a magnesium alloy increaseswhen the alloy is immersed in water or periodicallysubjected to moisture. Corrosion may also beaccelerated by dissimilar metal couples and whenconductive contaminants are dissolved in the water.Corrosion of magnesium alloys can be greatlydiminished by the use of the proper protective finish,such as magnesium conversion coating and paint.Some magnesium parts in current aircraft have beenoriginally protected by anodizing processes. Coatings

Figure 3-22. Hot Corrosion on Engine ComponentsFigure 3-21. Hot Corrosion on Fasteners

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Table 3-1. Effects of Corrosion on Metals

SYOLLA HCIHWOTKCATTAFOEPYTELBITPECSUSSIYOLLA

FOECNARAEPPATCUDORPNOISORROC

yollAmunimulA sserts,noitailofxe,ralunargretni,gnittipecafruSgnitterfdna,gnikcarceugitafdnanoisorroc redwopyargotetihW

yollAmuisengaM gnittipotelbitpecsusylhgiH ,sdnuomekilwons,yredwop,etihWecafrusnostopsetihwdna

woL&nobraCleetSyollA

)seires0008-0004(

;gnittipdnanoitadixoecafruSnoisorrocralunargretnidnaecafrus )tsur(edixonworb-hsiddeR

sleetSsselniatS)seires004-003(

eniramnignittipemos;noisorroceciverCralunargretni;gnikcarcnoisorroc;stnemnorivne

004(noisorrocecafrus;)seires003(noisorroc)seires

,nworb,derasemitemos;ecafrushguoRniatskcalbro

yollAmuinatiT

detaeperrodednetxe;tnatsisernoisorrocylhgiHnitluseryamstnevlosdetanirolhchtiwtcatnoc

.seitreporplarutcurtss'latemehtfonoitadarged.tnemelttirbmeesuacnacslootdetalpmuimdaC

.erutarepmetwoltastcudorpnoisorrocelbisivoNevobapolevedsedixoecafrusderoloC

)C°073(F°007

sadesu(muimdaCgnitalpevitcetorpa

)leetsrofnoisorrocecafrusmrofinU kcalbronworbottisopedyredwopetihwmorF

.ecafrusehtfognilttom

)etalp(muimorhC stiperehwleetsfognitsursetomorp(gnittiP)etalpnirucco

gnitalpfogniretsilb;stcudorpnoisorrocelbisivoNgnitfildnagnitsuroteud

yollAesab-lekciN)lenoM,lenocnI(

;seitilauqtnatsisernoisorrocdoogsahyllareneGretawaesnignittipotelbitpecsus tisopedyredwopneerG

lekciNsselortcelE)gnitalpasadesu( gnitalpecafrusfognikalfdnagnittiP

noisorrocsetomorptub,edorroctonseodlekciNehtniruccostiperehwlatemesabmunimulafo

gnitalp

,yollAesab-reppoCeznorB,ssarB noisorrocralunargretnidnaecafruS tisopedyredwopneerg-eulbroeulB

revliS ruflusfoecneserpnihsinratlliW mlifkcalbotnworB

dloG tnatsisernoisorrocylhgiH secafrusevitcelferfogninekradesuacstisopeD

niT htworgreksihwottcejbuS stisopedekil-reksihW

Figure 3-23. Aluminum Surface Corrosion Products Figure 3-24. Magnesium Corrosion Products

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of this type are thicker than those applied by immersionor brush on conversion coating. Anodized finishes cannotbe restored in the field. Care should be taken to minimizeremoval of these coatings.

3-10.4. STEEL. Ferrous (iron) alloys are used tomanufacture many aircraft components and massivestructures and assemblies in aircraft ground supportequipment, such as missile gantries, silo crib structures,frames and bodies of trailers and vans, and lesserstructural parts such as brackets, racks, and panels.Iron and steel are also used as component leads,magnetic shields, transformer cores, brackets, racks,and general hardware in avionic systems. If unprotected,ferrous alloy surfaces (with the exception of stainlesssteels) are easily corroded in the presence of moisture.Corrosion of steel is easily recognized because thecorrosion product is red rust (Figure 3-25). Whenferrous alloys corrode, a dark corrosion product usuallyforms first. When moisture is present, this coating isconverted to red rust, which promotes further attack byabsorbing moisture from the air. Ferrous alloy surfacesof structures or assemblies are normally painted, orplated with nickel, tin, or cadmium and painted, toprevent corrosion.

3-10.5. STAINLESS STEEL. Stainless steels, orcorrosion resistant steels (as they are more properlydescribed) are alloys of iron containing large amountsof chromium and nickel. Stainless steels are used forgears, bearings, and high strength bolts, and formountings, racks, brackets, and hardware in avionicsystems. The main reason for the existence of stainlesssteels is their resistance to corrosion. Stainless steelsare much more resistant to common rusting, chemicalaction, and high temperature oxidation than ordinarysteels, due to the formation of an invisible oxide film, or

passive layer, on the surface of these alloys. Corrosionand heat resistance are the major factors in selectingstainless steels for a specific application. However, itshould be well understood that stainless steels are notthe cure-all for all corrosion problems, due to serviceconditions which can destroy the oxide film on theirsurfaces. Stainless steels are susceptible to crevicecorrosion and stress corrosion cracking in moist, saltladen environments. Exposure to saltwater can causepitting. The corrosion product of stainless steel is aroughened surface with a red, brown, or black stain.Corrosion treatment of stainless steel should be limitedto cleaning. Stainless steels can cause galvaniccorrosion of almost any other metal with which they arein contact if proper techniques of sealing and protectivecoating are not used. Stainless steels may be magneticor non-magnetic. The magnetic steels are identified bynumbers in the American Iron and Steel Institute (AISI)400 Series (e.g. 410, 430). These steels are not ascorrosion resistant as the non-magnetic steels, whichare identified by numbers in the AISI 300 Series (e.g.304, 316).

3-10.6. TITANIUM. Titanium and titanium alloys findnumerous uses in aircraft, engines, and missiles attemperatures up to 1000°F (540°C). Above 1000°F,titanium readily absorbs gases from the surrounding airand becomes very brittle. Under certain conditions,chlorides and some chlorinated solvents may inducestress corrosion cracking of certain titanium alloys.Titanium and its alloys are highly corrosion resistant. Anoxide film forms on their surfaces almost immediatelyupon contact with air, which is extremely adherent tothe surfaces and provides a protective barrier. This isidentical to the way aluminum forms a protective oxidefilm on its surface. Even at temperatures approaching1000°F, titanium retains its strength and corrosionresistance. When titanium is heated, oxides having

Figure 3-25. Steel Corrosion Products

Figure 3-26. Color Changes in Titanium Due to Heating

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different colors form on the surface (see Figure 3-26).A blue oxide coating will form at 700° to 800°F (370° to425°C), a purple oxide at 800° to 950°F (425° to510°C), and a gray or black oxide at 1000°F (540°C) orhigher. These are protective discolorations and shouldnot be removed. Since titanium is the less active member(cathodic) of most dissimilar metal couples, it can greatlyaccelerate corrosion of a dissimilar metal coupled to it.Electrical insulation between titanium and other metalsis necessary to prevent galvanic corrosion of the othermetal. Titanium in contact with a corroding metal canabsorb hydrogen and become brittle. Frequentinspection of such areas is required to insure thatinsulation failure has not allowed corrosion to begin.

3-10.7. CADMIUM. Cadmium is used as a coating toprotect steel hardware, such as bolts, washers, andscrews, and as plating on electrical connectors. It isused as a plating on high strength steel parts (e.g.landing gear) to improve resistance to corrosion fatigue.Cadmium may also used to provide a compatible surfacewhen a part is in contact with other materials. Cadmium,when coupled with steel, is anodic and protects the steelby galvanic action. Corrosion on cadmium is evidentby white to brown to black mottling of the surface.When cadmium plate shows mottling and cracks in thecoating, the plating is still performing its protectivefunction as an anodic material (see Figure 3-27). Thecadmium plate on iron or steel is still protecting thebase metal until signs of rust begin to appear. Eventhen, any mechanical removal of corrosion products

should be limited to metal surfaces from which thecadmium has been depleted. Care should be taken notto remove undamaged cadmium plate adjacent to thecorroded area.

3-10.8. CHROMIUM. Chromium is used as a protectiveplating. Chromium plating is also used to provide asmooth, wear-resistant surface and to reclaim wornparts. When corrosion resistance in a marineenvironment is required, a nickel under-coat is used.The degree of protection is dependent upon platingthickness. Chromium forms a continuous oxide coatingthat can be polished to a high luster and still protect notonly itself but any underlying metal. Chromium coatingscontain cracks, and corrosion may originate at the basemetal below these separations. Figure 3-28 shows theresults of a failed chromium plate.

3-10.9. NICKEL. Nickel is important as a plating metal,an additive to stainless steel, and a base for nickelalloys. Pure nickel is used as an electroless coating andis subject to pitting corrosion. Flaking of the nickelcoating can also occur when an underlying metalcorrodes. When added to stainless steel alloys, thestress corrosion resistance increases with nickelcontents above 10%. Nickel based alloys are used inhigh temperature areas (engines, afterburners), butthey are subject to hot corrosion attack and embrittlementwhen sulfur containing gases are present.

3-10.10. COPPER AND COPPER ALLOYS. Copperand copper-based alloys (brass and bronze) areconsidered corrosion resistant, with corrosion usuallylimited to staining and tarnish. Copper and copper-based alloys are generally used in avionic systems ascontacts, springs, leads, connectors, printed circuitboard (PCB) conductors, and wires. Generally, changesin surface conditions are not dangerous and shouldordinarily have no effect on the part. Copper corrosionis evidenced by the accumulation of blue or blue-green

Figure 3-27. Cadmium Plated Surface Conditions

Figure 3-28. Failed Chromium Plate

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corrosion products on the corroded part (seeFigure 3-29). Sometimes copper or copper alloysurfaces tarnish to a dull gray-green color (patina) andthe surface will remain relatively smooth. Thisdiscoloration is the result of the formation of a fine-grained, airtight copper oxide crust. This crust offersgood protection for the underlying metal in ordinarysituations. However, exposure of copper and copperalloys to moisture or salt spray causes the formation ofblue-green salts, indicating active surface corrosion,which should be removed. When coupled with mostmetals used in aircraft construction, copper is the lessactive metal and greatly accelerates corrosion of iron,steel, aluminum, and magnesium. Examples are usuallyfound in electrical components and in areas wherecopper bonding strips or wires are fastened to analuminum chassis or structural components. Protectivepaint coatings are seldom required because of theinherent resistance of the metal. However, paint finishesmay be applied for decorative purposes or if the normaltarnish or green patina on the copper is objectionable.

