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NACE Standard RP0198-98 Item No. 21084 Standard Recommended Practice The Control of Corrosion Under Thermal Insulation and Fireproofing Materials — A Systems Approach This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he has adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this NACE International standard is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this standard intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this standard in specific instances. NACE International assumes no responsibility for the interpretation or use of this standard by other parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations by individual volunteers. Users of this NACE International standard are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in relation to this standard prior to its use. This NACE International standard may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this standard. Users of this NACE International standard are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard. CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication. The user is cautioned to obtain the latest edition. Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International Membership Services Department, P.O. Box 218340, Houston, Texas 77218-8340 (telephone +1 281/228-6200). Approved 1998-2-20 NACE International P.O. Box 218340 Houston, Texas 77218-8340 +1 281/228-6200 ISBN 1-57590-049-1 ©1998, NACE International
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Page 1: NACE RP198

NACE Standard RP0198-98Item No. 21084

StandardRecommended Practice

The Control of Corrosion Under Thermal Insulationand Fireproofing Materials — A Systems Approach

This NACE International standard represents a consensus of those individual members who havereviewed this document, its scope, and provisions. Its acceptance does not in any respectpreclude anyone, whether he has adopted the standard or not, from manufacturing, marketing,purchasing, or using products, processes, or procedures not in conformance with this standard.Nothing contained in this NACE International standard is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus,or product covered by Letters Patent, or as indemnifying or protecting anyone against liability forinfringement of Letters Patent. This standard represents minimum requirements and should in noway be interpreted as a restriction on the use of better procedures or materials. Neither is thisstandard intended to apply in all cases relating to the subject. Unpredictable circumstances maynegate the usefulness of this standard in specific instances. NACE International assumes noresponsibility for the interpretation or use of this standard by other parties and acceptsresponsibility for only those official NACE International interpretations issued by NACEInternational in accordance with its governing procedures and policies which preclude theissuance of interpretations by individual volunteers.

Users of this NACE International standard are responsible for reviewing appropriate health,safety, environmental, and regulatory documents and for determining their applicability in relationto this standard prior to its use. This NACE International standard may not necessarily addressall potential health and safety problems or environmental hazards associated with the use ofmaterials, equipment, and/or operations detailed or referred to within this standard. Users of thisNACE International standard are also responsible for establishing appropriate health, safety, andenvironmental protection practices, in consultation with appropriate regulatory authorities ifnecessary, to achieve compliance with any existing applicable regulatory requirements prior to theuse of this standard.

CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and maybe revised or withdrawn at any time without prior notice. NACE International requires that actionbe taken to reaffirm, revise, or withdraw this standard no later than five years from the date ofinitial publication. The user is cautioned to obtain the latest edition. Purchasers of NACEInternational standards may receive current information on all standards and other NACEInternational publications by contacting the NACE International Membership ServicesDepartment, P.O. Box 218340, Houston, Texas 77218-8340 (telephone +1 281/228-6200).

Approved 1998-2-20NACE InternationalP.O. Box 218340

Houston, Texas 77218-8340+1 281/228-6200

ISBN 1-57590-049-1©1998, NACE International

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Foreword

This NACE standard recommended practice provides the current technology and industrypractices for mitigating corrosion under thermal insulation and fireproofing materials, a problemtermed corrosion under insulation (CUI) in this standard. Because this corrosion problem hasmany facets and impacts several technologies, a systems approach has been adopted. Thisstandard is intended for use by corrosion-control personnel and others concerned with thecorrosion under insulation and/or fireproofing of piping and other plant equipment. This willconcern chiefly the chemical process, refining, and power generation industries.

This standard was prepared by NACE Work Group T-5A-30a on Corrosion Protection UnderInsulation, with the assistance of Task Group T-6H-31 on Coatings for Carbon and AusteniticStainless Steel Under Insulation and ASTM(1) Committee C-16.40.3 on Corrosion UnderInsulation. Work Group T-5A-30a supports NACE Task Group T-5A-30 on Corrosion UnderThermal Insulation, a component of NACE Unit Committee T-5A on Corrosion in ChemicalProcesses. This standard is issued by NACE Group Committee T-5 on Corrosion Problems in theProcess Industries, the sponsor of T-5A. Task Group T-6H-31 supports NACE Unit CommitteeT-6H (Coating Materials for Atmospheric Service), a component of NACE Group Committee T-6on Protective Coatings and Linings.

The work group has organized the standard into sections by function. Each section was writtenby specialists in that subject. These specialists are industry representatives from firms producing,specifying, designing, and using thermal insulation and fireproofing products on refinery andpetrochemical equipment and piping.

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In NACE standards, the terms shall, must, should, and may are used in accordance with thedefinitions of these terms in the NACE Publications Style Manual, 3rd ed., Paragraph 8.4.1.8.Shall and must are used to state mandatory requirements. Should is used to state that which isconsidered good and is recommended but is not absolutely mandatory. May is used to state that

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which is considered optional.

_________________(1) American Society for Testing and Materials (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.

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NACE InternationalStandard

Recommended Practice

The Control of Corrosion Under Thermal Insulationand Fireproofing Materials — A Systems Approach

Contents

1. General..................................................................................................................... 12. Corrosion Mechanisms.............................................................................................. 13. Mechanical Design.................................................................................................... 74. Protective Coatings................................................................................................. 165. Insulation, Fireproofing, and Accessory Materials.................................................... 196. Inspection and Maintenance.................................................................................... 23

References.................................................................................................................. 27 Bibliography ................................................................................................................ 28 Figure 1: Effect of Temperature on Steel Corrosion in Water ....................................... 3 Figure 2: Typical Vessel Attachments Where Water May Bypass Insulation ................. 8 Figure 3: Attachment to Piping Where Water May Bypass Insulation............................ 8 Figure 4: Vessel Insulation Support Ring, the Problem and the Solution..................... 10 Figure 5: Vertical Vessel Bottom Support Ring Minimizing Water Accumulation......... 10 Figure 6: Vessel-Stiffening Ring Insulation Detail ....................................................... 11 Figure 7: Center Nozzle at Top Head of Vessel .......................................................... 11 Figure 8: Common Nameplate Insulation Detail.......................................................... 12 Figure 9: Seal-Welded Cap on Insulation for Personnel Protection............................. 12 Figure 10:Double-Pipe Heat Exchanger Insulation Penetrated by C-Channel Support . 13 Figure 11:Protrusions Through Jacketing..................................................................... 13 Figure 12:Pipe Supports Without Protrusions .............................................................. 14 Figure 13:Cold Service Pipe Support Without Continuous Vapor Barrier ..................... 14 Figure 14:Cold Service Pipe Support with Continuous Vapor Barrier ........................... 15 Figure 15:Pipe Insulation Penetrated by Column Fireproofing ..................................... 16

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____________________________(2) Materials Technology Institute (MTI), 1215 Fern Ridge P

Section 1: General

1.1 Corrosion under insulation (CUI) has been occurringfor as long as hot or cold equipment has been insulatedfor thermal protection, conservation, or processstabilization. The destructive results and nature of thecorrosion mechanism are not referenced in the literatureuntil the 1950s. As more problems have beenexperienced, concern and interest have built around thissubject. Many articles and symposium papers have beenpublished since 1983 as interest and activity in CUI haveincreased. The increased activity was driven largely bymany occurrences of severe CUI resulting in majorequipment outages, production losses, and unexpectedmaintenance costs in refineries, gas plants, and chemicalplants.

1.2 To correct these problems, companies have devel-oped their own criteria and approaches to the preventionof CUI. When comparing the various approaches, it isevident that there are many similarities, some differences,some new ideas, and some old ideas that have stood thetest of performance. This standard incorporates theexperience of many companies throughout the oil, gas,and chemical industries.

1.3 The first ASTM standard relevant to CUI was ASTMC 692,1 adopted in 1971 and originally titled “Evaluatingthe Influence of Wicking Type Thermal Insulations on theStress Corrosion Cracking Tendency of AusteniticStainless Steels.”

1.4 A symposium was held jointly by NACE, ASTM, andMaterials Technology Institute (MTI)(2) on this subject withspeakers from industries worldwide in October 1983. Thepapers were published in 1985 as ASTM Publication STP

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arkway, Suite 1

1.5 The first NACE report on CUI was written in 1989 byTask Group T-6H-31 as publication 6H189.3 NACE TaskGroup T-5A-30 was organized shortly thereafter to serveas a forum for further discussion regarding CUI. Inaddition to reviews of the corrosion mechanisms,perspectives on such CUI topics as methods formitigation, insulation materials, and inspection were oftenexchanged. While corrosion engineers were becomingknowledgeable about CUI, ASTM Committee C-16 waspreparing standards for testing insulation with apropensity to cause chloride stress corrosion cracking(SCC) of austenitic stainless steel. These two groupsinteracted but proceeded to develop their standards andinformation separately.

1.6 Although most of the attention has been focused oncorrosion under thermal insulation, fireproofing materialsalso function, at least in part, as insulation appliedbetween the critical steel structure and a potential fire.Other fire protection mechanisms initiated as endo-thermic reactions within the fireproofing material during afire, such as sublimation, hydro-regeneration, andintumescence, are known to augment the insulating roleof the fireproofing. The mechanisms also add uniqueconsiderations to the discussion of the chemistry at thewet steel interface. A discussion of corrosionmechanisms, the root cause of failure, and corrosionprevention is the same for corrosion under both insulationand fireproofing.

1.7 The consensus is that the basic solution to prevent-ing CUI is the use of a high-quality protective coating. Itis the recommendation of this committee that wheneverCUI is a consideration, a protective coating should be

employed to protect the equipment before it is insulated.

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Section 2: Corrosion Mechanisms

Temperature

2.1 Carbon Steel

Carbon steel corrodes, not because it is insulated, butbecause it is contacted by aerated water. The role ofinsulation in the CUI problem is threefold. Insulationprovides:

(a) An annular space or crevice for the retention ofwater and other corrosive media;

(b) A material that may wick or absorb water; and

(c) A material that may contribute contaminants thatincrease or accelerate the corrosion rate.

The corrosion rate of carbon steel may vary because therate is controlled largely by the metal temperature of thesteel surface and contaminants present in the water.These factors and others are reviewed below.

2.1.1 Effects of Water, Contaminants, and

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larly detrimental because their respective metal

2.1.1.1 Sources of Water Under Insulation

The two primary water sources involved in CUIof carbon steel are:

(a) Infiltration from external sources; and(b) Condensation.

