BOILER AND HRSG TUBE FAILURESPPChem 11 00 11

LESSON 2:

Corrosion Fatigue

R. Barry Dooley and Albert Bursik

586 PowerPlant Chemistry 2009, 11(10)

PPChem PPChem 101 – Boiler and HRSG Tube Failures

INTRODUCTION

Fatigue damage occurs in general when a boiler tube issubject to repeat cyclic or fluctuating loading although thestress produced is below the material yield strength. Thetypes of fatigue damage include, e.g., corrosion, thermal,mechanical, vibration, and creep fatigue. It is important todetermine which form of fatigue is active, because meas-ures to avoid repeat failures differ as the case arises. In thislesson, the focus is exclusively on corrosion fatigue.

Corrosion fatigue occurs by the combined synergisticactions of cyclic loading and a corrosive environment. It isa discontinuous process with crack initiation and growthduring transient periods. The excessive stresses may becaused during boiler operation by the restraint at tubeattachments and by load changes (in particular during coldstarts or forced cools) or during shutdown or restart of circulation boilers by thermal stratification of water alongthe tube length. Poor water chemistry and its excursionsinfluence both initiation and propagation of corrosionfatigue. The key issue here is the breakdown of the protec-tive magnetite layer. The most decisive chemistry parame-ter is the pH (low pH excursions).

IDENTIFICATION

Pinhole thick-edged leaks are by far the most predominantform of corrosion fatigue failures. In much fewer cases,corrosion fatigue emanates as a long thick-edged crack.Note that not all BTF with a thick-edged fracture surfaceresult from corrosion fatigue. Thick-edged fractures alsooccur when thermal fatigue, mechanical fatigue, low tem-perature creep cracking, circumferential cracking, andhydrogen damage are active. The most important physicalfeature of a corrosion fatigue failure is multiple parallelcracks initiated on the inside of the tube. Upon metallurgi-cal examination the cracks are transgranular as they prop-agate through the tube wall.

FEATURES OF FAILURES

Figure 1 shows a typical multiple array of corrosion fatiguecracks initiated from the inside surface along the neutralaxis of an economizer tube. The initiation sites of thesecracks are associated with surface defects like pits orother discontinuities. The wide cracks have irregular pro-files and are filled with iron oxides.

In Figure 2, corrosion fatigue failure of a low pressure econ-omizer tube of an HRSG is depicted.

Corrosion fatigue cracks may have different appearances:

– pinhole leak

– thick-edged crack

– thick-edged blow-out or rupture

The pinhole leak caused by corrosion fatigue may be con-fused with a mechanical fatigue crack. In contrast to corro-sion fatigue, the mechanical fatigue cracks initiate on theoutside surface and are associated with welds or weld dis-continuities (e.g., the toe of a weld).

Figure 1:

Multiple array of corrosion fatigue cracks.

PPChem 101 – Boiler and HRSG Tube Failures

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Thick-edged cracks are generally associated with attach-ments. However, they may be of considerable length andextend beyond the attachment.

The thick-edged rupture is characterized by cracking downboth sides of the tube along the weld lines of the mem-brane. This relatively rarely occurring form may cause cat-astrophic damage (an entire tube section fails) and is aserious safety issue if it occurs on the cold side of the tubeand in high traffic areas.

LOCATION OF FAILURES

Conventional Boilers

In waterwalls, the predominant locations are those atwhich large stresses develop during transient operatingconditions as thermal expansion is constrained by tubeattachments (Figure 3). Typical locations include attach-ments in windbox casing, buckstay attachments and scal-lop bar attachments. In economizer tubing, locations atbends, welds with the potential for high residual stresses(e.g., fin welds), and locations at attachments are mostthreatened.

Endangered locations are, for example, lug mounted tie-bars connected to tubes. Corrosion fatigue is often initi-

Figure 2:

Corrosion fatigue of a low pressureeconomizer tube (HRSG).

Figure 3:

Typical multiple array of corrosion fatigue cracks (initiated from the inner surface at an attachment).

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PPChem PPChem 101 – Boiler and HRSG Tube Failures

ated at the end of membraneswhere the membrane stops be -cause of a tight bend in the tube orwhere the tubes are bent to form anopening such as for sootblowers ora mandoor.

Figures 4 and 5 show some typicalexamples.

Heat Recovery Steam Generators

As in conventional boilers, the mostlikely failure locations are at welds,at bends, and at attachments.These are locations where signifi-cant thermal stresses may developbecause of restrained thermal ex -pansion. Jeopardized are also tube-to-header connections where – dueto significant thickness transients –local thermal stresses may develop(a thin tube changes temperaturemore rapidly than a thick header).

Large transients or temperature dif-ferences may have different causes,e.g., uneven distribution of gas flowand non-uniform pressure drop inthe individual tube sections due todesign failures.

All Steam Generators

Corrosion fatigue cracks may alsodevelop in steam-touched tubingwhen fluid of a significantly lower (orhigher) temperature than the tubingis introduced. This may occur duringstand-by due to inappropriate boilerlayup.

MECHANISMS OF FAILURE

The synergistic effects of stress andenvironment cause corrosionfatigue. Corrosion fatigue is alsoknown by a number of other names designating basicallythe same mechanism, for example stress-assisted crack-ing or stress-assisted pitting. Sometimes the latter isaligned along original extrusion marks on the tube innersurface. Stress corrosion cracking is not the same mecha-nism as it requires a continuous application of stress and ismost often a continuous cracking process.