3-10.11. SILVER. Silver is used as a plating materialover copper in waveguides, miniature andmicrominiature circuits, wires, contacts, high frequencycavities, tank circuits, and RF shielding. Silver does notcorrode in the ordinary sense, although it will tarnish inthe presence of sulfur. The tarnish appears as a brownto black film. The tarnish is silver sulfide and may ormay not be detrimental to circuit electricalcharacteristics, depending on the application. Whensilver is plated over copper there can be an acceleratedcorrosion of the copper. This occurs through galvanicaction at pinholes or breaks in the silver plating. Oneexample of this is the deterioration of silver plated

copper wire. This problem is compounded becausethe wire insulation prohibits detection of breaks in thesilver plating until damage is extensive. This “redplague” is readily identifiable by the presence of abrown-red powder deposit on the exposed copper.Silver plating over nickel plate does not exhibit the redplague phenomenon.

3-10.12. GOLD. Traditionally considered the bestcoating for corrosion resistance and solderability, goldis used on printed circuits, semiconductors, leads, andcontacts. Gold is usually applied in a thin layer overnickel, silver, or copper. Gold is a noble metal (puremetal in nature) and does not normally corrode; however,a slight deposit will appear as a darkening of reflectingsurfaces. Tarnish removal is critical on gold componentsbecause of the very thin coatings used. Gold plated oversilver or copper in thin layers accelerates corrosion overthe less noble metals (silver or copper). This occurs atpores or pinholes in the gold. This corrosion is readilyidentified as darkening of the silver or blue-green depositson the copper. "Purple plague" is a brittle gold-aluminumcompound formed when bonding gold to aluminum.The growth of such a compound can cause failure inmicroelectronic interconnection bonds.

3-10.13. TIN. The use of tin in solder is a well-knownapplication. However, tin plating is also common onavionic RF shields, filters, crystal covers, and automaticswitching devices. Tin has the best combination ofsolderability and corrosion resistance of any metalliccoating. However, tin has a tendency to grow “whiskers”on tin plated wire and other plated applications.

3-11. DEGRADATION OF NON-METALS. Non-metallic materials (plastics, elastomers, paints andadhesives) are not subject to electrochemical corrosion,since ions are not easily formed from non-metallicmaterials and since the electrical conductivity of non-metals is extremely low. The degradation of non-metalsdepends on the chemical makeup of the material andthe nature of the environment. In general, aircraft non-metallic materials are selected for their performanceproperties (flexibility, transparency, strength, electricalresistance). Also, resistance to heat, impact, abrasion,ultraviolet radiation, moisture, ozone and otherdetrimental gases, as well as operational fluids suchas hydraulic fluid, lubricants, cleaners, and deicingfluids, must be considered. However, the use ofunauthorized maintenance chemicals and procedurescan accelerate degradation. In almost all cases, thedeterioration of a nonmetallic material permits moistureintrusion. This creates physical swelling, distortion,

Figure 3-29. Corroded Circuit Card

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mechanical failure through cracking, and altering ofelectrical characteristics. The most common nonmetalsused in aircraft systems and their modes of deteriorationare listed in Table 3-2. Examples of nonmetallicdeterioration are discussed in the following paragraphs.

3-11.1. GRAPHITE/CARBON FIBER COMPOSITES.Graphite or carbon fiber composites are materials whichconsist of reinforcing fibers in a matrix made of organicresin, usually epoxy. They are an important class ofaviation materials because of their high strength-to-weight ratios and high stiffness. Composite materialsare found in aircraft structure, connectors, enclosures,EMI seals and gaskets. Since carbon is the least activemetal in the galvanic series, it will accelerate thecorrosion of any aircraft metal to which it is coupled.Insulation between graphite or carbon epoxycomposites and other metals is necessary to preventdissimilar metal attack on the attached part.

3-11.2. ENCAPSULANTS AND CONFORMALCOATINGS. Encapsulants and conformal coatings areused to envelop an avionic component, module or

assembly. These materials are considered nearly aseffective as hermetic sealing. Typical materials used forthis purpose are epoxy, polyurethane, silicone rubber,acrylic, and varnish. Because these materials areorganic, they are susceptible to moisture, varyingtemperature, and fungus. Moisture accumulates whenincorrect repair procedures cause imperfections andbubbles, or when the coating is applied too thick. Hightemperatures may cause corrosive vapor to outgasfrom encapsulants and conformal coatings. Thisproblem is of special concern if a circuit componentburns on a conformal coated circuit board. Thesegaseous vapors can penetrate under circuit boardcoatings and cause major corrosion damage.

3-11.3. POTTING COMPOUNDS. Potting compoundsare used to encase a part or component, such as in anelectrical connector. Generally, potting compoundsare considered to be a good seal against moisturewhen correctly applied. Problems occur when pottingcompounds are cured too quickly, not mixed properly,or the surface is not cleaned completely. Aging ofcertain potting compounds can cause the pottingcompound to harden and become brittle, or soften and

Table 3-2. Effects of Deterioration on Nonmetals

LAIRETAM HCIHWOTKCATTAFOEPYTELBITPECSUSSILAIRETAM

FOECNARAEPPANOITAROIRETED

cilyrcA stnevlos,erutsiom,thgilVU gnikcarc,noitarolocsiD

evisehdA erutsiom,tnevlos,thgilVU,triD gnileep,gnikcarC

cimareC taehemertxE gnikcarc,noitarolocsiD

htolC wedlim,toryrD tsud,sraet,noitarolocsiD

gnitaoclamrofnoC ssehctarcs,erutsioM gnilbbub,gnikalf,gnileeP

kroC toryrd,wedlim,erutsioM gnileep,tsud,noitarolocsiD

remotsalE gnilcycevissecxe,thgilVU,taeH noitarolocsid,gnizarc,skcarC

noitaluspacnE erutsiom,thgilVU gnidnobsid,gnileep,gnikcarC

tleF wedlim,erutsioM ssenesool,noitarolocsiD

ssalG )nekorb(egamadlacinahcem,taeH derolocsid,dekcarC

setanimaL stnevlos,erutsiom,thgilVU noitanimaled,dnobsid,derolocsiD

tniaP ytidimuh,taeh,erutsioM gnikcarc,gnileep,selbbuB

citsalP noisarba,taeh,thgilVU noitamrofed,skcarc,noitarolocsiD

sremyloP stnevlos,taehemertxE noitamrofed,noitarolocsiD

sdnuopmoCgnittoP taeh,erutsiom,thgilVU noitamrofed,skcarc,noitarolocsiD

)evisorrocnon(VTR taeh,thgilVU,erutsioM noitarolocsid,gnidnobed,gnileeP

stnalaeS taeh,thgilVU,erutsioM noitarolocsid,gnidnobed,gnileeP

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revert. Usually, the process of potting involves the useof a mold to form the potting compound. These moldsare made of plastic and may become the source ofmoisture intrusion. All molds should be removed afterthe potting compound has cured (refer to Volume III).

3-11.4. LAMINATE CIRCUIT BOARDS. Laminatecircuit boards typically use encapsulants or conformalcoatings as sealers. In some cases, the laminate is notsealed along the edge or at the contact tabs. Thisallows the laminate board to absorb moisture anddelaminate. Overcleaning with abrasive materials maydamage the resin surface of the laminate board. Hightemperatures caused by the burning of a circuitcomponent may char the laminate surface and increasemoisture intrusion.

3-11.5. RUBBERS AND ELASTOMERS. Rubbers andelastomers are used for insulation, seals, gaskets,caps, tubing, films, and coatings. Natural rubber, siliconerubber, and polyurethane are normally susceptible toattack by fungi, microbes, ozone and ultraviolet light.Polyurethane and silicone rubber are to some extentpermeable to moisture.

3-11.6. TAPES. Some pressure sensitive tapes areeffective moisture barriers. Problems arise with cloth orpaper-based tapes. These materials absorb moistureand support fungal attack. This is particularly true whenwicking action takes place on the cloth or paper material.Some tapes deteriorate and outgas, emitting an acidthat is corrosive to metals.

3-11.7. OILS. The lubricants used in some electronicequipment are capable of minimizing moisture intrusionand corrosion attack. However, at high temperaturesand pressures, oils can chemically react with impuritiesto produce acids. Oil can hold a limited amount of water.Excess water will separate from the oil if the holdingcapacity is reached at a given temperature.

3-11.8. MATERIAL INCOMPATIBILITY OFNONMETALLICS. The complexity of modern aircraftand avionics makes it difficult to predict what problemsmay result from reactions between materials.Incompatibility of materials can cause deterioration ofthe nonmetallic substances. This may result in therelease of chemicals or gases that react with othercomponents. In some cases, cleaning solutions, hightemperature, or lubricants will cause a reaction innonmetallic substances. Common examples ofnonmetallic materials incompatibility are as follows:

3-11.8.1. Solvents. Some encapsulants, conformalcoatings, and acrylic plastics soften or dissolve whenthey come in contact with cleaning solvents.

3-11.8.2. Heat. The heating of conformal coatings forremoval or repair may outgas corrosive vapors ontometal components. Shrinkage elastomers (heat-shrinkable tubing) can damage adjacent circuitry whenheat guns are applied to shrink the tubing. Certain oils(such as silicones) and greases “creep” as temperatureincreases. This causes contamination of adjacentsurfaces, degradation of organic coatings, and attractionof particulates.

3-11.8.3. Acid Creation. Some commercial conformalcoating strippers contain acids that attack PCB laminatesand discolor or corrode copper. Certain roomtemperature vulcanizing (RTV) silicone sealants containacetic acid that is highly corrosive to metal components(a list of authorized RTV silicone sealants is provided inVolume III). Degradation of polyvinylchloride (PVC)gives off acid fumes that are corrosive to most materialsused in avionics. Some solid film lubricants containuninhibited molybdenum disulfide which, with moistureand heat, may form sulfuric acid.

