CHAPTER 3 FORMS OF CORROSION

CHAPTER 3CHAPTER 3

FORMS OF CORROSIONFORMS OF CORROSION

Chapter Outlines

3.1 Galvanic or Two-Metal Corrosion3.2 Crevice Corrosion3.3 Pitting Corrosion3.4 Intergranular Corrosion 1

GALVANIC CORROSIONGALVANIC CORROSION

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3.1 Galvanic or Two-Metal CorrosionGalvanic or Two-Metal Corrosion also called ' dissimilar metal corrosion‘.

Takes place when two metals are in physical contact with each other and are immersed in a conducting fluid.

corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte.

Examples: 1. Plate and screw of different electrical potentials due to

differences in processing2. Multiple component implant using different metals for each

component3. Copper and steel tubing are joined in a domestic water heater,

the steel will corrode in the vicinity of the junction

The following fundamental requirements have to be met for galvanic corrosion:

1. Dissimilar metals (or other conductors, such a graphite). 2. Electrical contact between the dissimilar conducting materials

(can be direct contact or a secondary connection such as a common grounding path).

3. Electrolyte (the corrosive medium) in contact with the dissimilar conducting materials.

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Noble, cathodic end

PlatinumGoldGraphiteTitaniumSilverHastelloy C18-8 austenitic stainless steels (passive condition)Iron-chromium alloys (passive condition)Inconel (passive)NickelMonelCupronickel alloysBronzesCopperBrassesInconel (active)Nickel (active)TinLead18-8 Austenitic stainless steels (active)13% Chromium stainless steel (active)Cast ironMild steel and ironCadmiumAluminum alloysZincMagnesium and magnesium alloys

Active, anodic end

The relative nobility of a material can be predicted by measuring its corrosion potential. The well known galvanic series lists the relative nobility of certain materials in sea water. A small anode/cathode area ratio is highly undesirable. In this case, the galvanic current is concentrated onto a small anodic area. Rapid thickness loss of the dissolving anode tends to occur under these conditions. Galvanic corrosion problems should be solved by designing to avoid these problems in the first place.

Fig. Galvanic corrosion between stainless steel screw and Aluminium.

Fig. Galvanic corrosion between Steel and Brass.

Fig. Anodic- cathodic behavior of steel with zinc and tin outside layers exposed to the Atmosphere.(a) zinc is anodic to steel and corrodes (b) steel is anodic to tin and corrodes (the tin layer was perforated before the corrosion began)

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Note that the area ratio of the anode: cathode is an important variable affecting the dissolution current density (and hence corrosion rate) pertaining to the anode. The area ratio is also important when considering the relative amount of current "available" from the cathodic reaction.

The following have been described as "main main factorsfactors" influencing galvanic corrosion rates in Skanaluminium's on-line publication "Alubook - Lexical knowledge about aluminium".

• Potential Difference between materials• Cathode Efficiency• Surface areas of connected materials (area ratio)• Electrical resistance of the connection between the materials and of the electrolyte.

Fig. Brass on Weathering SteelBrass on Weathering Steel - rust forms in discrete crystallites that are fine, red and diffusely reflecting, like hematite. The massive re-crystallized layer is a shiny blue, approaching the blue-black of secular hematite.

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CREVICE CORROSIONCREVICE CORROSION

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3.2 Crevice CorrosionCrevice Corrosion Crevice corrosion is a localized form of corrosion usually associated with a

stagnant solution on the micro-environmental level.

Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.

Occurs under gaskets, rivets and bolts, between valve disks and seats.

Well-known examples of such geometries including flanges, gaskets, disbonded linings/coatings, fasteners, lap joints and surface deposits.

Crevice corrosion is initiated by changes in local chemistry within the crevice:

• Depletion of inhibitor in the crevice• Depletion of oxygen in the crevice • A shift to acid conditions in the crevice• Build-up of aggressive ion species (e.g.

chloride) in the crevice

Rivets : a metal pin for passing through holes in two or more plates or pieces to hold them together, usually made with a head at one end, the other end being hammered into a head after insertion. Depletion : to decrease seriously or exhaust the abundance or supply of

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Mechanism- Chronology of Crevice Corrosion

Stage 1At time zero, the oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere.