Water infiltrates from such external sources asthe following:

(a) Rainfall;(b) Drift from cooling towers;(c) Condensate falling from cold service

equipment;(d) Steam discharge;(e) Process liquids spillage;(f) Spray from fire sprinklers, deluge systems,

and washdowns; and(g) Condensation on cold surfaces after vapor

barrier damage.

External water enters an insulated systemprimarily through breaks in the weatherproofing.The weatherproofing breaks may be the result ofinadequate design, incorrect installation,mechanical abuse, or poor maintenance prac-tices.

Condensation results when the temperature ofthe metal surface is lower than the atmosphericdew point. While infiltration of external watercan be reduced and sometimes prevented,insulation systems cannot be made vapor tight,so condensation as a water source must berecognized in the design of the insulationsystem.

2.1.1.2 Contaminants in Water Under Insulation

The role of contaminants is twofold:

(a) Contaminants can increase the conductiv-ity and/or corrosiveness of the waterenvironment; and

(b) Contaminants can reduce the protectionoffered by the corrosion product scale onthe carbon steel surface.

There are two primary classes of contaminantsin water under insulation:

(a) Contaminants external to the insulationmaterials; and

(b) Contaminants leached from the insulationmaterials.

Chlorides and sulfates are the principal contami-nants found under insulation. Whether theirsource is external or internal, they are particu-

salts are highly soluble in water, and theseaqueous solutions have high electrical conduc-tivity. In some cases, hydrolysis of the metalsalts can cause localized corrosion because ofdevelopment of low pH in anodic areas.

External contaminants are generally salts thatcome from sources such as cooling tower drift,acid rain, and atmospheric emissions. Theexternal contaminants are waterborne or air-borne and can enter the insulation systemdirectly through breaks in the weatherproofing.External contaminants also enter the insulationmaterials indirectly by depositing on the jacketsurface. Subsequent wetting then carries theconcentrated salts to breaks in the weather-proofing. The salts enter the insulation systemby gravity or the wicking action of absorbentinsulation. The salt concentrations graduallyincrease as water evaporates from the carbonsteel surface.

Contaminants contained in the insulationmaterials are well documented. Chloride isgenerally one of the contaminants, unless theinsulation product is declared “chloride free.”Chlorides can be present in almost all com-ponents of the insulation system, including theinsulation, mastic, and sealant. As water entersthe insulation system, the contaminants areleached from the material and concentrate aswater evaporates from the carbon steel surface.If the insulation materials contain water-leachable acidic compounds, then the pH of thewater will be lowered, resulting in increasedcorrosion.

2.1.1.3 Effect of Temperature

Service temperature is an important factoraffecting CUI of carbon steel because twoopposing factors are involved:

(a) Higher temperature reduces the time wateris in contact with the carbon steel;however,

(b) Higher temperature tends to increase thecorrosion rate and reduce the service lifeof protective coatings, mastics, andsealants.

Figure 1 illustrates the corrosiveness of waterversus temperature. In an open system, theoxygen content of the water decreases as thetemperature increases.4 As a result, aboveapproximately 80°C (176°F), the corrosion rateof carbon steel in aerated water begins todecrease. However, in a closed system, thecorrosion rate of carbon steel in water continues

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to increase as the water temperature increases.4

Field measurements of the corrosion rate ofcarbon steel corroding under insulation confirmthat the rate increases with temperature in amanner similar to that of a closed system.5 Thisis relevant to the corrosion mechanism occurringunder insulation, where the thin film of water,while not under pressure, is oxygen-saturated.Thus, the same oxygen cell corrosion mechan-ism is taking place as in a closed system. Thecorrosion rates from field measurements aresomewhat greater than laboratory rates, due tothe airborne or insulation-carried salts in thefield. Such salts can influence the corrosion ratebecause of their high solubility in water and theattendant increase in the conductivity of thewater film.

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Inspection of equipment has shown that carbonsteel operating in the temperature range of -4°C(25°F) to 150°C (300°F) is at the greatest riskfrom CUI. Equipment that operates continuouslybelow -4°C (25°F) usually remains free of corro-sion. Corrosion of equipment operating above150°C (300°F) is reduced because the carbonsteel surface is warm enough to remain dry.However, corrosion will tend to occur at thosepoints of water entry into the insulation systemwhere the temperature is below 150°C (300°F)and when the equipment is idle.

The service temperature of equipment oftenvaries, and the corrosion rate of carbon steelunder insulation will be affected by:

Figure 1Effect of Temperature on Steel Corrosion in Water

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(a) Intermittent or variable operation of equip-ment;

(b) Temperature variations along the height orlength of the equipment;

(c) Temperature at which attachments toequipment operate; and

(d) Idle or mothballed conditions.

2.1.2 Effects of Insulation Material

2.1.2.1. Effects of Types of Insulation

CUI of carbon steel is possible under all types ofinsulation. The insulation type may only be acontributing factor. The insulation characteris-tics with the most influence on CUI are:

(a) Water-leachable salt content in insulationthat may contribute to corrosion, such aschloride, sulfate, and acidic materials infire retardants;

(b) Water retention, permeability, and wetta-bility of the insulation; and

(c) Foams containing residual compoundsthat react with water to form hydrochloricor other acids.

Because CUI is a product of wet metal exposureduration, the insulation system that holds theleast amount of water and dries most quicklyshould result in the least amount of corrosiondamage to equipment.

Corrosion can be reduced by careful selection ofinsulation materials. Materials that may becheaper on an initial cost basis may not be moreeconomical on a life-cycle basis if they allowcorrosion. For more detailed information aboutinsulation materials, refer to Section 5.

2.1.2.2 Role of Weather Barrier and VaporBarrier Materials

Weather barriers and vapor barriers are appliedto insulation to keep the insulation dry. Masticsand sealants are materials used to close open-ings around protrusions in the insulation system.Weather barrier and vapor barrier materials arecritical components in the insulation system,because they must seal and protect the insula-tion. Their durability against mechanical abuse,ultraviolet (UV) degradation, water, and chemi-cals is of prime importance. In addition, thesematerials must not contain leachable compo-

___________________________ Metals and Alloys in the Unified Numbering System (latest revision), a jod the Society of Automotive Engineers Inc. (SAE), 400 Commonwealth D

nents that increase the corrosiveness within theinsulation system.

In the long term, the weather barriers and vaporbarriers break down or are damaged to the pointthat they can no longer keep the insulation dry.Therefore, maintenance and inspection ofweatherproofing are essential to ensure theintegrity of the insulation/fireproofing system.

For more information on this subject, refer toSection 5.

2.1.2.3 Effect of Design

Equipment design and mechanical details havean important influence on CUI of carbon steel.Several undesirable design features that influ-ence CUI include:

(a) Shapes that naturally retain water, such asflat horizontal surfaces, vacuum rings, andinsulation support rings;

(b) Shapes that are difficult or impractical toweatherproof properly, such as gussets,I-beams, and other structural components;

(c) Shapes that funnel water into the insula-tion, such as angle-iron brackets;

(d) Other items that cause interruption in theweatherproofing, such as ladder brackets,nozzle extensions, decking, and platformand pipe supports; and

(e) Protrusions through insulation on coldservice equipment where temperaturegradients from cold to ambient will occur.

The more breaks there are in equipment surface,the more likely that water will enter or bypassinsulation and drain poorly from equipment.Therefore, high-quality protective coatings mustbe used to protect steel and should be includedin the design specifications.

For more detailed information on this subject,refer to Section 3.

2.2 Austenitic Stainless Steel

The stainless steel alloys susceptible to SCC aregenerally classified as the 18-8s: austenitic alloyscontaining approximately 18% chromium, 8% nickel, andthe balance iron. Besides the basic alloy UNS(3) S30400,these stainless alloys include (among others) themolybdenum-containing grades (UNS S31600 andS31700), the carbon stabilized grades (UNS

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S32100 and S34700), and the low carbon grades (UNSS30403 and S31603).

To combat SCC, many variations of the basic 18-8stainless steels have been developed. These are thehigher-nickel, chromium, and molybdenum-containingalloys (super stainless steels), and the lower-nickel,higher-chromium duplex alloys. These alloys are moreresistant to SCC and have been found to be resistant toSCC under thermal insulation.

2.2.1 External Stress Corrosion Cracking (ESCC)

2.2.1.1 Mechanism of ESCC

ESCC occurs in austenitic stainless steel pipingand process equipment when chlorides or otherhalides in the environment or insulation materialare transported in the presence of water to thehot surface of stainless steel, and are thenconcentrated by evaporation of that water. Thismost commonly occurs beneath thermal insula-tion, but the presence of insulation is not arequirement. Thermal insulation primarily pro-vides a medium to hold and transport the waterwith its chlorides to the metal surface.

2.2.1.2 Tests and Standards Related to ESCC

Many of the early experiences of ESCC underinsulation occurred under wicking insulation.Tests showed that if this wicking insulationcontained leachable chlorides, then waterpermeating the insulation, extracting chlorides,and transporting them to the stainless steelsurface would cause SCC.

Out of these experiences came ASTM C 692 in1971, as discussed in Section 1. This standardwas followed in 1977 by ASTM C 8716 and thefinal ASTM specification in this series, ASTMC 795.7

These three specifications are notable becausethey established the concepts that:

(a) Wet insulation containing chlorides willcause ESCC; and

(b) Application of silicate to inhibit chloride inthe insulation would be effective inpreventing ESCC.

It is now understood that these concepts, whilecorrect, are too limited and not always effective.ESCC failures have been reported under non-wicking insulation. In cases of nonwickinginsulation, the water is under the insulation,having entered around it. The chlorides dis-solved in the water are from the external sourcesor the atmosphere, not the insulation materials.

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When external water and chlorides enter aroundan inhibited, wicking insulation material, ESCCcan develop due to lack of silicate available onthe wetted stainless steel surface. Plant experi-ence shows that the inhibitor is not alwaysleached out of the insulation in sufficient quanti-ties, nor is the inhibitor always in the right placeto inhibit the concentrated external chlorides.Sometimes the inhibitor may be leached sothoroughly under severely wetted conditions thatit may be transported away from the surfacesneeding inhibition.

The original wicking test as specified in the initialpublication of ASTM C 692 has been modifiedand now includes the drip test.8 The drip testcan be used to evaluate the stress crackingpotential of all types of insulation, wicking andnonwicking, as well as mastics and sealants.

One additional specification related to thismatter is ASTM C 929,9 which deals with thehandling of certain insulating materials.