3-11.8.4. Form Change. Some potting compoundsrevert to liquid form under certain conditions. Thisreversion causes a maintenance problem and reducesthe moisture protection in electrical connectors.

3-11.8.5. Oil and Lubricants. Application of conformalcoatings, adhesives, and paint finishes is difficult whensilicone oil film is present on base material. Some solidfilm lubricants containing graphite are corrosive. Graphiteand moisture will promote galvanic corrosion in manymetals. Some lubricants will attack neoprene, plastics,rubber, organic materials, and most paints.

3-11.8.6. Plastics. Plastics can be damaged by solarheating and cosmic radiation. The damage is a visibledarkening, discoloration, or color fading of the plastic.Lower pressures and vacuum can cause outgassingand loss of plasticizers/ flexibilizers. This can cause achange in structural properties such as loss of strength.The damage is appears as embrittlement and crazingof the surface, as well as loss of electrical properties.

3-12. EFFECTS OF ENVIRONMENT ONCORROSION. Corrosion of aviation equipment occursin both natural and man-made environments. Inaddition, the aircraft operational and maintenanceenvironment contributes to unique corrosion conditions.

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Natural conditions in the environment which affect thecorrosion process are moisture, temperature, saltatmospheres, ozone, sand, dust, and solar radiation.Corrosion may also be promoted by biological sources,including animals, insects, and microorganisms. Man-made conditions, which also affect the corrosionprocess, include industrial pollution, manufacturingoperations, packaging, storage conditions, andshipment. By understanding these conditions,maintenance personnel will be better able to preventaircraft damage.

3-13. NATURAL ENVIRONMENTS.

3-13.1. MOISTURE. Moisture is present in air as a gas(water vapor) or as finely divided droplets of liquid (mistor fog). It often contains contaminants such as chlorides,sulfates, and nitrates, which increase its corrosive effects.Moisture enters all areas of an aircraft that air can enter.Enclosed areas which are not sealed allow air to enterand leave as the pressure between the interior andexterior changes. These pressure differences occurwhen the aircraft changes altitude, when atmosphericpressure changes, and when the temperature of airinside an enclosed area changes.

3-13.1.1. Condensed Moisture. Moisture will condenseout of air when the air becomes too cool to hold all of themoisture in it. The dew found on aircraft after a cool nightis the result of condensation. Condensed moistureusually evaporates as the surrounding air warms, butleaves contaminants, including salts, behind. This canresult in the build-up of soils and salt contamination.Condensed moisture and its contaminants can also betrapped in close fitting wettable joints, such as fayingsurfaces. Some gasket and packing materials will absorbseveral times their weight in water and, when heated,can transmit this retained moisture into the sealed area.Moisture can accumulate in such areas throughsuccessive cycles of warming and cooling. In addition,moisture can be drawn along poor bond lines by capillaryaction (wicking). Temperature and humidity conditionscan vary widely in separate sections of aircraft dependingon the success of environmental sealing, condensation,and location near heat-generating equipment.

3-13.1.2. Effect of Moisture. Electrolyte formationresults from condensation of moisture. All non-metalsabsorb some moisture, which may cause changes indimensional stability, dielectric strengths, ignitionvoltages, and volume insulation resistances. In general,organic matrix composites are adversely affected bymoisture and may suffer a loss of strength and stiffness

from exposure. Hermetic sealing (liquid and vaporproof at normal temperatures and pressures) isrecommended for moisture-critical items such ascapacitors and quartz crystals.

3-13.2. SALT ATMOSPHERES. Salt forms a strongelectrolyte when dissolved in water, which causes rapidcorrosion of unprotected metal surfaces. The primarysource of the world’s salt is the ocean, which is 3.5% to3.9% salt. Normal sea winds can carry from 10 to 100pounds of seasalt per cubic mile of air. Since dissolvedsalts are strong electrolytes, it is easy to understandwhy corrosion is such a severe problem in shipboardand coastal environments.

3-13.3. OZONE. Ozone is a particularly active form ofoxygen which is formed naturally during thunderstorms,by arcing in electrical devices, and by photochemicalreactions in smog. When ozone is absorbed byelectrolyte solutions in contact with metals, it increasesthe rate of corrosion. It also oxidizes many nonmetallicmaterials, being particularly harmful to natural andcertain types of synthetic rubber. Rubber seals storednear welding equipment have experienced completedegradation.

3-13.4. SOLAR RADIATION. The two ranges of solarradiation most damaging to materials are ultraviolet (therange that causes sunburn) and infrared (the range thatmakes sunlight feel warm). On earth, maximum solarradiation occurs in the tropics and equatorial regions,but considerable damage occurs in the temperate zonesas a result of solar heating, photochemical effects, andcombinations of these two phenomena. Non-metals,especially organic and synthetic materials, are stronglyaffected by sunlight. Both natural and synthetic rubberdeteriorate rapidly when exposed to sunlight. Afterextended exposure, plastics darken, paints lose theirprotective characteristics, polymers undergo markeddecreases in strength and toughness, and colors fade,removing essential color coding. Most electronicequipment is housed in enclosed structures and isprotected from solar radiation. Extra care must be takenin the selection and surface treatment of parts, such ascables and harnesses, that are exposed to exteriorenvironments.

3-13.5. TEMPERATURE. High temperature eitherimproves or impairs the performance of avionicequipment, depending on conditions. Corrosion andother harmful processes (outgassing, decomposition)increase as temperature rises. However, moderateincreases in temperature may prevent condensation.

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Most fungus growth is inhibited by temperatures above104°F (40°C). Low temperatures pose no real threat ofcorrosion except that as temperature drops relativehumidity rises. The greatest problem in extremely lowtemperatures, as in polar regions, is the shrinkage ofseals and gaskets. This results in leakage andembrittlement of organic materials. If temperaturesvary sufficiently, moisture may condense to form liquidwater in the equipment. Similarly, the introduction ofcooling air may cause condensation. If moistureintrusion occurs, subsequent freezing can cause theentrapped moisture to expand, resulting in structuraldamage.

3-13.6. PRESSURE. Most military equipment isintended for service in low pressure (high altitude)environments. These environments create the familiarcorona, arcing, and poor cooling problems in avionicequipment. Another problem is that of cyclic low andhigh pressures. These varying pressures create leakyseals and cause breathing. Breathing promotescondensation and creates a corrosive environment.Low pressure also causes outgassing (loss of volatilecomponents, such as plasticizers) of plastics and otherorganic materials. Very low pressure causes changesto the physical and chemical properties of somematerials.

3-13.7. SAND, DUST, AND VOLCANIC ASH. Some ofthe least recognized contributors to corrosion are sand,dust, and volcanic ash. They often contain a number oftar products, ashes, and soot. Sand, dust and volcanicash are hygroscopic and, when present on internal orexternal surfaces of aircraft or electronic parts, canabsorb and hold moisture. This provides an electrolytefor corrosion and the growth of fungus. The presence ofsand, dust or volcanic ash may also affect the operationof electrical contacts, prevent proper action of rotatingmotor-drive devices, and cause malfunctions ofindicating instruments. Sand and dust are extremeproblems in the deserts, since dry, powdery sand anddust are easily carried by wind. During sandstorms,they can penetrate sealed equipment as well as internalareas of airframes. In arid regions such as deserts,small sand particles are often blown as high as 10,000feet by the siroccos (hot, dust laden winds). Sand anddust on surfaces can cause an abrasive action when thesurfaces are moved or vibrated. This removes theprotective oxide coatings and leaves the metal exposed.Volcanic ash contains chlorides and sulfates, whichare extremely corrosive in the presence of moisture.Although small amounts of sand, dust, or ash may be

unnoticed by operating personnel, they may besufficient to initiate or promote corrosion.

3-13.8. CLIMATE. Warm, moist air, normally found intropical climates, tends to accelerate corrosion. Cold,dry air, normally found in arctic climates, tends toreduce corrosion rates. Corrosion does not occur invery dry conditions. For this reason, desiccants areused in shipping containers to produce very dry localenvironments. The operational climate extremes havealways been considered in aircraft design. However,certain areas within an aircraft, such as the cockpit andair conditioned equipment bays, may be subjected toclimatic conditions very different from external areas ofthe aircraft. Relatively warm, dry air that has beencooled by air conditioners, thus reducing its ability tohold moisture, and ducted into interior areas of theaircraft, can release sufficient moisture to acceleratecorrosion. It is imperative, therefore, to consider notonly the operational environment but also theenvironments in which the equipment will be fabricated,transported, reworked, or repaired.

3-13.8.1. Desert. The hot, wind-swept desert creates asevere maintenance problem because powdery dustcan penetrate even supposedly sealed components.High daytime temperatures, high humidities (in areassuch as the Persian Gulf), ultraviolet radiation, and finedust are the four most serious, destructive elements ofthe desert climate. Nonmetallic materials suffer themost damage from the hot desert climates, where airtemperatures during the day may reach 124°F (50°C).Temperatures inside closed containers may be 100°F(38°C) higher than external air temperatures.

3-13.8.2. Temperate Zones. The temperate orintermediate climate zone encompasses most of theNorth American and European continents. These areasat various times of the year may approximate theextremes of polar, desert, or tropical temperatures andhumidity. The temperate zone temperatures range from-25° to 59°F (-32° to 15°C) in the winter and from 59° to125°F (15° to 52°C) in summer. The relative humidityalso fluctuates between five and 100%. The most criticalareas are coastal locations: during the warm periods ofthe year the relative humidity approaches 100% atnight and the air has high concentrations of salt.Moisture from this salt-laden air can condense onequipment during early evening and morning hours,thereby causing serious corrosion. Because of itsrelatively mild temperatures, the temperate zone isalso the most heavily populated. Consequently, the

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smoke, smog, ozone, and corrosive fumes associatedwith heavy industry are also present.