Stage 2Because of the difficult access caused by the crevice geometry, oxygen consumed by normal uniform corrosion is very soon depleted in the crevice. The corrosion reactions now specialize in the crevice (anodic) and on the open surface (cathodic).

Stage 3The crevice development a few more accelerating factors fully develop: 1. The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off

protons (acid) and forming corrosion products. The pH in a crevice can reach very acidic values, sometimes equivalent to pure acids.

2. The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. See, for example, how the corrosion of steel is affected as a function of water pH.

3. The corrosion products seal even further the crevice environment.4. The accumulation of positive charge in the crevice becomes a strong attractor to

negative ions in the environment, such as chlorides and sulfates, that can be corrosive in their own right.

Fig. Schematic illustration (initial stage) of the mechanism for crevice corrosion between two riveted sheets.

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Fig. Pack rust is a form a localized corrosion typical of steel components that develop a crevice into an open atmospheric environment. This expression is often used in relation to bridge inspection to describe built-up members of steel bridges which are showing signs of rust packing between steel plates.

Zebra mussels- an example of marine environment

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Underside of panel where severe corrosion was foundClose-up picture showing the severity of corrosion9

Crevice Corrosion Testing

ASTM G78ASTM G78Standard Guide for Crevice Corrosion TestingStandard Guide for Crevice Corrosion Testing

A good example of how crevice corrosion can be reproduced and accelerated in a laboratory environment is the formation of occluded cells with multiple crevice assemblies (MCAs), as described in the ASTM G78 Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments.

In this test, washers make a number of contact sites on either side of the specimens. The number of sites showing attack in a given time can be related to the resistance of a material to initiation of localized corrosion, and the average or maximum depth of attack can be related to the rate of propagation. The large number of sites in duplicate or triplicate specimens is amenable to probabilistic evaluation.

The susceptibility to localized corrosion becomes quite visible once a specimen equipped with these Teflon washers has been exposed to a corrosive environment for an extended period of time.

... after 30 days in 0.5 FeCl3 + 0.05 M NaCl 10

PITTING CORROSIONPITTING CORROSION

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3.3 Pitting Pitting CorrosionCorrosion Pitting corrosion is a localized form of corrosive attack that produces holes

or small pits in a metal.

the bulk of the surface remains unattacked.

Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films.

Localized pitting attack is found where these passive films have broken down.

Pitting attack induced by microbial activity, such as sulfate reducing bacteria (SRB) also deserves special mention.

Pitting can be one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates a metal without causing it to lose a significant amount of weight.Special case of crevice corrosion:

1. Initiated by inclusions, scratches, or handling damage instead of deep cracks

2. The presence of static flow conditions and reduced oxygen availability are less important than in crevice corrosion 12

Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable

membrane of corrosion products. Pits can be either hemispherical or cup-shaped.

Pitting is initiated byPitting is initiated by:

1. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)

2. Localized damage to, or poor application of, a protective coating. 3. The presence of non-uniformities in the metal structure of the component, e.g.

nonmetallic inclusions.

A local cell that leads to the initiation of a pit A local cell that leads to the initiation of a pit can be caused by an abnormal anodic site can be caused by an abnormal anodic site surrounded by normal surface which acts as surrounded by normal surface which acts as a a cathodecathode, or by the presence of an abnormal , or by the presence of an abnormal cathodic site surrounded by a normal surface cathodic site surrounded by a normal surface in which a pit will have disappeared due to in which a pit will have disappeared due to corrosion. corrosion.

Fig. Local Cathode on a corroded piece of material

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FIG The pitting of a 304 stainlesssteel plate by an acid-chloride solution.