In summary, the ASTM specifications C 692,C 795, C 871, and C 929 standardize theselection and evaluation of insulation materialswith regard to their propensity to cause SCC ofaustenitic stainless steels.

These standards do not treat the other aspectsof the ESCC problem. If a noncrack-producinginsulation is placed in service in a chlorideenvironment, then a stress cracking failurebecomes a possibility. Thus, relying solely onmaterials tested and approved according toASTM standards may put stainless steel equip-ment in jeopardy. This limitation has not beenunderstood among the engineering, construc-tion, and user groups in the petrochemical andrefining industries, among others.

2.2.1.3 Sources, Levels, and Forms of Chlorides

When the ESCC mechanism was first identified,many believed the primary source of chlorideswas the insulation itself. While some insulationsdo contain appreciable chloride levels, testingand plant experience have shown that thechlorides more frequently come from coastalatmospheres, nearby chloride-containing chemi-cal process units, wash water and fire protectiondeluge systems, and process spillage. Chlorideconcentration need not be high in the water, asthe hot metal surface concentrates the chloridesby evaporating to a level sufficient to causecracking.

2.2.1.3.1 Sources

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Sources of chlorides fall into two categories:insulating materials and external sources. Asystems approach will develop strategies tocombat both categories.

2.2.1.3.1.1 Insulating materials includeinsulation, mastics, sealants, adhe-sives, and cements. Failures after onlya few years of operation are typicallyassociated with insulating materialscontaining high levels of leachablechlorides.

2.2.1.3.1.2 External sources includerain, coastal fog, wash water, fire anddeluge system testing, and processleaks or spills. Failures due to intro-ducing chlorides from external sourcestend to occur after five years or more ofservice. These sources account formost of the chloride-induced failures.

2.2.1.3.2 Levels

Experience has shown that insulatingmaterials with as little as 350 ppm chloridehave been identified near ESCC locations.Deposits near ESCC events have beenfound with as little as 1,000 ppm chloride. Itis useful to consider these levels whendetermining acceptable chloride levels forinsulating materials.

2.2.1.3.3 Forms

Sodium chloride is the most prevalentchloride salt found in CUI events. Whenfound in sufficient quantities, it causes SCCof austenitic stainless steel. Other sourcesof chloride ions known to be aggressiveinclude chlorine, hydrogen chloride gas,hydrochloric acid, hydrolyzed organic chlo-rides, and thermally decomposed polyvinylchloride (PVC). Likewise, acidic conditionsin combination with chloride will be moreaggressive than neutral or basic conditions.It is useful to consider these observationswhen specifying insulating materials.

2.2.1.4 Effect of Temperature

Temperature has a twofold effect. First, asstated above, at elevated temperature waterevaporates as it contacts the hot stainless steelsurface. This evaporation can concentrate thechloride salts, allowing them to be deposited onthe metal surface. Second, as temperatureincreases, the rate of the corrosion reactionincreases, and the time decreases for initiationand propagation of ESCC.

Most ESCC failures occur when metal tempera-ture is in the “hot water” range: 50°C to 150°C(120°F to 300°F). Failures are less frequentwhen metal temperature is outside this range.Below 50°C (120°F) the reaction rate is low, andthe evaporative concentration mechanism is notsignificant. Above 150°C (300°F), water is notnormally present on the metal surface, andfailures are infrequent. Equipment that cyclesthrough the water dew point is particularlysusceptible. Water present at the low tempera-ture evaporates at the higher temperature.During each temperature cycle the chloride saltsdissolved in the water concentrate on thesurface.

2.2.1.5 Role of Stress

In order for SCC to develop, sufficient tensilestress must be present in the stainless steel. Ifthe tensile stress is eliminated or sufficientlyreduced, the cracking will not occur. Thethreshold stress required to develop crackingdepends somewhat on the cracking medium.Most mill products, such as sheet, plate, pipe,and tubing contain enough residual processingtensile stresses to develop cracks withoutapplied stresses. When 18-8 stainless steelsare cold formed and welded, additional residualstresses are imposed. As the total stress rises,the potential for SCC increases. Attempts tocontrol SCC by reducing the tensile stress bythermal treatment are not practical.

2.2.2 Effects of Types of Insulation

The solution to ESCC of stainless steel does not liewith the type of insulation chosen. Industry experi-ence and testing have shown that cracking occursunder all types of insulation materials. Insulationsthat absorb water are particularly troublesome in thatthey hold water and slowly allow the concentrationmechanism to proceed. Insulations that do notabsorb water are frequently specified in an attempt tolessen the problem; but without other preventivemeasures, cracking may still occur.

Polyurethane foam, polyisocyanurate foam, andphenolic foam do not provide immunity to ESCC,especially when used in the hot water range.Residual chlorine or bromine compounds used inmanufacturing the foam may leach out and hydro-lyze, forming an acidic condition that accelerates thecracking of 18-8 stainless steels.

For more detailed information on insulationmaterials, refer to Section 5.

2.2.3 Effects of Mastics and Sealants

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If water could be excluded, the insulation would staydry, and ESCC would not occur. While this soundslike a reasonable approach toward prevention, inpractice it is extremely difficult to prevent wateringress. In fact, once insulation becomes wetted, theweather barriers, mastics, and sealants make waterescape difficult, so the insulation remains wet. Also,mastics and sealants may contain water-leachablechlorides that can contribute to ESCC problems.

For more information on mastics and sealants, referto Section 5.

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2.2.4 Effect of Design

Design steps to minimize water ingress are beneficialbut not normally adequate to prevent cracking.Some amount of water entry into the insulationsystem will eventually occur. High-quality immersiongrade protective coatings as outlined later in thisstandard shall be specified to protect the stainlesssteels.

For additional information on design, see Section 3,and for protective coatings, see Section 4.

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Section 3: Mechanical Design

3.1 Poorly designed or applied insulation systems andprotrusions through thermal insulation permit water tobypass the insulation, thereby corroding the substratemetal.10 Metal also corrodes when weather barriers andvapor barriers break down after vessels and piping areput in service and are exposed to the weather. This oftenresults in structural failures, unplanned or extendedshutdowns, and unscheduled replacement of equipment.Insulation system life can be prolonged, and substratemetal corrosion can be reduced by better design ofprotrusions, attachments, and supports associated withvessels and piping.

3.2 Thermal Insulation System Design

Equipment and piping are insulated for any of thefollowing reasons:

(a) Heat conservation and/or freeze protection;(b) Process control;(c) Personnel protection;(d) Sound control;(e) Condensation control; and(f) Fire protection.

Insulated surfaces for carbon steel operating continuouslyabove 150°C (300°F) or below -4°C (25°F) and foraustenitic stainless steel operating continuously above150°C (300°F) or below 50°C (120°F) do not presentmajor corrosion problems. However, equipment andpiping operating either steadily or cyclically betweenthese temperatures can present significant corrosionproblems. These problems are aggravated by selectinginadequate insulation materials and by improper insula-tion design. Guidelines for proper design to control corro-sion in thermal insulation systems are presented below.

3.2.1 Specification Requirements

Insulation specifications are critical requirements forinsulation system design and installation work.

They control material and application requirements.Loosely written specifications with insufficientmaterial descriptions and application requirementsmay result in costly repairs during construction orafter the plant is operational.

Common specification flaws to be avoided are:

(a) Incorrect application of materials: e.g., open-cell or wicking-type insulation materials, suchas calcium silicate, and fibrous productsspecified for below-ambient temperature appli-cations.

(b) Product specification by using a generic name

without stating the properties required for theintended service.

(c) Improper and unclear application methods:

e.g., incorrect multilayer schedules, lack ofexpansion joints, missing vapor barriers, andincorrect insulation securement methods.

A specification needs to be complete and detailed. Itmust clearly describe materials, application, andfinishing requirements. If a service needs specialattention from an insulation standpoint, it should bestated in the specification. For more information oninsulation materials, see Section 5.

3.3 Effect of Equipment and Piping Attachment Design

The design of equipment and piping attachments is animportant part of insulation system design. The shape,geometry, and orientation of attachments can allowmoisture or rainwater to bypass the insulation and toconcentrate at the attachment point. Examples of suchattachments are shown in Figures 2 and 3. Attention todetails such as these is important in order to produce ahigh-quality insulation system.

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8

Platform Support

Lifting Lugs

PlatformBracket

InsulationSupport Ring

Nozzle orManway

InsulationSupport Ring

Skirt AccessOpening

Nozzle

Davit

Pipe Bracket

Support Ring orStiffener Ring

Ladder Support

Figure 2Typical Vessel Attachments Where Water May Bypass Insulation

Pressure Gauge

Caulking Compound

Note: Jacketing Seam at Top

Pipe

Insulation

Metal Jacketing

Figure 3Attachment to Piping Where Water May Bypass Insulation

Attachment relies on caulking compound only.

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Some attachment shapes can be modified to be easier toseal, but this is not always possible. Although structuralsteel angles are among the most difficult shapes toweatherproof, they are widely used in industry. Some-times pressure vessel codes dictate which attachmentshapes can be used. Caulking compounds used atjacketing penetrations keep rain water out only until thecompounds fail because of weathering or burn off due tohigh equipment operating temperatures. Therefore,insulation designs relying only on these compounds mayfail prematurely.

There are several problems frequently encountered wheninsulating vessels and piping:

3.3.1 Problems Insulating Vessels

(a) The lip or rim on bucket-type insulation supportrings on vessels may act as a moisture dam,leading to severe corrosion and pitting of thevessel. A relatively inexpensive alternativedesign—a flat bar bolted onto welded clipswhich is shown in Figure 4—can minimizemoisture accumulation.

(b) In a problem similar to (a), vertical vessel

bottom support rings can accumulate moistureat the metal-to-insulation interface if the inter-face is unprotected. The design principleshown in Figure 4 is extended to this applica-tion in Figure 5. An economical flat cut bar canbe used. The flashing ring, which can be fieldfabricated, protects the insulation and fire-proofing by deflecting water over and down thesupport ring edge.

(c) Stiffening rings extending beyond the insulation

may allow moisture intrusion. Built-up insula-tion and jacketing with “Pittsburg seams” andproper overlaps, as shown in Figure 6, canprevent the intrusion.

(d) Uninsulated nozzles located on the top heads

of vertical vessels can divert water underinsulation. The caulking compound and metalflashing normally used in this situation do verylittle to keep out the water. This problem canbe remedied by extending the nozzle beyondthe insulation and jacketing as shown in Figure7, and then by insulating the nozzle up to thecap. The cap depends on a seal weld aroundthe nozzle, not on sealants, to prevent waterpassage past the insulation.