3-13.8.3. Tropics. The greatest challenge to the aircraftindustry is the design of equipment that is protectedfrom corrosion and deterioration in the heat and humiditythat prevails in the tropics. Even though theyencompass only a small portion of the earth’s landarea, the tropics demand the greatest amount ofconsideration from the standpoint of corrosion treatmentand control. The tropical environment is sustained bylong periods of heavy rainfall, during which as many as100 inches of rain may fall. Relative humidities of up to100% at ambient air temperatures of 85°F (29°C) poseformidable threats of corrosion. When high humidityand temperature conditions are combined with salt-laden air, the corrosive environment becomes extremelysevere. Extended periods of high heat and humiditycontribute to rapid corrosion of metals, cracking andflaking of rubber and plastic materials, and deteriorationof seals. Equipment, whether stored or in use, requiresspecial protective containers and frequent preventivemaintenance. Many items become covered with fungi ina matter of hours. For effective operation of electronicequipment in the tropics, special efforts must be made.The critical combination of high temperatures,condensation, high relative humidity, and contaminantssuch as salt and sand may cause catastrophic failure ofequipment. Intensive preventive maintenance and thebest possible protective techniques are necessary foraircraft and their components.

3-14. BIOLOGICAL CORROSION.

3-14.1. ANIMAL AND INSECT DAMAGE. Damage toaircraft and aircraft subsystems may be caused byinsects, birds, and various small animals, especially intropical environments. Equipment in storage is mostsusceptible to this type of attack. Insects and smallanimals may enter through vent holes or tears inpackaging and sometimes build nests. They like to feedon various organic materials, such as polyethylene,insulation, and wire coatings, which can result in systemor equipment failure. Another type of damage can occurwhen electrical insulation, varnishes, and circuit boardcoatings are eaten by insects. When bare wires orcircuit components are exposed, more areas becomeavailable for corrosion and shorting to occur. Moistureabsorbed by nests plus excretions from animals maycause corrosion and deterioration that goes unnoticeduntil equipment is put to use and fails. Damage may alsooccur when organic materials, such as upholstery, areshredded for nests or consumed as food.

3-14.2. MICROORGANISMS. Microbial attack, as theterm is used in this manual, includes the action ofbacteria, fungi, or molds. Microorganisms are nearlyeverywhere and outnumber all other types of livingorganisms. Those organisms causing the greatestcorrosion problems are bacteria and fungi (seeFigure 3-30). Damage resulting from microbial growthcan be caused by: (1) the tendency of the growth to holdmoisture, which then causes corrosion; (2) digestion ofthe substrate as food for the microorganism; or (3)corrosion of the surface beneath the growth by secretedcorrosive fluids. Modern avionic equipment, becauseof complexity, dense packaging, and higher sensitivity,is more susceptible to damage from microbes thanearlier systems. Condensed moisture can provideconditions that promote the growth of molds, bacteria,and fungi. Once established, these growths continueto absorb and hold moisture. Acid secretions from themicroorganisms are strong electrolytes. Theseelectrolytes corrode the underlying metal. Somenonmetals provide nutrients that can accelerate growth.The presence of bacteria and fungi can readily beidentified by damp, slimy, and bad smelling growths.These vary in color from black, bluegreen, green, toyellow. Table 3-3 lists some common materials andthe corresponding effect of moisture and fungi.

3-14.2.1. Bacteria. Bacteria may be either aerobic oranaerobic. Aerobic bacteria require oxygen to live.They can accelerate corrosion by oxidizing sulfur toproduce sulfuric acid or by oxidizing ammonia toproduce nitric acid. Bacteria living adjacent to metalsmay promote corrosion by depleting the oxygen supplyor by releasing metabolic products. Anaerobic bacteria,on the other hand, can survive only when oxygen is not

Figure 3-30. Biological Growth on Helicopter Wall

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present. The metabolism of these bacteria requiresthem to obtain food sources by oxidizing inorganiccompounds such as iron, sulfur, hydrogen, and carbonmonoxide. The resultant chemical reactions causecorrosion.

3-14.2.2. Microbial Growth Requirements. Fungi makeup one class of microorganism that feeds on organicmatter. Low humidity levels will inhibit the growth ofmost species of fungi and bacteria. Ideal growthconditions for most fungi and bacteria are temperaturesbetween 68° and 104°F (20° and 40°C) and relativehumidities between 85 and 100%. It was formerlybelieved that microbial attack could be prevented byapplying moisture-proof coatings to nutrient materials

or by drying the interiors of compartments withdesiccants. However, some moisture-proof coatingsare attacked by microorganisms, especially if thesurface on which they are used is already contaminated.Some microorganisms can survive in spore form forlong periods while dry, and can become active whenmoisture is available. When desiccants becomesaturated and unable to absorb moisture in the affectedarea, microorganisms can begin to grow. Dirt, dust,and other airborne contaminants are the leastrecognized contributors to microbial attack. Unnoticed,small amounts of airborne debris may be sufficient topromote fungal growth.

Table 3-3. Effects of Moisture and Fungi on Various Materials

LAIRETAMROTRAP TCEFFE

stroppus,srehsaw:rebiF nignitluser,ngilasimottroppusehtsesuactahtgnillewssesuacerutsioM.ignufybdeyortseD.strapdetroppusfognidnib

srotalusni,spirtslanimret:rebiF .klatssorcdnasrevohsalfgnisuac,shtapegakaellacirtcelesmroferutsioM.ignufybdeyortseD.tsoleraseitreporpgnitalusnI

,sdraob,spirtslanimret:scitsalpdetanimaLliocdnastekcosebut,slenapdraobhctiws

srotcennocdna,smrof

esuacshtapegakaeL.tsolebotseitreporpgnitalusnisesuacerutsioMdnaecafrusnoworgignufdnasrucconoitanimaleD.klatssorcdnasrevohsalf

erutarepmetemertxerednunoitcartnocdnanoisnapxE.segdednuora.segnahc

,sdraoblanimret:scitsalpdedloMebut,rotcennoc,slenapsdraobhctiws

smrofliocdnastekcos

,ignuffosretroppuserasecafrusfosegdednuorgro,dewas,denihcaMneewtebecnatsisersecuderhtworglagnuF.srevohsalfdnastrohsgnisuac

.sselesuerastrapehttahttnetxenahcusotcitsalpnodetnuomstrap

esolullecdna,repap,nenilnottoC,gnibbew,sgnirevoc,noitalusni:sevitavired

scirtceleid,snoitanimal,gnitleb

,seitreporpcirtceleiddnagnitalusnifotnemeriapmirossolsesuacerutsioM.ignufybdeyortseD.klatssorcdna,srevohsalf,gnicragnisuac

citsalp,sgnisuohdnasesuoh,sesac:dooWstsam,srellif .ignufdnaerutsiomybdesuaceranoitanimaleddna,gnillews,toryrD

steksag,sesac,sparts:rehtaeL gnisuac,slairetamevitcetorpdnagninnatyortsedignufdnaerutsioM.noitaroireted

swodniw,sesnel:ssalG.stcesnidaeddnaseceftcesni,kcarttcesni,tsudcinagronoworgignuF

ybraenedorrocdnaytilibisiverucsbossalgnohtworglagnufdnasetimdaeD.straplatem

noitangerpmirof:xaWesuac,ignuffohtworgehttroppusnaelctoneratahtsexawgnitibihni-ignuF

foecnartnetimrepdna,seitilauqevitcetorpdnagnitalusnifonoitcurtsed.stiucriclacirtcelesecnalabnudnastrapsyortsedtahterutsiom

slateM

dnasugnuF.noisorrocdiparesuacropaverutsiomdnaerutarepmethgiH,noisorrocdeepstahtstcudorprehtodnadicaecudorphtworglairetcab

gnivomfonoitarepoehthtiwserefretnisihT.noitadixodna,secafrusfognihcteriarosetalp,sroticapac,slanimretneewtebtsudsesuacdna,swercs,strap

.srevo-cradna,ytivitisnesnissol,esionesuacyamnrutnihcihw,sresnednoc

ralimissid,lateM ,tneserpsierutsiomnehW.slaitnetopnoisorroctnereffidevahyamslateM.sedorroc)edona(slatemehtfoeno

stniojderedloS pusdeepshcihw,erutsiomsdlohsdraoblanimretnoxulfgniredloslaudiseR.ignuffohtworgdnanoisorroc

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3-14.2.3. Microbial Nutrients. Since fungi, bacteria,and other microorganisms are classified as living, it waspreviously thought that only materials derived fromliving organisms could provide food for microorganisms.Thus, wool, cotton, feathers, and leather are known tobe microbial nutrients. To a large extent this rule ofthumb is still valid, but the increasing complexity ofsynthetic materials makes it difficult, if not impossible, todetermine from the name alone whether a material willsupport growth of microorganisms. Many otherwiseresistant synthetic materials are rendered susceptibleto microbial attack by the addition of chemicals, whichchange the properties of the material. In addition,different species of microorganisms have differentgrowth requirements. The service life, size, shape,surface smoothness, cleanliness, environment, andspecies of microorganism involved all determine thedegree of microbial attack on the affected item.

3-15. MAN-MADE ENVIRONMENTS.

3-15.1. INDUSTRIAL POLLUTANTS. Carbon frominternal combustion engine exhaust, nitrates fromagricultural fertilizers and industrial processes, ozonefrom electrical motors and welding operations, sulfurdioxide from engine exhaust and industrial and shipsmoke stacks, and sulfates from automobile exhaustare significant pollutants. Corrosive solutions form whenthese contaminants are absorbed by condensedmoisture. Many of the fumes and vapors emitted byships and from factories can greatly accelerate metalcorrosion. Industrial atmospheres may exist over largeareas, since wind may carry these corrodents manymiles from their source. The combination of thesepollutants contributes to the deterioration of non-metallicmaterials and severe corrosion of metals.

3-15.2. MANUFACTURING, ASSEMBLY ANDREPAIR. During the manufacture, assembly, or repairof aircraft and subsystem components, many factorsthat might lead to corrosion may be introduced. The useof unsuitable materials and improper material processingcan cause corrosion. Shearing or hole-punchingoperations on some metal alloys, especially high-strength aluminum, may introduce stresses thateventually lead to corrosion. Assembly or repair ofavionic components in areas contaminated by fumes orvapors from adjacent operations (such as welding,paint spraying, and solvent cleaning) may result inentrapment of fumes in the equipment and may causecorrosion. Air conditioned avionic shops are inherentlylower in temperature than surrounding spaces. Inshops without humidity control, the air conditioningsystem can create condensation. This is of particular

concern after working hours because the absence ofshop personnel (body heat) and the lack of heatgenerated by operating various equipment create lowertemperatures. Thermostats should be adjusted toprevent excessive cooling and condensation afterworking hours.