Alloying can have a significant impact on the pitting resistance of stainless steels. Conventional steel has a greater resistance to pitting than stainless steels, but is still

susceptible, especially when unprotected. Aluminum in an environment containing chlorides and aluminum brass (Cu-20Zn-2Al) in

contaminated or polluted water are usually susceptible to pitting. Titanium is strongly resistant to pitting corrosion. Proper material selection is very effective in preventing the occurrence of pitting

corrosion. Another option for protecting against pitting is to mitigate aggressive environments and environmental components (e.g. chloride ions, low pH, etc.).

Inhibitors may sometimes stop pitting corrosion completely. Further efforts during design of the system can aid in preventing pitting corrosion, for

example, by eliminating stagnant solutions or by the inclusion of cathodic protection.

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Fig. Corrosion Pits are the primary source of leaks in water handling systems

Sewer Explosion due to Pitting Corrosion

An example of corrosion damages with shared responsibilities was the sewer explosion that killed 215 people in Guadalajara, Mexico, in April 1992. Besides the fatalities, the series of blasts damaged 1,600 buildings and injured 1,500 people.

This example of a pitted surface was produced by exposing a specimen of aluminum A92519 to 3.5% NaCl during seven days. The width of the picture is approximately 1 mm. 

Pitting: corrosion of a metal surface, confined

to a point or small area, that takes the form of cavities.

 Pitting factor: ratio of the depth of the deepest pit

resulting from corrosion divided by the average penetration as calculated from weight loss.

 Pitting resistance equivalent number (PREN):

an empirical relationship to predict the pitting resistance of austenitic and duplex stainless steels. It is expressed as PREN = Cr + 3.3 (Mo + 0.5 W) + 16N

*Sewer: an artificial conduit, usually underground, for carrying off waste water and refuse, as in a town or city.

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THROUGH PITS SIDEWAY PITS

Corrosion Pit Shapes

Narrow, deep

Shallow, wide

Elliptical

Vertical Grain Attack

Subsurface

Undercutting

Horizontal grain attack

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Aircraft Location of Failure

Cause Incident Severity

Place Year

From

Bell Helicopter

Fuselage, longeron

Fatigue, corrosion and pitting present

Serious AR, USA

 1997

NTSB

DC-6 Engine, master connecting rod

Corrosion pitting Fatal AK, USA

 1996

NTSB

Piper PA-23 Engine, cylinder

Corrosion pitting Fatal AL, USA

 1996

NTSB

Boeing 75 Rudder Control

Corrosion pitting Substantial damage to plane

WI, USA

 1996

NTSB

Embraer 120

Propeller Blade

Corrosion pitting  Fatal and serious, loss of plane

GA, USA

 1995

NTSB

Gulfstream GA-681

Hydraulic Line

Corrosion pitting Loss of plane, no injuries

AZ, USA

 1994

NTSB

L-1011 Engine, compressor assembly disk

Corrosion pitting Loss of plane, no injuries

AK, USA

 1994

NTSB

Embraer 120

Propeller Blade

Corrosion pitting Damage to plane, no injuries

Canada  1994

NTSB

Embraer 120

Propeller Blade

Corrosion pitting Damage to plane, no injuries

Brazil  1994

NTSB

F/A-18 Trailing-edge Flap (TEF) Outboard Hinge Lug

Corrosion pitting, fatigue

Loss of TEF Australia

 1993

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INTERGRANULAR INTERGRANULAR CORROSIONCORROSION

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3.4 3.4 Intergranular Intergranular CorrosionCorrosion

Intergranular corrosion refers to preferential (localized) corrosion along grain boundaries.

or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected.

This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries.

This selective dissolution may lead to the dislodgement of grains.

Intergranular corrosion in sensitized stainless steels and exfoliation in aluminum alloys represent industrially significant examples of this form of damage.

Also known as “knife- line attack”

classic example is the sensitization of stainless steels or weld decay

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FIG. Intergranular corrosion of a failed aircraft component made of 7075-T6 aluminum (picture width = 500 mm) 

Fig. Severe problem in the welding of stainless steels, when it is termed weld decay.

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