(e) The bracket supporting the nameplate on a

vessel can permit water intrusion past theinsulation where the bracket penetrates it. This

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is illustrated in Figure 8. A shorter bracket thatdoes not extend beyond the insulation can beused for permanent identification of the vessel,and a duplicate nameplate can be mounted onthe outside of the metal jacketing or bottomskirt for in-service identification.

3.3.2 Problems Insulating Piping

(a) Insulation of piping for personnel protectioncan incur water entry at the termination if it isunsealed. Seal-welding a cap, as demon-strated in Figure 9, can prevent water entry.An alternative is to forego the insulation anduse expanded metal on stand-offs.

(b) Using angle iron or C-channel to support

double pipe exchangers creates many pro-trusions through the insulation. See Figure 10as an illustration. These protrusions, difficult toseal, afford entry points for moisture. Using atubular support provides a surface andprotrusion-to-insulation contour that is easier toseal.

(c) Rod hangers or clamps supporting piping by

direct contact make protrusions through thejacketing as shown in Figure 11. Water canenter past the insulation when the caulkingcompound dries enough to crack or to separatefrom the insulation. However, load-bearingsupports that contact only the jacketing, asshown in Figure 12, allow a continuous weatherbarrier.

(d) In a problem similar to (c), when insulated

piping rests directly on structural beams, theweather barrier must be cut around the steel.This breaks the weather barrier continuity andallows moisture intrusion. However, pipingsupported as shown in Figure 12 keeps theweather barrier continuous. The insulation andjacketing are free to move with the piping, andwater intrusion is reduced.

(e) In another problem similar to (c), the vapor

barrier of piping in cold service is notcontinuous when the piping is supported asshown in Figure 13. Instead, the integrity ofthe insulation system relies on joint sealantsand caulking compounds used at theinsulation-to-pipe support interface. Thesecompounds cannot maintain their seal whenthe piping moves, and moisture may intrude.Figure 14 shows the design of a piping supportfabricated with a built-in vapor barrier thatremains continuous despite piping movement.

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Corrosion Problems with Bucket-Type Insulation Support Ring

Vessel

Flat bar

Welded clip

Flat Bar-Type Support Ring Eliminates Moisture Accumulation

Figure 4Vessel Insulation S upport Ring, the Problem and the Solution

Figure 5Vertical Vessel Bottom S upport Ring Minimizing Water Accumulation

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Figure 6

Vessel-Stiffening Ring Insulation DetailSeams prevent water intrusion.

Figure 7Center Nozzle at Top Head of Vessel

Extended and Insulated with Seal-Welded Cap

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Figure 8Common Nameplate Insulation Detail

Water may enter through bracket penetration.

Figure 9Seal-Welded Cap on Insulation for Personnel Protection

Cap prevents water entry.

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Figure 10Double-Pipe Heat Exchanger Insulation Penetrated by C-Channel Support

Water entry points shown

Figure 11Protrusions Through Jacketing

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Figure 12Pipe Supports Without Protrusions

Figure 13Cold Service Pipe Support Without Continuous Vapor Barrier

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Figure 14Cold Service Pipe Support with Continuous Vapor Barrier

(a) Also in cold service, insulation and vapor

barriers are often penetrated for improvedaccess to equipment lying close to theinsulation, such as instrument connections,drain-valve hand wheels, and valve packingglands. These penetrations can allow moistureintrusion and condensation. The problem isavoided by extending valve stems andinstrument connections above the insulation.

(b) Clearance for insulation between piping and

adjacent structures can be insufficient due toincorrect pipe spacing, unexpected thickness ofsteel column fireproofing, and unexpectedpiping movement. This inadequate clearanceoften permits moisture to bypass weather-proofing and vaporproofing, as shown in Figure15. The only cure is to design adequate spacefor insulation. Design considerations shouldinclude effects of adjacent structures, pipingmovement, and expansion joints.

(c) Electrical conduit suspended from piping or

penetrating its insulation presents insulationsealing difficulties for both hot and cold service.Moreover, in extremely hot service, the conduitmay suffer overheating damage; in coldservice, the conduit may corrode. The remedyis to avoid penetrating piping insulation; e.g.,suspend conduit from structural members.

(d) Providing adequate piping clearances, attend-

ing to attachment geometry, and understanding

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incidental corrosion can preclude many of theproblems described above. Knowledge ofvarious insulation materials and their installa-tion requirements, along with knowledge ofequipment and piping, is necessary for corro-sion control.

3.4 Weather Barrier and Vapor Barrier Design

In insulation system design, selection of weather barriersand vapor barriers is as important as selection of thermalinsulation. While it is easy to say, “Keep water out,” inpractice, keeping water out is not always feasible.Weather barriers and vapor barriers break down due tochemical attack, sunlight, mechanical damage, andgalvanic corrosion. Caulking compounds and masticsused during construction for sealing jacket seamsdegrade in sunlight and at temperatures exceeding thematerials’ recommended use limits. Vapor barriers alsodegrade in sunlight, creating cracks and open seams thatallow moisture penetration.

In cold service, thermal insulations rely on vapor barriersto keep out moisture. With the possible exception of anall-welded metal sleeve enclosure, there is no perfectvapor barrier. Mastic vapor barriers without metaljacketing require periodic inspections to check for signs ofmechanical damage, aging, cracking, delaminations, andbroken seals. Unattended repairs shorten insulation lifeand promote corrosion. Metal jackets should be avoidedover vapor barriers on cold service insulation unlessneeded for protection of the insulation.

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In warm service, weather barriers are normally metallic.They are fabricated from roll jacketing and can havemany seams. Sometimes seams are installed on topsurfaces or have improper overlaps. Overlap seams aremore vulnerable to foot traffic damage on horizontal lines,vessel heads, and tank tops when thin metallic jacketingis used over fibrous insulation.

The use of dissimilar metals in metallic jacket design inthe presence of moisture should be avoided as this oftencauses galvanic corrosion.

3.5 Insulation System Design

Insulation designs for rigid and semirigid materials mayrequire expansion joints, depending on operatingtemperatures and sizes of equipment and piping. Failureto employ these joints at the required locations in theinsulation can lead to its uncontrolled movement. As a

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result, weather barriers and vapor barriers break down.This can allow migration of water into the insulation andlead to corrosion. Normally, insulation design for flexiblematerials, such as fibrous blanket, does not requireexpansion joints.

System designers may fail to allow for movement ofinsulation caused by piping expansion. For example,based on the coefficients of thermal expansion at -73°C(-100°F) and 20°C (70°F), cellular glass insulationexpands about the same amount as carbon steel,whereas cellular foam expands about nine times morethan carbon steel. When the insulated system cools,joints will compress in cellular glass but open in cellularfoam. Therefore, cellular foams (such as a polyurethanesystem) require more expansion joints. Also, to controlthe lateral migration of water vapor, polyurethaneinsulated systems need more frequent vapor stops thando cellular glass systems.

Figure 15Pipe Insulation Penetrated by Column Fireproofing

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c

Section 4: Prote

4.1 Scope

4.1.1 This section presents information for theselection of protective coatings for carbon steel andaustenitic stainless steel under thermal and/or noise

tive Coatings

reduction insulation systems and cementitious fire-proofing. Protective coatings have been recognizedand accepted and are recommended as a highlyeffective method of protecting insulated metallicsubstrates such as these steels from corrosion.

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Attempts to prevent water from entering insulatedsystems have not been successful, and corrosionprotection techniques such as inhibitors and cathodicprotection have been less effective than protectivecoatings in mitigating corrosion under insulation.

4.1.2 Coating systems considered in this section arethin film liquid-applied coatings, fusion-bondedcoatings, metallizing, and wax-tape coatings. Thesesystems have a history of successful use. Othersystems may also be satisfactory.

4.1.3 Insulation covering is not addressed in thissection.

4.1.4 Failures with inorganic zinc coatings under wetinsulation are not discussed in this standard; thesubject has been addressed in NACE Publication6H189.

4.1.5 Coating manufacturers or project specifica-tions should be consulted regarding suitability ofspecific products for carbon steels and austeniticstainless steels under insulation systems.

4.2 Coating Austenitic Stainless Steel Under ThermalInsulation

4.2.1 Austenitic stainless steel can be subject toESCC when covered with insulation. Also, if acoating contains a low-melting-point metal, thenliquid metal cracking (LMC) of the steel may be a riskif the coating is heated above the melting point of themetal it contains. Consequently, the criteria for acoating system used to prevent ESCC and LMC ofaustenitic stainless steel are as follows:

4.2.1.1 The coating system shall not containfree, soluble chlorides or other halides aftercuring. Compounds of chlorides or other halideswithin the cured-resin chemical molecule are notconsidered harmful unless they are subject torelease through aging within the expectedservice temperature range.

4.2.1.2 Due to the risk of LMC, the coating shallnot contain zinc, lead, copper, or their com-pounds in its formulation.

4.2.1.3 The coating shall be selected for theexpected service temperature range if this rangecould allow moisture to occur on substrate sur-faces. This is especially true with processesusing intermittent thermal cycling through thedew point.

4.2.2 Table 1 lists protective coating systems foraustenitic stainless steel equipment. Maximumservice temperature and duration of the proposed

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application should be considered in selecting acoating system. For other coatings, the manufac-turer should be consulted regarding expected coatingperformance.

4.2.3 Aluminum foil wrapping has been used toprevent ESCC of stainless steel under insulation.

4.3 Coating Carbon Steel Under Thermal Insulation andCementitious Fireproofing

4.3.1 The coating systems recommended for use oncarbon steel operating below 150°C (300°F) underthermal insulation are typically tank lining systemsformulated to prevent corrosion. Other coatings maybe used at the buyer’s discretion.

4.3.2 Epoxy protective coatings, as a class ofmaterials, are recommended for use on carbon steelunder cementitious fireproofing.

4.3.3 If galvanized steel under cementitious fire-proofing has been corroding, coating the galvanizedsteel should be considered. The manufacturer ofproprietary cementitious fireproofing should beconsulted regarding the compatibility of the fire-proofing with galvanized steel.

4.3.4 Users who steam-purge lines shall select acoating capable of withstanding the surface tempera-ture for the duration of the purging. The coatingmanufacturer should be consulted for specifictemperature resistance information.