3-15.3. SOLDER FLUX CORROSION. The simple taskof soldering a component in a circuit board can causecorrosion, often due to failure to remove flux residues.Most metals exposed to the atmosphere develop a thinfilm of oxide. This film is not visible and solder alonecannot dissolve it. During soldering operations, theaddition of flux removes the oxide film and preventsfurther metal oxidation. Although most flux is burnedaway during the soldering process, some residuesremain and must be removed. Residues from solderfluxes can degrade circuitry by causing solder joints tocorrode, causing corrosive flux vapors that settle onadjacent components, reducing insulation resistance,changing the resistivity of the solder joint, and attractingdirt and other contaminants that may absorb moisture.Refer to ANSI/J-STD-001 for general solder information.Refer to ANSI/J-STD-004, -005, -006 for solder fluxrequirements for securing connections in electrical orelectronic equipment. Refer to NAVAIR 01-1A-505(Navy), TO 1-1A- 14 (Air Force), or TM 55-1500-323-24(Army) for additional information on solder fluxes andsoldering techniques.

3-15.4. EQUIPMENT HANDLING. Equipment removedfrom aircraft for maintenance or inspection may beexposed to various environments. Equipment thatrequires handling or protection against the environmentshould be preserved and packaged accordingly. Insome cases, special containers are furnished for thepurpose of protecting the equipment. Where containersor special packaging are not furnished, steps shall betaken to provide the protection required. For example,a printed circuit board (PCB) that has been repaired butnot coated with conformal coating requires protectionagainst electrostatic discharge and contaminants. Itshould be placed in an antistatic bag and transportedusing a covered antistatic tote tray or fast pack from therepair station to the test bench. Components withconductive coatings shall be protected from abrasion,damage, and electrostatic discharge. Corrosion thatdevelops as a result of improper handling or inadequatepackaging, storage, and shipment is extremelydestructive. Some of the same design characteristicsthat support corrosion in equipment on board aircraftalso lead to deterioration in inadequately packaged,stored, and shipped components. All uninstalled avionicequipment should be preserved and stored in shock

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and moisture resistant packaging with an activedesiccant. The following unacceptable conditions shouldbe corrected:

a. Equipment with a high replacement rate that iskept unprotected.

b. Equipment moved on an open flat bed vehiclebetween the removing/installing activity and the repairshops that is unprotected.

c. Equipment exposed to the environment awaitingtransfer to the repair shop or depot activity that isunprotected.

3-15.5. PACKAGING. Packaging is intended to protectequipment or components against corrosion,deterioration, and physical damage during transportationand storage. Avoid the use of unsuitable materials, orcorrosion attack may occur as a result of improperpackaging materials. Proper packaging techniques arenot necessarily limited to the manufacturing or shippingactivity but concern the shop technician as well. Certainwoods, cottons, foams, and papers absorb moistureand are susceptible to mold and fungus attack. Thesematerials, and shredded newspaper, excelsior, andfiberboard, may emit acidic vapors. Corrosive vaporsare not only a product of wood but other incompletelycured organic materials such as glues, paints, varnishes,resins, and preservatives. Outgassing (decomposition)of organic materials such as adhesives, gaskets,sealants, wire insulation, sleevings, tubing, plastics,and circuit board varnishes also may produce corrosiveorganic acid vapors. When corrosive vapors arereleased in a confined space or in a piece of avionic

Table 3-4. Effects of Airframe Fluid Intrusion

DIULFFOEPYT TCEFFE

leuFenignE .sremylopemosfognillewsrogninetfoS

diulFciluardyH lacirtcelenosmlifevitalusnifonoitcudortni;noisehdagnitaocfokcaL.stcatnocrotcennoc

stnacirbuL .slairetamteksagdnalaesemosskcattA

tnalooCcirtceleiD .slaescinagroskcattA

diulFgnici-itnA .gniriwlacirtcelenokcattadnanoitasnednocdesaercnI

stnanimatnoCsuoeuqA)stnaccised,noitasnednoc,eniru,retaweerf(

evisorroc;saeraeglibnisdiulffogniloopgnisuacnoitasnednocdesaercnInosmlifevitalusnifonoitcudortni;selpuoccillatemibdetcetorpnufokcatta

.secafrustcatnocrotcennoclacirtcele

sdiulFecnanetniaM)sreppirts,srenaelc,stnegreted,stnevlos(

gnikcarcdnasgnitaoccinagroemosfonoisehdadecuderro/dnagninetfoS.noitalusnieriwemosfo

equipment, serious corrosion attack will occur in thepresence of moisture. Instructions for proper packagingare given in MIL-STD-2073-1.

3-15.6. STORAGE. Even traces of corrosive vapor inpackages containing aircraft parts may result in seriouscorrosion. Moreover, the natural breathing of packagesmay introduce moisture into the parts and equipment.Some packing materials have been known to decomposeand emit corrosive vapors during periods of prolongedstorage. Avionic equipment should not be stored inwooden boxes, fiberboard containers, or ventedcontainers, especially during air shipment. Refer toNAVAIR 15-01-500 (Navy), TM 743-200-1 (Army), orT.O. 1-1-17 (Air Force) for additional storage information.

3-15.7. SHIPMENT. During shipment, materials suchas plastics and lubricants are often exposed toenvironments that were not considered during the designstage. Materials shipped by air are subjected to changesin atmospheric pressure and can lose volatilecomponents by outgassing. The vibration andmechanical shocks associated with shipment by truckcan damage protective coatings or platings. Shipmentby ocean vessel may expose the equipment to corrosivemarine environments, vibrations and shock from enginesor sea conditions, and residual corrosive vapors fromprevious shipments. Equipment should be properlypackaged in accordance with MIL-STD-2073-1 tominimize damage during shipment. Packagingdamaged during shipment should be repaired as soonas possible.

3-15.8. AIRCRAFT FLUIDS. Many fluids can bepresent in various areas of an airframe. Table 3-4 lists

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the types of fluid intrusion and the possible effects.Some of these fluids are corrosive to metals, whileothers are destructive to seals. The destruction ofseals may lead to fluid penetration into areas that areconsidered protected. Some fluids are from externalsources, while others are present due to internal leaksor servicing spills.

3-15.9. OPERATIONAL AND MAINTENANCEENVIRONMENT. All personnel should familiarizethemselves with their local environment, such asmoisture, temperature, atmospheric pressure, salt,water, sand, and dust. The type of aircraft, aircraftmission, and ground operations also influence the rateof corrosion.

3-15.9.1. Type of Aircraft. Fixed-wing and rotary-wingaircraft have some airframe flexibility, which results inseal deterioration. This allows moisture/fluid intrusion.Vulnerable areas include gaskets on access doors/panels; fuselage flexure points; vents, ducts, and staticpressure sensors; equipment bay door andelectromagnetic interference (EMI) gaskets; radomeand antenna seals; and opening for steps.

3-15.9.2. Effects of Aircraft Mission. A number of fieldcorrosion problems are a result of the mission of theaircraft. For example, during a search and rescue (SAR)operation, helicopter flights are performed with thedoors and windows open for optimum visibility. Thiscauses water to enter the cabin, bilge areas, and lowermounted avionic equipment. During low-level over water

flights involved in anti-submarine warfare (ASW),moisture is introduced into aircraft via the fine oceanspray prevalent at lower altitudes. This moisturesaturates wiring harnesses, connectors, antennas,waveguides, and switches, and causes frequentequipment failure. Flight operations in tropicalenvironments can produce conditions that support thegrowth of fungus and other microorganisms.

3-15.9.3. Protection of Aircraft During GroundOperations. Equipment can also be damaged duringmaintenance periods. Many problems are encounteredwhile aircraft are parked. In general, the majority ofaircraft ground time is spent with the aircraft opened upor unbuttoned. It is often necessary for canopies andaccess panels to be open during certain maintenancefunctions. This produces situations where moisture,rain, or ocean spray may soak cockpit and internalavionic components. Inspection and timely corrosioncontrol is essential for proper operation and full lifecycle. Maintenance operation time varies widely betweentypes of equipment and depends largely on reliabilityand troubleshooting time. Most front-line combat aircraftaverage 1 to 2 hours per day in flight operations. Insome cases, aircraft may be nonoperational for extendedperiods while waiting for spare parts. This increases thevulnerability of the entire system to corrosion. Some ofthe potential problems associated with increasedmaintenance operation time are as follows:

a. The avionic system(s) involved may be open(radomes up, equipment bay doors open, canopiesraised) for extended periods.

b. Maintenance may damage seals and the lockingintegrity of fasteners, scratch protective finishes, andotherwise impart wear and tear to the aircraft.

c. Components may be moved several miles andexposed to the environment between the removing/installing activity and the repair shops.

3-25/(3-26 Blank)

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GLOSSARY

A

A-SCAN - A data presentation method for ultrasonicinspection. Data is displayed on a cathode ray tube(CRT). Sound energy amplitude is plotted on the verticalaxis and distance (or time of flight) on the horizontalaxis.

Abrade - To prepare a surface by roughening throughsanding or mechanical means.

Accelerator - A compound that hastens a reaction,especially one that reduces the curing or hardeningtime of compounds.

Acetic Acid - A clear, colorless organic acid with adistinctive sharp odor (e.g. vinegar). Sometimes usedas a solvent.

Acidic - Acid forming or having acid characteristics.

Active Metal - A metal ready to corrode, or beingcorroded.

Additive - A substance added in small amounts tosomething else for a particular purpose. For example,substances are added to fuel and lubricants to preventcorrosion, gum formation, varnishing, sludge formation,or knocking.

Adhesion - The chemical and/or mechanical bondingof a material to a surface.

Adhesion Promoter - A material applied to a surfaceto chemically enhance the adhesion of a paint orsealant to the surface.

Adhesive - Substance capable of holding materialstogether by surface attachment. Adhesive is a generalterm for glue, cement, or paste.

Aeration Cell - An electrolytic cell in which the drivingforce results from a difference in the amount of oxygenin solution from one point to another. Corrosion isaccelerated in areas where the oxygen concentration isleast; for example, in a stuffing box or under packing.