4.3.5 Inorganic zinc coatings or galvanizing shall notbe used under thermal insulation in the 50 to 150°C(120° to 300°F) service temperature range for long-term or cyclic service. Zinc provides inadequatecorrosion resistance in closed, sometimes wetenvironments.

4.3.6 Thermally sprayed aluminum coatings haveperformed successfully in marine and high-temperature environments.

4.3.7 Wax-tape coatings may be used to preventcorrosion of carbon steel during a dry cycle or whencycling through dew points. Tape applicationprocedures should follow those prescribed in NACEStandard RP037510 for wax-tape coating systems.

4.3.8 Table 2 lists protective coating systemsrecommended for carbon steel equipment. The usershould select the system appropriate for the expectedtemperature range. Maximum service temperatureand its duration should be considered. For othercoatings, the manufacturer should be consultedregarding expected coating performance.

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Table 1Protective Coating Systems for Austenitic Stainless Steels Under Thermal Insulation

SUBSTRATE TEMPERATURERANGE(A)

SURFACEPREPARATION

SURFACEPROFILE(B)

PRIME COAT(C) FINISH COAT(C)

Austenitic StainlessSteel System No. 1

-45 to 60°C(-50 to 140°F)

NACE No. 3(D) 25 to 50 µm(1 to 2 mils)

130 µm (5 mils) of high-build (HB) epoxy

N/A

Austenitic StainlessSteel System No. 2

-45 to 150°C(-50 to 300°F)

NACE No. 3 25 to 50 µm(1 to 2 mils)

150 µm (6 mils) ofepoxy/phenolic or high-temperature-rated amine-cured coal tar epoxy

150 µm (6 mils) ofepoxy/phenolic or high-temperature-rated amine-cured coal tar epoxy

Austenitic StainlessSteel System No. 3

-45 to 370°C(-50 to 700°F)

NACE No. 3 25 to 50 µm(1 to 2 mils)

50 µm (2 mils) of air-driedmodified silicone coating

50 µm (2 mils) of air-driedmodified silicone coating

Austenitic StainlessSteel System No. 4(E)

-45 to 760°C(-50 to 1,400°F)

NACE No. 3 40 to 65 µm(1.5 to 2.5 mils)

100 µm (4 mils) siloxane 100 µm (4 mils) siloxane

(A) The temperature range shown for a coating system is that over which the system is designed to maintain its integrity and capability to perform asspecified when correctly applied. However, the user may determine whether any coating system is required, based on corrosion characteristics ofstainless steel at certain temperatures.(B) A typical minimum and maximum surface profile is specified for each substrate. Acceptable profile range may vary, depending on substrate andtype of coating. Coating manufacturer’s recommendations should be followed.(C) Coating thicknesses are typical dry film values. Temperature ranges are typical for the coating system. For protective coatings not listed,specifications and coating manufacturer’s recommendations should be followed.(D) NACE No. 3/SSPC-SP 6 (latest revision), “Commercial Blast Cleaning” (Houston, TX: NACE, and Pittsburgh, PA: SSPC).(E) This system is not recommended for cyclic service characterized by rapid temperature fluctuations.

Table 2Protective Coating Systems for Carbon Steels Under Thermal Insulation and Cementitious Fireproofing

SUBSTRATE TEMPERATURERANGE(A)

SURFACEPREPARATION

SURFACEPROFILE(B)

PRIME COAT(C) INTERMEDIATECOAT(C)

FINISH COAT(C) REMARKS

Carbon SteelSystem No. 1

-45 to 60°C(-50 to 140°F)

NACE No. 2(D)50 to 75 µm(2 to 3 mils)

130 µm (5 mils) high-build (HB) epoxy

N/A 130 µm (5 mils) HBepoxy

N/A

Carbon SteelSystem No. 2

-45 to 60°C(-50 to 140°F)

NACE No. 2 50 to 75 µm(2 to 3 mils)

N/A N/A 300 µm (12 mils)fusion-bonded epoxy(FBE)

In-shopapplication only

Carbon SteelSystem No. 3

-45 to 60°C(-50 to 140°F)

NACE No. 2 50 to 100 µm(2 to 4 mils)

180 to 250 µm (7 to10 mils) metallizedaluminum

15 to 20 µm (0.5 to0.75 mil) of MIL-P-24441/1(E) epoxypolyamide (EPA)followed by 75 µm (3mils) of MIL-P-24441/1 EPA

75 µm (3 mils) ofMIL-P-24441/2(F)

EPA

U.S. NavyStandard DOD-STD-2138(G)

Carbon SteelSystem No. 4

95°C (200°F)maximum

NACE No. 2 50 to 75 µm(2 to 3 mils)

25 to 50 µm (1 to 2mils) moisture-curedurethane aluminumprimer

50 to 75 µm (2 to 3mils) moisture-curedmicaceousaluminum urethane

Two 75-µm(3-mil) coats ofacrylic urethane

N/A

(Table continued on next page)

(A) The temperature range shown for a coating system is that over which the system is designed to maintain its integrity and capability to perform asspecified when correctly applied. However, the user may determine whether any coating system is required, based on corrosion characteristics ofcarbon steel at certain temperatures.(B) A typical minimum and maximum surface profile is specified for each substrate. Acceptable profile range may vary, depending on substrate andtype of coating. Coating manufacturer’s recommendations should be followed.(C) Coating thicknesses are typical dry film values. Temperature ranges are typical for the coating system. For protective coatings not listed,specifications and coating manufacturer’s recommendations should be followed.(D) NACE No. 2/SSPC-SP 10 (latest revision), “Near-White Metal Blast Cleaning” (Houston, TX: NACE, and Pittsburgh, PA: SSPC).(E) MIL-P-24441, Part 1 (latest revision), “General Specification for Epoxy-Polyamide Paint” (Philadelphia, PA: Department of Defense).(F) MIL-P-24441, Part 2 (latest revision), “General Specification for Epoxy-Polyamide Paint” (Philadelphia, PA: Department of Defense).(G) U.S. Navy/DOD STD 2138 (latest revision), “Metal Sprayed Coatings for Corrosion Protection Aboard Navy Ships” (Philadelphia, PA:Department of Defense).

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Table 2 ContinuedProtective Coating Systems for Carbon Steels Under Thermal Insulation and Cementitious Fireproofing

SUBSTRATE TEMPERATURERANGE(A)

SURFACEPREPARATION

SURFACEPROFILE(B)

PRIME COAT(C) INTERMEDIATECOAT(C)

FINISH COAT(C) REMARKS

Carbon SteelSystem No. 5

-45 to 150°C(-50 to 300°F)

NACE No. 1(H)50 to 75 µm(2 to 3 mils)

150 µm (6 mils)epoxy/phenolic orhigh-temperature-rated amine-curedcoal tar epoxy

N/A 150 µm (6 mils)epoxy/phenolic orhigh-temperature-rated amine-curedcoal tar epoxy

N/A

Carbon SteelSystem No. 6

120 to 540°C(250 to 1,000°F)(with intermittentcycling60 to 120°C[140 to 250°F])

NACE No. 2 50 to 100 µm(2 to 4 mils)

150 to 200 µm (6 to8 mils) metallizedaluminum

N/A Silicone seal coatper manufacturer'srecommendation

N/A

Carbon SteelSystem No. 7

480°C (900°F)maximum

NACE No. 2 50 to 75 µm(2 to 3 mils)

250 to 380 µm (10 to15 mils) metallizedaluminum per DODSTD-2138

N/A Two 40-µm(1.5-mil) coats ofTT-P-28(I) high heatsilicone paint

N/A

Carbon SteelSystem No. 8

120 to 540°C(250 to 1,000°F)(in continuousservice above120°C [250°F])

NACE No. 2 25 to 50 µm(1 to 2 mils)

75 µm (3 mils)inorganic zinc (IOZ)

N/A N/A N/A

Carbon SteelSystem No. 9(J)

-45 to 650°C(-50 to 1,200°F)

NACE No. 2 40 to 65 µm(1.5 to 2.5mils)

100 µm(4 mils) siloxane

N/A 100 µm (4 mils)siloxane

N/A

Carbon SteelSystem No. 10

60° C (140°F)maximum

SSPC-SP 2(K)

and/or SSPC-SP3(L)

N/A thin film ofpetrolatum orpetroleum waxprimer

N/A 1 to 2 mm(40 to 80 mils)petrolatum orpetroleum wax tape

N/A

Carbon SteelUnderCementitiousFireproofingSystem No. 11

Ambient NACE No. 3(M)25 to 50 µm(1 to 2 mils)

130 µm (5 mils) ofhigh-build epoxy orcoal tar epoxy

N/A N/A N/A

(H) NACE No. 1/SSPC-SP 5 (latest revision), “White Metal Blast Cleaning” (Houston, TX: NACE, and Pittsburgh, PA: SSPC).(I) TT-P-28 (latest revision), “Heat-Resistant Aluminum Paint (1,200°F [650°C])” (Philadelphia, PA: Department of Defense).(I) This system is not recommended for cyclic service characterized by rapid temperature fluctuations.(J) SSPC-SP 2 (latest revision), “Hand Tool Cleaning” (Pittsburgh, PA: SSPC).(K) SSPC-SP 3 (latest revision), “Power Tool Cleaning” (Pittsburgh, PA: SSPC).(L) NACE No. 3/SSPC-SP 6 (latest revision), “Commercial Blast Cleaning” (Houston, TX: NACE, and Pittsburgh, PA: SSPC).

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Section 5: Insulation, Fireproofing, and Accessory Materials

5.1 Scope

This section describes the properties of industrial insula-tion, insulation accessories, and fireproofing materialsthat affect corrosion. Other performance properties ofthese materials are not characterized. Emphasis isplaced on service performance characteristics, exposureto operating temperatures, and the ability to excludewater over the design life of the system.

5.2 Insulation Materials

Commonly used industrial insulation materials aredescribed and grouped generically. No attempt is madeto describe every commercial product available on themarket. Differences between specific commercial prod-ucts within a generic type are not addressed.

Insulation materials for use on austenitic stainless steelmaterials should be qualified as appropriate according toASTM C 795. Some users specify stricter chloride limitsthan those given in ASTM C 795. Some users specify amaximum chloride content in addition to those

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measurements given in ASTM C 795, such as 100 ppmfor perlite, 200 ppm for calcium silicate, and 25 ppm formineral man-made fiber insulation. Also, the ratio ofsodium silicate to chlorides might be specified as 20 to 1for calcium silicate and mineral fiber or 200 to 1 forperlite.