Aerobic - A process which occurs in the presence ofoxygen.

Air Curing - Thermosetting or curing at ordinary roomtemperature without the addition of heat.

Aircraft Controlling Custodian - Navy commandresponsible for specific aircraft (AIRLANT, AIRPAC,CNAVRES, CNATRA, or NAVAIR).

Alkaline - Having a pH of more than 7.

Alloy - A combination of two or more metals.

Anaerobic - A process which occurs in the absence ofoxygen.

Anion - A negatively charged ion that migrates towardthe anode in an electrolyte. The chloride ion in sea wateris an anion.

Anode - The positively charged electrode of anelectrolytic cell at which oxidation or corrosion occurs.It may be a small area on the surface of a metal or alloy,such as that where a pit develops, or it may be the moreactive metal in a cell composed of two dissimilar metals,(i.e., the one with the greater tendency to go intosolution).

Anodize - Application of a protective oxide film on ametal (such as aluminum) through an electrolyticprocess. This layer provides protection from corrosionand is a good base for paint.

Anodic Protection - Formation of a protective film onmetals by externally applied anodic current. Apotentiostat is used to maintain the potential differencebetween the metal and a reference electrode. Atsomewhat higher values of potential, the current dropsto a very low value, the metal becomes passive, andcorrosion is greatly reduced.

Application Time - The length of time after mixing thatsealant remains suitable for application to the substrate.

Aqueous - Relating to, like, containing, or dissolved inwater.

Assembly Time - The maximum length of time aftermixing or thawing that sealant remains suitable toassemble parts, including final tightening of fasteners,to assure proper sealing.

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Atomization - The formation of tiny droplets of liquid, asin the paint spraying process.

Austenitic - A form of stainless steel that is essentiallynonmagnetic and not hardenable by heat treatment.Austenitic stainless steels have the highest corrosionresistance of any of the steel alloys.

B

Base Compound - The major component of a two-partcuring-type sealant that contains the polymer (usuallyreferred to as Part B).

Bilge - The lowest point of an aircraft’s inner hull. Thisarea is where cable runs, wire bundles, coaxial cables,lights, and antennas are typically installed.

Bleeding - A defect in which pigment from a lower coatof paint diffuses into an upper coat and discolors it.

Brush Coat - The thin coating of sealant usually appliedover fasteners, seams, and various parts and smallopenings.

C

Capillary Action - The action by which the surface of aliquid, in contact with a solid, is raised or lowered. Thisis caused by the relative attraction of the liquid moleculesfor each other (surface tension) and those of the solid.The “wicking” of a fluid up a cloth is an example ofcapillary action.

Carbonize - To reduce or convert into carbon, usuallyby high heat or partial burning.

Cathode - The negatively charged electrode of anelectrolytic cell, where the action of the corrosion currentcauses a reduction reaction. This results in the nearlycomplete elimination of corrosion.

Cation - A positively charged ion of an electrolyte whichmigrates toward the cathode. Metallic ions, such as ironor copper, are cations.

Cathodic Protection - Corrosion protection achievedby supplying electrons to the metal to be protected. Thesource of the protective current may be a sacrificialmetal, such as magnesium, zinc, or aluminum, whichcreates a galvanic coupling. The current may also besupplied by a power source (rectifier, generator, orbattery) attached to an appropriate anode and the metal

to be protected. As the current passes to the metal,corrosion is suppressed.

Catalyst - The component of a two-part curing-typesealant that causes the polymer to react.

Caustic Embrittlement - Aluminum and its alloys arerapidly attacked by even dilute alkali solutions. Theresult of the combined action of tensile stress andcorrosion due to the alkali solution causes causticembrittlement.

Cavitation - The sudden formation and collapse of lowpressure bubbles in a liquid stream by means ofmechanical forces. Cavitation is often encounteredaround propellers, rudders, struts, and in pumps.

Cell - In corrosion processes, a cell is a single unit ofelectrolysis that is responsible for corrosion. It consistsof an anode and a cathode immersed in an electrolyteand electrically joined together. The anode and cathodemay be two separate metals or dissimilar areas on thesame metal.

Chalking - Deterioration of an organic coating duringexposure to the environment that results in a powdery,chalky residue on the painted surface.

Channel Seal - A seal formed by injecting a noncuringsealant material into a groove machined in one fayingsurface of the mating or overlapping structure afterassembly.

Checking - Fine cracks in a surface film due to excessiveshrinkage of the film or expansion of the surface.

Chemical Conversion Coating - A chemical treatmentof a metal surface, such as aluminum or magnesium,which results in a protective (corrosion resistant) layeron the metal’s surface. Types of application includeimmersion, brushing, or spraying with the selectedchemical solution. Coatings covered by MIL-C-5541are examples. The protective layer also greatly enhancespaint adhesion

Chlorides - Certain compounds of chlorine that areextremely corrosive. Most pitting corrosion is associatedwith chlorides. Many varieties are present in seawaterand contribute to making seawater corrosive.

Clean - Free from dirt or pollution.

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Clear Water - Colorless water containing no visiblesuspended particles.

Combustible Liquid - Any liquid capable of ignitingand burning at or above 100°F, but below 200°F.

Concentration Cell - An electrolytic cell consisting ofan electrolyte and two electrodes of the same metal oralloy that develops a difference in potential as a resultof a difference in concentration of ions (most often metalions) or oxygen at different points in a solution.

Conformal Coating - A closely adhering moisture andgas barrier applied to circuit boards to prevent corrosionand breakdown of electrical insulation.

Corona - A faint glow adjacent to the surface of anelectrical conductor at high voltage.

Corrosion Fatigue - A reduction in the ability of a metalto withstand cyclic stress due to its exposure to acorrosive environment.

Corrosion Rate - The speed of corrosion attack. It isusually expressed in terms of weight loss per unit of timeor depth of penetration per unit of time.

Couple - Two or more metals or alloys in electricalcontact with each other. These usually can act as theelectrodes of a cell if they are immersed in an electrolyte.

Cracking - (1) Breaking of a metal or alloy in a brittlefashion along a narrow path or network. (2) Localizedbreaking of a paint film to expose the underlying material.

Crazing - The formation of surface cracks, often as afine network, that changes the properties of the film.Crazing does not usually penetrate into the underlyingsurface.

Crevice Corrosion - Corrosion occurring within acrevice. The crevice may be formed at the matingsurfaces of two or more parts of the same or differentmetals, or by a metal and nonmetallic material. Stainlesssteel and aluminum alloys are particularly susceptibleto crevice corrosion. Crevice corrosion is an example ofconcentration cell corrosion.

Critical Avionic Components - Miniature ormicrominiature circuits, including the components,printed circuit boards, tunable coils, tuned circuits, anddevices with gold/silver plated connectors or contacts.

Critical Humidity - The relative humidity, under aspecific set of conditions, at which a metal or an alloy willbegin to corrode. In the presence of hygroscopic(moisture absorptive) solids of corrosion products, thecritical humidity will be lowered. For example, steel willnot corrode if the relative humidity is less than 30% in amarine atmosphere.

Cure - The process by which a coating or sealant isconverted from the liquid to the solid state. Enamelpaints cure; lacquers do not cure.

Cure Rate - The length of time required for a coating orsealant to obtain the minimum hardness required by thespecification.

D

Deionized Water - Water which has had various organicand inorganic materials removed by means of an ionexchange process.

Deposit Attack - When foreign material (dirt, corrosionproducts) is deposited on the surface of a metal, it mayshield the metal from the oxygen necessary to regeneratea protective oxide layer. An oxygen concentration cell isformed, and serious corrosion may result.

Desiccant - A drying agent which acts by absorbingmoisture.

Distilled Water - Water which has had various organicand inorganic materials removed by means of anevaporation and condensation (distillation) process.

Dry to Tape - The drying time required to allow a coatingthe ability to resist marring by adhesive tape.

Durability - Ability of components to function andsustain stresses in field service for a specified period oftime with economical maintenance. This is measured interms of minimum acceptable failure free lifetime (MFL)and expected maximum lifetime (EML) including repair(avionics components), and mean time between failure(MTBF) (aircraft components).

E

Elastomer - A synthetic material with the elasticproperties of natural rubber.

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Electrochemical Corrosion - Corrosion which occurswhen current flows between cathodic and anodic areason metallic surfaces.

Electrode - A metal or alloy that is in contact withelectrolyte and serves as the site where electricitypasses in either direction between the electrolyte andmetal.

Electrolysis - Chemical changes, especiallydecomposition, in the solution or electrolyte due to thepassage of an electric current. Its relation to corrosionarises only if the corrosion process alters the make-upof the electrolyte.

Electrolyte - Any substance which, in solution or whenfused, disassociates into electrically charged ions thatrender the liquid capable of conducting a current. Solubleacids, bases, and salts, such as sea water, areelectrolytes.

Electromagnetic Interference (EMI) - Radiationgenerated from electromagnetic fields which areproduced by radar antennas, Radio Frequency (RF)antennas, shipboard transmitters, certain poorlydesigned avionics units, electric motors, and lightningand other natural effects. This type of radiation caninterfere with aircraft avionics systems causing electricalmalfunctions.

Electromotive Force (EMF) Series - A list of elementsaccording to their standard electrode potentials. Themore negative the potential the greater the tendency ofthe metal to corrode. A hydrogen gas electrode is thestandard reference and its potential is designated aszero. All potentials are positive or negative with respectto the hydrogen electrode. This series does not indicaterates of corrosion.

Electronic Countermeasure - An offensive or defensivetactic using electronic or reflecting devices. It is used toreduce the military effectiveness of enemy equipmentinvolving electromagnetic radiation.

Electronic Warfare - Warfare directed at the electroniccapabilities of the enemy, to detect and prevent hostileuse of the electromagnetic spectrum. Electronic warfareincludes electronic countermeasures and countercountermeasures.

Electrostatic Spraying - A system of applying a coatingin which the coating droplets from an air, air-assistedairless, or airless spray gun are given an electrical

surface charge. These electrically charged droplets areattracted to an electrically grounded workpiece.

Embrittlement - Severe loss of ductility in a metal alloythat results in a sudden, brittle fracture.

Emulsion - One liquid dispersed throughout a secondliquid with which the first liquid will not mix to form ahomogeneous solution.