Using references in ASTM C 795, ASTM C 692 specifiesthe test methods for qualifying materials. The dripmethod provides a technique that closely simulatesinsulated systems. Modifications of this method andapparatus may be useful in the testing of coatings incombination with insulation materials over a temperature-controlled substrate.

ASTM material specifications refer to various testmethods for use in characterizing insulation materials andaccessories. Manufacturers should be encouraged toprovide test information, preferably performed by anindependent third party. This information can be veryuseful in characterizing specific commercial materials.

5.2.1 Calcium Silicate

Calcium silicate pipe and block insulation is specifiedin ASTM C 533.11 It is a rigid pipe and block insula-tion composed principally of hydrous calcium silicateand usually incorporates a fibrous reinforcement.

Calcium silicate is intended as a high-temperatureinsulation. At ambient temperatures it can absorb upto 400% of its weight when immersed in water. It ishygroscopic and will absorb 20 to 25% by weightwater in humid conditions from water vapor presentin air. For this reason, most manufacturers publisha lower temperature limit, typically 150°C (300°F), forits use outdoors.

Calcium silicate when wet is alkaline, having a pH of9 to 10. High pH may be detrimental to coatingssuch as alkyds and inorganic zinc.

Most problems with calcium silicate are associatedwith use at temperatures lower than recommendedcyclic temperature services with an ambient tempera-ture for the majority of the time, and on equipmentsubject to extended shutdown.

5.2.2 Expanded Perlite

Expanded perlite block and pipe insulation is speci-fied in ASTM C 610.12 It is composed of expandedperlite, inorganic silicate binders, fibrous reinforce-ment, and silicone water-resistant additions. It is arigid material furnished in block and pipe coverforms.

Expanded perlite is used as a moderate-to-high-temperature insulation. At lower temperatures, theadditives for water resistance provide protection from

20

absorption of water. At elevated temperaturesaround 315°C (600°F), some additives burn out, andwater resistance is reduced. ASTM C 610 includes atest method for determining the effect of temperatureon water resistance.

5.2.3 Man-Made Mineral Fibers

ASTM groups commercial glass and mineral fiberinsulation materials into a single category, generallydescribed as rocks, slag, or glass processed from amolten state into a fibrous form and including organicbinders. Generally, these materials are used fromambient to high temperatures. The upper tempera-ture limits vary, depending on the specific fiber andbinder. Typically, mineral fibers have a highertemperature limit. Several ASTM specificationsaddress various forms.

Water absorption characteristics of these productsvary greatly. Fiber length and orientation affect thesecharacteristics which, in turn, affect wicking, bindercomposition and quantity, and burn-out characteris-tics of the binder.

Ability of fibrous insulation to repel water varies fromproduct to product and depends on the type of binderused. Some binders break down in the presence ofheat and water. After binder breakdown, these prod-ucts can become excellent wicking material, trans-mitting moisture and corrosive solutions to the steelsurface. Fibrous products also allow water vapor topermeate. Their use in below-ambient temperatureapplications, even with a vapor barrier, has hadlimited success. Construction joints (overlaps andfield joints glued on themselves during installation ofvapor barrier sheet) or damaged sections of vaporbarriers allow moisture to migrate into the insulationsystem. With time and repeated thermal cycling,these vapor barrier joints fail, allowing the passage ofmoisture.

Compressive strength varies with density of thematerial and the effect of binder burn-out. Whilechange in compressive strength does not directlyaffect corrosion, materials with low compressivestrength result in an insulation system with typicalmetal jacketing that is vulnerable to physicaldamage, allowing water intrusion.

5.2.4 Cellular Glass

Cellular glass is specified in ASTM C 552.13 It is arigid block material that has been foamed undermolten conditions to form a closed cell structure. Itis commonly used in below-ambient to moderatetemperatures (-25°C to 200°C [-13°F to 392°F]). Acommon use is as insulation on electric-traced orsteam-traced lines for either freeze protection orprocess control.

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__(4) N

Cellular glass is water resistant and retains onlysmall amounts of water on cut or fractured surfaces.However, water entering through cracks or joints inthe insulation system can reach the metal surfaceand cause corrosion and ESCC.

5.2.5 Organic Foams

ASTM includes specifications for various types andforms of organic foam insulation. The types mostcommonly used in industrial applications includepolyurethane, polyisocyanurate, flexible elastomeric,and phenolic. Polystyrene and polyolefin are lesscommonly used because of temperature limitations.Organic foams are used in below-ambient tomoderate temperature applications and have watervapor ratings from 2.5 to 127 mm (0.1 to 5 perm-in.).

These materials, as do all insulations, contain vary-ing amounts of leachable chlorides, fluorides, sili-cate, and sodium ions as measured by ASTM C 871.The pH, chloride content, fluoride content, silicatecontent, and sodium content are obtained from theleachate produced by boiling pulverized foam inwater. Levels of leachable chlorides can range fromnondetectable to 200 ppm. The leachate pH canrange from 1.7 to 10.0. When the leachate is foundto be less than pH 6.0, special consideration shouldbe given to protect the substrate from acceleratedcorrosion.

5.2.5.1 Spray-applied polyurethane foam isspecified in ASTM C 1029.14 It is a rigid, closed-cell foam that is formed by a chemical reactionat the time of application.

5.2.5.2 Preformed polyisocyanurate foam isspecified in ASTM C 591.15 It is a rigid, closed-cell foam that is formed by a controlled chemicalreaction.

5.2.5.3 Preformed flexible elastomeric rubber isspecified in ASTM C 534.16 It is a flexible,closed-cell foam that is formed by an extrusionprocess.

5.2.5.4 Faced or unfaced phenolic foam isspecified in ASTM C 1126.17 It is a rigid, closed-cell foam that is formed by a controlled chemicalreaction.

5.2.5.5 Preformed polystyrene foam is specifiedby ASTM C 578.18 It is a rigid, closed-cell foamthat is formed by either an extrusion or moldingprocess.

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5.2.6 Ceramic Fiber

ASTM specifies ceramic fiber separately from man-made mineral fiber. It is typically used in high-temperature applications. Its use at lower tempera-tures is limited due to its high cost.

If the fiber is used at moderate temperatures, thewicking characteristics of the particular product formwill affect water absorption.

5.2.7 Prefabricated Systems

Many products on the market combine insulationmaterials with various accessories to produce pre-fabricated systems intended to enhance installationefficiencies and/or overall service performance. Allcomponents of a system must be considered for aparticular application. Of particular interest areminor components (accessory materials) that may bedetrimental to austenitic stainless steels (seeParagraph 5.3).

5.2.8 Historical Materials

Materials that are no longer manufactured, or arerarely used today, may be of concern in existingsystems. In particular, asbestos and magnesium-based materials may contain high levels of chlorides.

5.3 Insulation Accessory Materials

Insulation accessory materials include those componentsused to fabricate insulation materials into shapes that fitpipe and equipment, as well as components used toapply those shapes, provide weatherproofing, and sealprojections through the insulation system.

Materials such as cements, mastics, and coatings mayrequire mixing with water before use. In that case, waterquality is a concern. When used over austenitic stainlesssteel, maximum chloride content of the water must bespecified. Concentrations of less than 50 to 100 ppm aresuggested. The best practice is to use condensate orsome other high-purity water source.

Some users specify mastics and sealants that do notcontain PVC, brominated compounds, chlorinatedhydrocarbons, or acetic acid derivatives because thecompounds promote ESCC.

ASTM test methods for insulation materials used overaustenitic stainless steels are not always appropriate foraccessory materials. Some specifiers refer to NuclearRegulatory Commission(4) requirements.

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5.3.1 Cements

Cements are used to join insulation materials intouseful shapes. Hydrated silicates are used withcalcium silicate, perlite, and cellular glass. Waterquality is a concern. Asphaltic materials arecommonly used in low-temperature systems. Someasphaltics may not pass acceptance testing for usewith austenitic stainless steels. Materials containingchlorinated polymers, such as PVC, are not suitablefor insulating austenitic stainless steels.

5.3.2 Adhesives

Adhesives are used to bond insulation materials toequipment surfaces in some applications. Adhesivesare also a component of tapes, prefabricated pipecovers, and other prefabricated systems.

Some adhesives used on adhesive tapes have beenfound to cause cracking of austenitic stainless steels.The most common problem has been the use of tapeto temporarily position heat tracers or otherinsulation system components.

Adhesives used with labeling systems for identifi-cation are of concern due to the effects of chloridecontent and crevices.

5.3.3 Mastics and Coatings

Mastics and coatings are applied over insulationmaterials for weather protection and as cold servicevapor barriers where metal or other jacketing is notused. Irregular shapes, such as pumps and valves,are typical applications.

Weatherability and maintenance of these materialsmust be considered when used to provide primaryweather protection. Periodic inspection and repair ofdamage are necessary to maintain the usefulness ofthese materials.

5.3.4 Sealants and Caulks

Sealants and caulks are used to seal protrusionsthrough insulation systems and to provide vaporbarriers in below-ambient conditions.

Failure of sealant and caulking systems is a commonsource of water intrusion into insulation systems.Weatherability, maintenance, and suitability for theservice temperature must be considered. Periodicinspection and repair of damage are necessary tomaintain the usefulness of these materials.

5.3.5 Jacketing Materials

Jacketing materials are used to provide mechanicaland weather protection for insulation systems.

Commonly used materials include aluminum,aluminized steel, galvanized steel, stainless steel,fiberglass-reinforced plastic, thermoplastics, rein-forced fabrics, and tape systems.

Aluminum jacketing is economical, relatively corro-sion resistant, and easy to work with, and therefore,its use is widespread. Pitting corrosion from theinside surface due to entrapped moisture andreaction with wet insulation materials is a concern. Itis commonly supplied with an inner barrier ofthermoplastic film and/or kraft paper. It is availablewith various factory-applied coatings for additionalcorrosion resistance. Pitting and perforation ofaluminum jacketing negates its function as a weatherbarrier. Use of aluminum on high-temperature(above 540°C [1000°F]), high-alloy equipment isnormally restricted due to liquid metal crackingconcerns.

Stainless steel jacketing is available in types 302,304, and 316. Because it is more expensive thanaluminum jacketing, its use is limited to specializedapplications such as plant atmospheres corrosive toaluminum, areas where insulation is intended toserve as fireproofing, and use on high-temperature(above 540°C [1,000°F]), high-alloy equipment. Theconcerns previously discussed for ESCC of stainlesssteel equipment and piping are also a concern withstainless steel jacketing in the appropriate environ-ment or in contact with leachable chloride-containinginsulation. Stainless steel jacketing is commonlysupplied with an inner barrier of thermoplastic filmand/or kraft paper. When stainless steel jacketing isused, it should be used in conjunction with stainlesssteel bands and hardware to reduce the occurrenceof galvanic corrosion and, at high temperatures,LMC.