Enamel - A paint having a high gloss finish.

Encapsulant - The general term describing materialsused to encase or fill a void to prevent the entrance ofmoisture or fungus. Conformal coatings, fungus-proofcoatings, and potting compounds are all forms ofencapsulants.

Epoxy - A type of paint or resin, adhesive, or plasticnoted for high mechanical strength, good adhesion, andresistance to solvents, acids, alkalis, and corrosion.

Ester/Diester Oils - Oils containing synthetic materialsknown as esters or diesters, which are chemicallyformed by the reaction of an alcohol and an acid.Examples include jet engine oil (MIL-PRF-23699) andhydraulic oil (MIL-PRF- 83282). These synthetic oilscan attack certain plastics and paints.

Erosion Corrosion - Accelerated attack of a metal dueto the relative movement between a fluid and the metalsurface.

Etching - The use of a chemical solution or primer toprepare a surface for priming or bonding by removing alayer of the base metal.

Exfoliation - The separation of a material in flakes orlayers.

Expected Maximum Lifetime (EML) - The expectedmaximum period of time over which an avionics system,subsystem, module or component performssatisfactorily. This includes acceptable availability,operation, and support cost (specified number of repaircycles).

Extrusion Grade Sealant - A higher viscosity (8,000 -16,000 poise) sealant designed for application byextrusion by a sealant gun. This grade is usually usedfor forming fillets and sealing vertical surfaces. Usuallydesignated as a Class B sealant.

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F

Fairing - A shape that produces a smooth transitionfrom one angular direction to another.

Fatigue - The tendency of a material to fracture underrepeated cyclic stressing.

Faying Surface - The common surface between matingparts.

Faying Surface Seal - A preassembled seal installedbetween two mating surfaces, used to prevent corrosion.In combination with fillet seals, faying surface sealsprevent a leak path from extending through a fayingsurface into an interior area.

Feathering - To spread out (as paint) especially aroundthe edges of a particular area in order to blend in withadjoining matter and leave no clear lines of distinction.

Filiform Corrosion - Corrosion that develops undercoatings on metals, and appears as fine ragged hairlines,usually wavy or curved and randomly distributed.

Fillet Seal - A primary seal applied at the meeting of twoadjoining surfaces and along the edges of faying surfacesas a continuous bead after part assembly. It can beapplied over, along the edges, and between matingparts.

Film - A thin layer of material that may or may not bevisible to the eye.

Fish-Eyes - Craters distinguished by a center whichconsists of a uniform flat painted region, surrounded bya depression, followed by a ridge of paint. Fish-eyes arecaused by undispersed fluid globules in the paint or byairborne droplets that are deposited on the paintedsurface.

Flammable Liquid - Any liquid having a flash point of100°F or less.

Flashpoint - The minimum temperature at which aliquid gives off an ignitable vapor.

Flat - A surface with minimal reflection. Flat is theopposite of gloss.

Fretting Corrosion - Corrosion which occurs at theinterface of two mating surfaces, under load and

subjected to vibration and slip. The relative motionbetween the two surfaces is extremely small.

Fungus - A group of parasitic lower plants that feed ondead or decaying organic matter. Includes molds,mildews, smuts, mushrooms and some bacteria.

G

Galvanic - A chemical reaction involving two dissimilarmetals which produces direct current electricity.

Galvanic Corrosion - The accelerated corrosion of ametal that is associated with the flow of electrons froma less active metal in the same solution and in contactwith the more active metal.

Galvanic Couple - A closed electric circuit of twoconnected dissimilar metals joined by an electrolyte.

Galvanic Series - A list of metals and alloys arrangedin order of their tendency to corrode in a givenenvironment. The galvanic series is a more accuratepredictor of galvanic relationships than the electromotiveforce (EMF) series.

Gloss - The degree to which a surface reflects light.Gloss is the opposite of flat.

H

Hazardous Material - A material which may pose athreat to human health or the environment whenimproperly handled or disposed of.

Hazardous Waste - Waste which is characterized bythe Environmental Protection Agency (EPA) as 1)ignitable, 2) corrosive, 3) reactive, or 4) toxic, as definedin 40 CFR 261, or is a listed hazardous waste identifiedin that regulation.

Hydrogen Embrittlement - Loss of ductility of a metalcaused by the absorption of hydrogen ions. Hydrogenmay be generated as a by-product of corrosion orthrough various manufacturing operations (cleaning,plating). High strength steel alloys are most susceptibleto hydrogen embrittlement.

Hygroscopic - The property of readily absorbing andretaining moisture.

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I

Impingement Attack - A form of erosion corrosion thatis associated with turbulent flow of a liquid, as at theentrance of a condenser tube or around bends in a pipeline. Air bubbles can accelerate impingement attack.

Inhibitor - A chemical substance or mixture which,when added in small amounts to a solution, markedlydecreases corrosion.

Inorganic Coating - A coating composed of matterother than of plant or mineral origin (e.g. electroplate,chemical conversion, anodize, phosphate or oxide).

Integral Fuel Tank - A load carrying structure of anaircraft that is completely sealed to provide fuelcontainment. It may exist as a cavity in a wing and/or thefuselage.

Ion - An electrically charged atom, group of atoms, ormolecule. The sign of the charge is positive in the caseof cations and negative in the case of anions.

L

Lacquer - A paint that contains a synthetic resin andforms a film after solvent loss. The film is susceptible toattack by the same or similar solvents used in thelacquer.

Local Cell - A cell in which the driving force is due to adifference in potential between areas on a metal or alloysurface immersed in an electrolyte. The potentialdifference may be due to inclusions, lack of homogeneity,or varying concentration of the solution with respect tooxygen or metal ions.

M

Malfunction - Failure to function correctly, especially afailure causing a flight safety situation, a mission abort,or a failure to accomplish mission.

Matte Surface - A surface with a dull finish, as in thecase of an etched or sandblasted surface.

Microbes - Microscopic living plants or organisms,such as germs or bacteria.

Mil - One thousandth of an inch.

Mill Scale - The heavy oxide layer formed during hotfabrication or heat treatment of metals. The term is mostfrequently applied to the scale of mixed iron oxides oniron and steel.

Minimum Failure Free Lifetime (MFL) - The minimumperiod of time that an avionics system, subsystem,module or component performs satisfactorily withoutfailure.

Mottling - Appearance of spotting or blotches of differentcolor or shades of coloring.

N

Nitrates - Compounds including certain combinationsof nitrogen and oxygen. Present in many industrialpollutants.

Noble Metal - A metal usually found as uncombinedmetal in nature, characterized by excellent corrosionresistance and high cost. Also called precious metals.Platinum, gold, and silver are noble metals.

Noncritical Avionics Components - Any avioniccomponent that is not critical to the function of theaircraft, such as tubes, tube sockets, resistors,mechanical devices, knobs, various macroelectroniccomponents and hardware.

Non-Destructive Inspection - An inspection methodused to check the soundness of a material or a partwithout impairing or destroying the serviceability of thepart. Examples are ultrasound, x-ray, and liquidpenetrant.

O

Orange Peel - A surface bumpiness or waviness thatresembles the skin of an orange. Orange peel is oftencaused by poor leveling and is a common defect in bothspray and roll applied painted surfaces.

Organic Coating - A coating composed of matterderived from living organisms or carbon containingcompounds (e.g. paint, lacquer, plastic, grease,preservative).

Outgassing - Emission of a gas during the cure ordecomposition of organic material, usually increased inrate by higher temperatures.

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Overspray - Sprayed paint that misses the area beingpainted and falls upon the surrounding surface.

Oxidation - (1) A chemical reaction in which the atomsin an element lose electrons. Any corrosion processinvolves oxidation of the metal in a true chemical sense.(2) The combination of a metal with oxygen whichproduces oxides, e.g. the scaling of steel at hightemperatures.

Ozone - A triatomic (O3) form of oxygen formed naturally

from diatomic oxygen by electric discharge or exposureto solar ultraviolet radiation. The generation of theozone layer in the upper atmosphere helps to minimize/reduce the amount of harmful ultraviolet radiationproduced by the sun. Ozone is an unstable, powerfullybleaching, poisonous oxidizing agent used to purify anddeodorize air and sterilize water.

P

Passivation - The process or processes that cause ametal to become inert to a given corrosive environment.Passive metals have a very low corrosion rate.

Peel - A method of separating a bond of two materials,where the flexible material is pulled from the matingsurface at a 90° or 180° angle.

Phantom Gripe - An intermittent malfunction or failurewhich cannot be verified/identified for corrective action.

pH - A term used to express the effective hydrogen ionconcentration of a solution. Values range from 0 to 14.A pH of 7 indicates a neutral solution. Values lower than7 indicate an acidic solution, while values greater than7 indicate a basic solution.

Pitting - A form of extremely localized attack thateventually results in holes in the metal. Pits may varyfrom deep cavities of small diameter to relatively shallowdepressions. Pits may be isolated or so close togetherthat they look like a rough surface.

Plasticizer - A chemical added to rubber or resins toimpart flexibility and keep them soft and pliable.

Polyethylene - A thermal plastic (softens with heat)characterized by high impact strength and high electricalresistivity. One of several plastics used for wire coating,it is also used to create film and sheets for packaging.

Polyurethane - A type of paint or resin known for itstoughness, flexibility, weather resistance, chemicalresistance, and abrasion resistance. Commonly usedto make topcoats.

Pot Life - The usable or sprayable life of a coating orsealant after mixing has occurred.

Potentiostat - An electronic device that maintains ametal at a constant potential with respect to a referenceelectrode.

Potting Compound - A poured material which cures toa hard rubberlike consistency and creates a seal. Itadds moisture resistance and vibration resistance tothe item.

Powder Coating - A coating that is applied to thesurface as a dry, finely ground powder and then heatedabove its melting point so that the powder particles flowtogether or cure.

Primer Coat - The first coat of a protective paint system.Originally applied to improve adherence of succeedingcoat(s), it now usually also contains a corrosion inhibitor.

R

Relative Humidity - The ratio (expressed as a percent)of the actual moisture content of the air relative to themaximum moisture content of the air fully saturated atthe same temperature.

Retarders - A solvent added to a paint to slow down itsevaporation rate.