Galvanized steel or aluminized jacketing suffers fromiron-oxide staining as a result of corrosion at seams,screw holes, and other edges where the zinc oraluminum is unable to provide adequate coverage.In addition, galvanized jacketing cannot be used athigh temperatures greater than 370°C (700°F) aszinc is a low-melting-point metal. As with the jacket-ing materials mentioned above, galvanized andaluminized-steel jacketing is commonly supplied withan inner barrier of thermoplastic film and/or kraftpaper.

Plastic materials such as fiberglass-reinforced plasticand thermoplastics are not commonly used forjacketing because of their low melting temperatures,lack of resistance to mechanical abuse and ultra-violet radiation (sunlight), and corrosion by manychemicals. These materials are only used forspecialized applications and are more effective forindoor use.

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Reinforced fabric jacketing is typically used forremovable and reusable insulation covers, which aremade specifically for specific equipment items and

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Section 6: Inspection

____________________________(5)

American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20

piping components where conventional insulationmethods are impractical.

__________________________________

and Maintenance

6.1 Overview

Thermal insulation on plant process equipment creates aformidable barrier to easy inspection for corrosiondamage. Unfortunately, the very presence of thermalinsulation can set up corrosion problems that arecompletely unrelated to the product contained in the pipeor vessel.

In many instances, it is a simple task to detect andmeasure the effects of corrosion due to the process fluidsand gases on the inside surface of piping and equipment,yet a very difficult task to detect and measure the effectsof corrosion due to thermal insulation on the outsidesurface.

Removing all the insulation would be the ideal method forlocating and evaluating CUI, but it is time-consuming andexpensive. Visual inspection for evidence of moisture orcorrosion will help to predict where surface corrosionthreatens the piping system or equipment. At the veryleast, it can locate “suspect” areas for furtherinvestigation. All plant personnel can and should helpwith the visual inspection and then consult with thecompany experts.

6.2 Pre-Inspection Activities

A plan should be developed to inspect and recordwarning signs of CUI. It is helpful to begin with a plant orarea map indicating location of equipment. For processpiping, refer to API(5) 570.19

The map should be used as a point of departure toprioritize, inspect, and record suspect insulation. Thefollowing list should be used in setting priorities, and aseparate prioritizing checklist should be used for eachitem of equipment.

6.2.1 Location of Equipment

6.2.1.1 Is it indoors or outdoors?

Inside areas are less at risk, provided that theyare not near hose-down, safety shower, or fireprotection deluge systems.

6.2.1.2 Does the prevailing wind contain highhumidity or corrosive contaminants?

Equipment downwind of corrosive mists (e.g.,cooling tower, power plant, and seashore) ismore exposed to CUI factors.

6.2.1.3 Is equipment susceptible to mechanicaldamage?

Insulation systems bumped by tools or used asmechanical support for workers are more likelyto break down and allow water entry.

6.2.2 Temperature and Materials of Construction

6.2.2.1 How susceptible is the alloy to corrosionor to cracking at operating temperatures?

The probability of material failure varies withoperating temperature or range of temperature.The following are the temperature ranges ofgreatest concern.

6.2.2.1.1 For carbon steel, continuousprocess operation at temperatures between-4°C and 150°C (25°F and 300°F) or cyclingabove and below the dew point.

6.2.2.1.2 For type 300 series stainlesssteels, continuous process operationbetween 50°C and 150°C (120°F and 300°F)or cycling above and below the dew point.

6.2.3 Age of Equipment

6.2.3.1 How long has the equipment been inservice since last insulated?

Because CUI is an insidious problem, it ishelpful to check records for when the equipmentwas installed or last insulated. CUI problemsare commonly found to be significant after aboutfive years.

6.2.4 Coatings

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6.2.4.1 Is equipment coated?

Coated equipment has a better survival rate, butthe type of coating used should also be con-sidered. Coatings suitable for liquid immersionservice are usually specified, and guidelines forselection of protective coatings are found inSection 4. Insulated equipment that has beencoated is much easier to inspect.

6.2.5 Risk Potential — Process/Business/Environ-ment/Safety/Health

6.2.5.1 Are there exposed fittings?

Fixtures such as clips, nozzles, and inspectionports needing caulking are potential points forwater entry. A change of design is sometimesthe only solution.

6.2.5.2 What are the consequences of the leak?

In choosing the frequency of inspection, abusiness should consider the environmental andeconomic consequences of a leak. Also,OSHA(6) CFR 1910.119: Process SafetyManagement20 (or similar local standards)should be used as a guide.

6.2.5.3 What is the cost of downtime for repairsor replacement? Should key items of plantequipment be inoperable for several weeks?Several months?

6.3 Visual Inspection

6.3.1 New Construction

Design and specification documents should bereviewed to make sure they are complete andcorrect. Adequate resources must be devoted toensure that the design details are properly imple-mented. Visual inspection of the insulated equipmentand piping in the work area should be started usingthe site map, the prioritizing checklist, and aninspection work sheet. Equipment designated to becoated should be checked to verify that it has beencoated according to manufacturer’s or owner’sspecifications. Suspect areas should be recorded.The following guidelines should be adhered to as CUImay occur when the following recommendationshave not been followed:

(a) Keep insulation dry at all times.(b) Keep surfaces to be insulated clean and dry.

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Occupational Safety and Health Administration (OSHA), 100 Constituti

(c) Ensure that a full bedding coat of asphaltcutback is applied when required.

(d) Use the insulation thickness designated in theproject insulation specifications.

(e) Determine whether the insulation should besingle-layer or double-layer.

(f) Ensure that all joints are staggered, especiallyon double-layered systems.

(g) Ensure that a bedding coat has been appliedbetween the first and second insulation layers,for systems operating below -40°C (-40°F). Donot apply the coat to the substrate.

(h) Ensure that the insulation has no gaps greaterthan 3 mm (0.125 in.).

(i) Replace the affected section of insulation if thegap exceeds 3 mm (0.125 in.). Do not usefinishing cement to fill the gap.

(j) Use valve stem extension handles, whereapplicable, for insulated valves.

(k) For systems requiring a vapor barrier, ensurethat the vapor barrier has been applied to theexterior of the insulation before installing thejacket.

(l) Do not use screws to secure jacketing onsystems with vapor barriers.

(m) Ensure that insulation has been secured withthe specified wire, bands, or tape.

(n) Ensure that all insulation terminations haveend caps.

(o) Ensure that watershed angles are provided.(p) Ensure that installed insulation is protected

from rain and washdown until jacketing isinstalled.

(q) Ensure that the proper jacketing type and metalthickness is installed.

(r) Ensure that the jacketing is installed inwatershed fashion on horizontal runs.

(s) Ensure that the bands and breather springs arethe correct size and material. These areinstalled on the outside of the jacketing aroundthe equipment.

(t) Ensure that the bands are turned under orcaulked at the clips.

(u) Ensure that the nozzle openings and all otherprotrusions are flashed and caulked.

(v) Ensure that the system has been caulked.Caulking should be left beaded, not feathered.

(w) Order duplicate equipment nameplates forsystems operating below 0°C (32°F). Theseshould be banded, not screwed, to the outsideof the jacketing.

6.3.2 Equipment in Service

Using the site map, the prioritizing checklist, and aninspection work sheet, inspections of the insulated

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equipment and piping in the area should beconducted. Specific items of equipment that arecoated should be identified. Suspect areas should berecorded. Inspection personnel should be alert to thefollowing warning signs:

(a) Weathered, damaged, inelastic, or missingcaulking on piping, vessel heads, sidewalls,supports, and nozzles.

(b) Weathered, split, or missing mastic moisturebarriers on piping and vessels, and onsidewalls, above supports, and around nozzles.

(c) Punctured, torn, loose, dislodged, slipped,missing, or corroded metal jacketing.

(d) Stains, deposits, or holes in jackets andcovers.

(e) Unsealed piping terminations.(f) Gaps in jackets around pipe hangers, at the tip

of vertical pipe runs, and at other protrusionssuch as structural stainless steel supports.

(g) Swollen or blistered insulation.(h) Improper installation interfering with water run-

off.(i) Mildew or moisture at insulation support rings

or vacuum rings on vessels.(j) Unprotected insulation where parts have been

removed.(k) Unsealed metal wall thickness test points.(l) Flashing that does not shed water.(m) Open joints in jackets from physical damage.

6.4 Nondestructive Moisture and Corrosion DetectionTechniques

These techniques and devices can enhance visualinspection on any type of insulation. On pressure vesselsand piping, the CUI pattern may be nonuniform, and spotnondestructive evaluation (NDE) may be misleading.

6.4.1 Delmhorst moisture meter

6.4.2 Infrared thermography

6.4.3 Neutron backscatter device

6.4.4 Flash radiography

6.4.5 Electromagnetic (eddy current)

6.4.6 Ultrasonic testing (UT) of the equipment fromthe inside

6.4.7 Fluoroscopic imaging of piping

6.4.8 Profile radiography

6.5 Assessment of Damage

If investigations or observations indicate wet insulation,the extent of corrosion or structural damage to the

NACE International

equipment must be assessed. Insulation should beremoved or the corrosion should be evaluated by asuitable NDE technique. Some of the techniques arelisted in Paragraph 6.4.

The following procedure should be used for assessing thedamage:

6.5.1 Remove a patch of insulation, 120 to 150 cm2

(18 to 24 in.2) in area, from vessels or piping greaterthan 61 cm (24 in.) in diameter, or a section 0.9 m(3 ft) long from piping less than 61 cm (24 in.) indiameter, where there is probable corrosion damage.Site-specific requirements must be followed whenremoving asbestos, respirable ceramic fiber (RCF),or non-asbestos respirable fiber (NARF) insulation.

6.5.2 When repeat inspections are to be made at thesame point, use replaceable insulation plugs to closeinspection holes in the insulation.

6.5.3 Examine the equipment for thick rust depositson carbon steel and hard, crusty deposits onaustenitic stainless steel. Corrosion is often foundabove vacuum rings and insulation support rings,above and below manways, and below breaks in tophead moisture barriers.

6.5.4 If there is no corrosion and the insulation isdry, replace the insulation and seal thoroughly.

6.5.5 If there is no corrosion but the insulation iswet, remove the insulation to the point where it iscompletely dry. Eliminate the source of waterintrusion, using proper insulation installationtechniques.

6.5.6 If corrosion damage has occurred, remove allthe insulation from the damaged areas. The totalsystem must be inspected and cleaned. Thedamaged equipment or parts must be repaired asnecessary or replaced. The metal surface must beprotectively coated and reinsulated.

6.6 Equipment Inspection Methods

6.6.1 Carbon Steel

Ultrasonic thickness and pit depth measurementtechniques are usually used to determine theremaining wall thickness of pipe, tanks, pressurevessels, and other plant equipment when there isdirect access to the exterior surface. Testing shouldbe conducted using established test procedures suchas those found in API 510,21 570, and 653.22

6.6.2 Stainless Steel

6.6.2.1 Eddy Current Inspection

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Eddy current inspection is recommended forstainless steel surface inspection. Whenproperly used, it is a rapid, effective method fordetecting ESCC. Eddy current examinationmust be performed by qualified specialists.

6.6.2.2 Liquid Dye Penetrant Inspection (PT)

When eddy current examinations are notpractical, liquid dye penetrant inspection is auseful procedure for ESCC detection. The metalsurface must be as near ambient temperature aspossible. This procedure will not be effective atan elevated temperature. The periphery ofcracked areas should be examined for lessobvious cracking, especially if weld repair isbeing considered. Only halogen-free PTmaterials should be used.

6.6.3 Surface Preparation and Cleaning

One effective surface cleaning procedure for PT ofstainless steels involves aspirating a small amount ofgrit, such as No. 7 crushed flint, into a high-pressurewater-blast nozzle to remove deposits (but not thebrown stains occurring on stainless steel) andminimize dusting. Brown stains on stainless steeloften indicate ESCC. Heavy grit blasting or sandingmay smear over the cracks and decrease theeffectiveness of the PT.

6.6.3.1 The stainless steel surface should beprepared for PT by one or more of the followingtechniques to remove surface deposits and toavoid peening shut any ESCC.

(a) Hydroblasting — Conventional abrasiveblasting should not be used.

(b) Disc sanding — This can be done withcoarse grit and moderate pressure. Toomuch pressure will force grit into cracks.

(c) Flapper sanding — This can be done withcoarse grit and moderate pressure.

(d) Pencil grinding — This can be used toprepare fillet welds where a sander cannotreach.

6.6.3.2 The prepared area should be washedwith water, cleaned with chloride-free solvent,and wiped dry with a lint-free cloth. Red dyepenetrant should be applied by spray or brushon ambient temperature surfaces, allowing 15min to penetrate. Excess penetrant should bewiped off with a lint-free cloth soaked in chloride-free solvent. A very thin coating of developershould be applied followed by visual inspectionafter at least 10 min for indications of cracking.

__________________________merican National Standards Institute (ANSI), 11 W. 42nd St., New Yo

6.7 Repair

The extent of damage will determine the type and amountof repair required.

Before beginning repairs, a qualified corrosion/materialsspecialist should be consulted to assist in assessingdamage and choosing repair methods. Methods must beconsistent with good practices and code requirements.Examples of repair techniques and insulation refurbish-ment practices are:

6.7.1 Replacement of equipment may be necessaryif its integrity is affected by severe corrosion ofcarbon steel or by ESCC of austenitic stainless steel.

6.7.2 Repair of equipment that has corroded mustfollow the requirements of applicable codes andstandards. These include the ANSI(7) National BoardInspection Code (NB-23),23 API 510 for pressurevessels, API 653 for tanks, and API 570 for piping.

6.7.3 Replace deteriorated caulking with siliconecaulking compounds.

6.7.4 Replace flashing around vacuum and insula-tion support rings, and clips on vessels as well, withtypes that direct water away.

6.8 Shutdown and Mothballing

Some severe cases of CUI have occurred duringextended shutdowns or mothballing of equipment.Weather barriers deteriorate during these idle periods,and typically, no maintenance or repair is performed.

Carbon steel piping and equipment may be severelycorroded at ambient temperature when mothballed.Stainless steel is susceptible to ESCC by the water-leached salts when the equipment is brought on-line afteridle periods; however, it is not likely to corrode underinsulation during storage.

When plant management is uncertain whether equipmentwill be used again, funds or facilities to maintain weatherbarriers or to move equipment indoors may not beprovided. Stripping all insulation before mothballing isthe most cost-effective way of storing carbon steel andstainless steel piping and equipment. As a rule, rustingof exposed carbon steel is less severe and more uniformthan corrosion under wet insulation.

Stored equipment shall be abrasive-blasted, recoated,and reinsulated before use in a CUI environment.

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References

1. ASTM C 692 (latest revision), “Standard Test Methodfor Evaluating the Influence of Thermal Insulations onExternal Stress Corrosion Cracking Tendency of Austeni-tic Stainless Steel” (West Conshohocken, PA: ASTM).

2. W.I. Pollock and J.M. Barnhart, eds., Corrosion ofMetals Under Thermal Insulation, STP 880. (WestConshohocken, PA: ASTM, 1985).

3. NACE Publication 6H189 (latest revision), “A State-of-the-Art Report on Protective Coatings for Carbon Steeland Austenitic Stainless Steel Surfaces Under ThermalInsulation and Cementitious Fireproofing” (Houston, TX:NACE).

4. F.N. Speller, Corrosion — Causes and Prevention,2nd ed. (New York, NY: McGraw-Hill Book Co., 1935), p.153 and Fig. 25.

5. W.G. Ashbaugh, “Corrosion of Steel and StainlessSteel Under Thermal Insulation,” in Process IndustriesCorrosion, eds. B.J. Moniz, W.I. Pollock (Houston, TX:NACE, 1986), p. 761.

6. ASTM C 871 (latest revision), “Standard TestMethods for Chemical Analysis of Thermal InsulationMaterials for Leachable Chloride, Fluoride, Silicate, andSodium Ions” (West Conshohocken, PA: ASTM).

7. ASTM C 795 (latest revision), “Standard Specifica-tion for Thermal Insulation for Use in Contact withAustenitic Stainless Steel” (West Conshohocken, PA:ASTM).

8. W.G. Ashbaugh, “External Stress Corrosion Crackingof Stainless Steel Under Thermal Insulation,” MP 4, 5(1965): pp. 18-23.

9. ASTM C 929 (latest revision), “Standard Practice forHandling, Transporting, Shipping, Storage, Receiving,and Application of Thermal Insulation Materials For Usein Contact with Austenitic Stainless Steel” (WestConshohocken, PA: ASTM).

10. NACE Standard RP0375 (latest revision), “WaxCoating Systems for Underground Piping Systems”(Houston, TX: NACE).

11. ASTM C 533 (latest revision), “Standard Specifica-tion for Calcium Silicate Block and Pipe ThermalInsulation” (West Conshohocken, PA: ASTM).

12. ASTM C 610 (latest revision), “Standard Specifica-tion for Molded Expanded Perlite Block and Pipe ThermalInsulation” (West Conshohocken, PA: ASTM).

13. ASTM C 552 (latest revision), “Standard Specifica-tion for Cellular Glass Thermal Insulation” (WestConshohocken, PA: ASTM).

14. ASTM C 1029 (latest revision), “Standard Specifica-tion for Spray-Applied Rigid Cellular PolyurethaneThermal Insulation” (West Conshohocken, PA: ASTM).

15. ASTM C 591 (latest revision), “Standard Specifica-tion for Unfaced Preformed Rigid Cellular Polyiso-cyanurate Thermal Insulation” (West Conshohocken, PA:ASTM).

16. ASTM C 534 (latest revision), “Standard Specifica-tion for Preformed Flexible Elastomeric Cellular ThermalInsulation in Sheet and Tubular Form” (WestConshohocken, PA: ASTM).

17. ASTM C 1126 (latest revision), “Standard Specifica-tion for Faced or Unfaced Rigid Cellular Phenolic ThermalInsulation” (West Conshohocken, PA: ASTM).

18. ASTM C 578 (latest revision), “Standard Specifica-tion for Rigid, Cellular Polystyrene Thermal Insulation”(West Conshohocken, PA: ASTM).

19. API Standard 570 (latest revision), “Piping InspectionCode: Inspection, Repair, Alteration, and Rerating of In-Service Piping Systems” (Washington, DC: API).

20. CFR 1910 (latest revision), “Occupational Safety andHealth Standards” (Washington, DC: OSHA).

21. API Standard 510 (latest revision), “Pressure VesselInspection Code: Maintenance Inspection, Rating, Repair,and Alteration” (Washington, DC: API).

22. API Standard 653 (latest revision), “Tank Inspection,Repair, Alteration, and Reconstruction” (Washington, DC:API).

23. NB-23 (latest revision), “National Board InspectionCode: Manual for Boiler and Pressure Vessel Inspectors”(New York, NY: ANSI).

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Bibliography

Ashbaugh, W.G. “ESCC of Stainless Steel underThermal Insulation.” Materials Protection 4, 5 (1965):p. 18.

Ashbaugh, W.G. “Stress Corrosion Cracking of ProcessEquipment.” Chemical Engineering Progress, 66, 10(1970): p. 44.

Delahunt, J.F. “Corrosion of Underground InsulatedPipelines.” Journal of Protective Coatings and Linings3, 1 (1986): p. 36.

Haney, E.G. “The Zinc-Steel Potential Reversal inCathodic Protection.” CORROSION/81, paper no.216. Houston, TX: NACE, 1981.

Mattsson, E. “The Atmospheric Corrosion Properties ofSome Common Structural Metals—A ComparativeStudy.” CORROSION/82, plenary lecture. Houston,TX: NACE, 1982.

Montle, S., W.G. Ashbaugh, and J.F. Delahunt. “ProblemSolving Forum.” Journal of Protective Coatings andLinings 2, 1 (1985): p. 10.

NACE Publication 6A176. “Inorganic Zinc Coatings forImmersion (Tank Lining) Service.” Houston, TX:NACE, 1976.

NACE Publication 6B161. “Zinc Filled Inorganic Coat-ings.” Houston, TX: NACE, 1961.

SSPC Publication PS 12.00. “Guide for Selecting Zinc-Rich Painting Systems.” Pittsburgh, PA: SSPC,1991.

Zinc: Its Corrosion Resistance. New York, NY: Inter-national Lead Zinc Research Organization, Inc.,1971.

ISBN 1-57590-049-1NACE International