Reversion - The process in which a cured materialreverts toward its precured condition, e.g. a curedpotting compound reverts to a sticky, liquid-likeconsistency.

Room Temperature Vulcanizing (RTV) - A process fortreating or curing of synthetic rubber or plastic materialswhich occurs at room temperature.

S

Sacrificial Anode - A metal that is anodic to the metalbeing protected. It corrodes preferentially whengalvanically coupled to the protected metal and isconsumed (sacrificed) during protection.

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Seal - An adhesive agent, such as sealant, used toclose or secure parts to prevent leaks. Seals may alsobe created by compressive interference, such as O-rings, plugs, and interference fit fasteners.

Sealant - A continuous thick film, with low cohesivestrength, which prevents the passage of liquids orgasses.

Shelf Life - The length of time an unopened containercan be stored at the recommended storage temperatureand still retain the properties required by the specification.

Solvent - A liquid substance capable of dissolving ordispersing another substance.

T

Tack-Free Time - The length of time required for acuring sealant to lose its surface tackiness.

Thixotropic - A property of certain gels to liquefy whenshaken or stirred and then return to a hardened stateupon standing.

Titration - A method or process of determining theconcentration of a dissolved substance by adding to ita standard reagent of known concentration in carefullymeasured amounts until a reaction (color change orelectrical measurement) is completed.

Top Coat -The final paint film applied to a surface.

Total Environment - The circumstances and conditionswhich surround and influence the equipment. The totalenvironment includes manufacturing, storage, shipping,mission, maintenance, and repair.

U

Ultraviolet (UV) Light - Light of a wavelength bandshorter than visible light but longer than X-ray radiation.Longer wavelength UV from the sun causes sunburn.Shorter wavelength UV from unfiltered UV lamps candamage unprotected eyes.

Unacceptable Response - A detrimental abnormalityin system performance.

Undesirable Response - A tolerated interruption ofnormal performance.

Uniform Surface Corrosion - Corrosive etching ofmetal involving only the surface.

V

Viscosity - The degree to which a fluid resists flowunder an applied force.

Void Seal - A seal used to fill holes, channels, and othervoids in a fuel tank. The void seal provides continuoussealing, while a fillet seal would be interrupted bystructural gaps.

Volatile Organic Compound (VOC) - Compounds inpaints and solvents that dissolve into the air. VOCcontent is restricted in many materials due toenvironmental regulations.

W

Water Spotting - The change in surface appearanceresulting from the action of water standing on the paintfilm or substrate. Spotting usually is caused by watersensitivity of the coating, although the defect can be theresult of dissolved material deposited as waterevaporates.

Wet Edge - The ability of a wet coating to blendsmoothly together in the overlap areas during application.

Wet Installed Fastener - Fasteners that have sealantapplied to the shank and under the head prior toinstallation to provide a corrosion barrier and secondaryseal.

Wrap Around - The phenomenon by which electricallycharged paint droplets curve around the rear side of theobject being painted.

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INDEX

A

Aircraft Fluids ............................................ 3-15.8, T3-4

Aircraft Preventive Maintenance Program.............. 2-4

Aluminum and Aluminum Alloys,Effect of Corrosion on ........................ 3-10.1, F3-23

Animal and Insect Damage................................ 3-14.1

Anodized Aluminum, Effect of Corrosion on ...... 3-10.2

Avionics Preventive Maintenance Program ............ 2-5

B

Bacteria ........................................................... 3-14.2.1

Biological Corrosion .............................................. 3-14

C

Cadmium, Effect of Corrosion on ........... 3-10.7, F3-27

Chemical Definitions ............................................... 3-5

Chromium, Effect of Corrosion on .......... 3-10.8, F3-28

Climate ............................................................... 3-13.8

Condensed Moisture ....................................... 3-13.1.1

Copper and Copper Alloys,Effect of Corrosion on ...................... 3-10.10, F3-29

Corrosion Prevention Philosophy .................. 2-2, F2-1

Corrosion

Crevice ................................................ 3-9.6, F3-16

Definition of ....................................................... 3-4

Development of ................................................. 3-7

Erosion ........................................................... 3-9.8

Exfoliation ................................ 3-9.5, F3-14, F3-15

Factors Influencing ............................................ 3-8

Fatigue .............................................. 3-9.10, F3-19

Filiform..................................... 3-9.7, F3-17, F3-18

Fretting .............................................. 3-9.11, F3-20

Galvanic ................................................ 3-9.2, F3-8

Hot ......................................... 3-9.12, F3-21, F3-22

Intergranular ................. 3-9.4, F3-11, F3-12, F3-13

Metals Affected by .................................. 3-10, T3-1

Pitting ........................................ 3-9.3, F3-9, F3-10

Preventive Maintenance ................................... 2-3

Stress .................................................. 3-9.9, F3-19

Theory ............................................................... 3-6

Types ................................................................. 3-9

Uniform Surface .................................... 3-9.1, F3-7

Corrosion Control Program ..................................... 2-6

Maintenance ........................................................ 2-6.3

Training ................................................................ 2-6.1

Corrosion Resistant Steel,Effects of Corrosion on ................................. 3-10.5

Corrosion-Related Failure Data Feedback ............. 2-7

Crevice Corrosion ..................................... 3-9.6, F3-16

D

Degradation of Non-metals .......................... 3-11, T3-2

Conformal Coatings ..................................... 3-11.2

Encapsulants ................................................ 3-11.2

Laminate Circuit Boards ............................... 3-11.4

Oils ............................................................... 3-11.7

Potting Compounds ...................................... 3-11.3

Rubbers and Elastomers ............................. 3-11.5

Tapes ........................................................... 3-11.6

Development of Corrosion ...................................... 3-7

E

Effects of Aircraft Mission ............................... 3-15.9.2

Effects of Airframe Fluid Intrusion ........................ T3-4

Effects of Corrosion on Metals ............................. T3-1

Effects of Deterioration on Nonmetals .................. T3-2

Effects of Moisture and Fungi onVarious Materials ............................................. T3-3

Effects of Environment on Corrosion .................... 3-12

Equipment Handling .......................................... 3-15.4

Erosion Corrosion ................................................ 3-9.8

Exfoliation Corrosion ..................... 3-9.5, F3-14, F3-15

Subject Paragraph,Figure, orTable Number


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F

Factors Influencing Corrosion ................................. 3-8

Fatigue Corrosion ................................... 3-9.10, F3-19

Filiform Corrosion ......................... 3-9.7, F3-17, F3-18

Fretting Corrosion ................................... 3-9.11, F3-20

G

Galvanic Corrosion ..................................... 3-9.2, F3-8

Galvanic Series of Metals and Alloys ................... F3-4

Gold and Gold Plating,Effect of Corrosion on ................................. 3-10.12

Graphite/Carbon Fiber Composites ................... 3-11.1

H

Hot Corrosion .............................. 3-9.12, F3-21, F3-22

I

Incompatibility of Non-Metals ............................ 3-11.8

Industrial Pollutants ........................................... 3-15.1

Intergranular Corrosion ...... 3-9.4, F3-11, F3-12, F3-13

Iron and Steel ..................................................... 3-10.4

M

Magnesium and Magnesium Alloys,Effect of Corrosion on ........................ 3-10.3, F3-24

Maintenance and Readiness Data Collection ..... 2-7.2

Maintenance Functions ............................... 2-5.3, F2-2

Man-made Environments ...................................... 3-15

Manual Change Procedures ................................... 1-8

Manufacturing .................................................... 3-15.2

Material Incompatibility of Nonmetallics ............ 3-11.8

Materials ................................................................. 2-9

Metals Affected by Corrosion ...................... 3-10, T3-1

Microorganisms............................. 3-14.2, T3-3, F3-30

Moisture ............................................................. 3-13.1



N

Natural Environments ........................................... 3-13

Nickel, Effect of Corrosion on ............................ 3-10.9

Non-metals, Degradation of ......................... 3-11, T3-2

O

Operational and Maintenance Environment ...... 3-15.9

Ozone ................................................................ 3-13.3

P

Packaging .......................................................... 3-15.5

Pitting Corrosion ............................. 3-9.3, F3-9, F3-10

Pressure ............................................................. 3-13.6

Preventive Maintenance ......................................... 2-3

Aircraft Program ................................................ 2-4

Avionics Program .............................................. 2-5

Protection of Aircraft DuringGround Operations .................................... 3-15.9.3

Purple Plague .................................................. 3-10.12

R

Red Plague ...................................................... 3-10.11

Reporting Errors andImprovement Recommendations ....................... 1-7

Requisitioning and Automatic Distribution .............. 1-9

S

Safety ...................................................................... 2-8

Materials Handling ......................................... 2-8.3

Responsibility of Personnel ............................ 2-8.2

Responsibility of Supervisors ......................... 2-8.1

Salt Atmosphere ................................................ 3-13.2

Sand, Dust, and Volcanic Ash ........................... 3-13.7

Shipment ............................................................ 3-15.7

Silver, Effect of Corrosion on ........................... 3-10.11

Index-3

NAVAIR 01-1A-509-1TM 1-1500-344-23-1

TO 1-1-689-1

01 March 2005



Solar Radiation .................................................. 3-13.4

Solder Flux Corrosion ........................................ 3-15.3

Specific Reporting Requirements ........................ 1-7.2

Steel, Effect of Corrosion on ................... 3-10.4, F3-25

Stainless Steel, Effect of Corrosion on .............. 3-10.5

Storage .............................................................. 3-15.6

Stress Corrosion ....................................... 3-9.9, F3-19

Symbols ............................................................... 1-6.4

T

Temperature ...................................................... 3-13.5

Theory of Corrosion ................................................ 3-6

Tin .................................................................... 3-10.13

Titanium, Effect of Corrosion on ............. 3-10.6, F3-23

Training and Qualification Requirements ............ 2-6.2

Type of Aircraft ............................................... 3-15.9.1

Types of Corrosion ................................................. 3-9

U

Uniform Surface Corrosion ......................... 3-9.1, F3-7

Usage and Conflicts ................................................ 1-6

W

Wording ................................................................ 1-6.3

Index-3/(Index-4 Blank)

Index-4

NAVAIR 01-1A-509-1TM 1-1500-344-23-1TO 1-1-689-1

01 March 2005


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