It is essential in addressing the root cause of corrosionfatigue that the importance of both the stress and the envi-

ronmental components be identified. Most often corrosionfatigue is driven by the application of a stress imposed bythe system or restraints (attachments etc. as above). Insome cases the cycle chemistry has an influence, but it isalways minor compared to the stress, which is required inthe description of the mechanism to produce a strain onthe inner surface that is great enough to crack (initiate cor-rosion fatigue) and continue to crack (repetitive initiation)the protective oxide layer (magnetite) on the inner surfaceof the tube.

Figure 4:

Typical scallop bar attachment on the waterwall surrounding a burner.

Figure 5:

Locations of pinhole corrosion fatigue leaks associated with the attachment for the dripshield on a boiler.

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PPChemPPChem 101 – Boiler and HRSG Tube Failures

Breakdown of Magnetite Protective Layer

The use of carbon steel or low-alloyed steel – materialsthermodynamically instable in water at operating tempera-tures – for boiler components exposed to high tempera-tures and pressures is only possible because a protectiveoxide layer is formed on the waterside surface of the tube.This protective layer consists mainly of magnetite (Fe3O4).

When the imposed strain is greater than the fracture strainof the oxide (magnetite), the oxide will crack in a regulararray. This cracking then allows boiler water (or evaporatorwater in HRSGs) to come in touch with the tube surface.This then causes more magnetite to grow on this exposedsurface at a relatively fast rate (parabolic growth law). Thisoxide will remain in place until the next application of straingreater than the fracture strain. This will crack the newlyformed magnetite at the bottom of the corrosion fatiguecrack. The cracks grow by a repetition of this process(called repetitive crack initiation); see Figure 6.

Rupture of the magnetite film acts as a stress concentrator.Generally, it is recommended to keep the strain level in themagnetite layer below 0.2 % in tension to avoid film rup-ture.

Addressing Root Causes of Corrosion Fatigue

Three issues are important when evaluating corrosionfatigue occurrence on a particular boiler and its rootcauses:

– "Geography" of failures and damageIt is important to find out where in the boiler corrosionfatigue failures have occurred and in what boiler areasnon-destructive evaluations have indicated damage bythe corrosion fatigue mechanisms. This "geography"helps to identify locations at which detailed monitoringshould be carried out.

Figure 6:

Typical corrosion fatigue cracks illustrating the discontinuous nature of the cracks (bulges along the crack length).

– "History" of failuresComparison of the number of failures in different opera-tion periods is also important. Each operation periodmight have a number of hot, warm, or cold starts, unittrips, two-shift cycles or forced cools. In this way, itbecomes obvious which of the operating spaces mightbe driving the corrosion fatigue mechanism.

– Operating spaceIt is important to delineate all the different types of oper-ating space which have been used on the boiler.

With knowledge of these three factors, we can then moveahead to address the root cause of the corrosion fatigueproblem on that particular boiler. This requires monitoring(temperature, strain, and waterwall movement) selectedendangered locations (recognized from the geography)through all the operating spaces identified by the history offailures.

Excessive Stresses/Strains

Restraint Stresses Breakdown of the magnetite layer isprobable at locations at which excessive strains may bedeveloped (geography and history) during particular opera-tion spaces. It is vital for any short-term and long-termactions targeting corrosion fatigue failures to identify thecritical regions (geography) which often exist at tubeattachments. Redesigning tube attachments in order toincrease the flexibility at the connection and/or improve-ment of weld profiles by grinding may be important; how-ever, any measures taken should ensure that the fracturestrain of the magnetite will not be reached at that locationwith the new design. Note that often not the design but achange in the operation space is required to avoid exten-sive strains and breakdown of the protective oxide layer.

Subcooling in Natural Circulation Boilers High strainshave occurred during stratification of water along thelength of boiler tubing during shutdown and restart of nat-ural circulation boilers. This requires that the boiler bemonitored at the top and bottom to determine the stratifi-cation amount during particular operating spaces (which inthis case may include shutdown).

Operational Aspects

Unit operation can have a significant effect on the occur-rence of corrosion fatigue failures. Based on the evaluationof the history of failures, the operating spaces, which mightdrive the corrosion fatigue mechanism, should be identi-fied. In this way, it becomes obvious which of the operatingspaces might be driving the corrosion fatigue mechanism.This could be a hot, warm or cold start, it could be theshutdown for any of these, and it could be a forced cool, ora trip or any other operating space. It is vital that an organ-ization recognizes the full extent of the operating spacesthrough which a unit has operated.

In the case of HRSG, the approach is parallel to that usedat conventional plants. The focus here should be on theoperating space and the thermal transient which is respon-sible for protective oxide layer breakdown.

Water Chemistry

It is a matter of common knowledge that boilers that havehad boiler water purity problems suffer more often fromcorrosion fatigue failures than those operated with correctchemistry. Units operated on older or incorrect phosphatetreatments experiencing hideout and hideout return havebeen at risk. Large swings in the boiler water pH and pHdepressions during shutdown and early startup are in par-ticular harmful. However, they are detrimental only if theyoccur at the same time that the strain is highest duringwhichever operating space is applicable. This is an impor-tant caveat because chemistry excursions can occur atany time, but only if they coincide with the application of astrain which is great enough to crack the oxide will they ini-tiate and reinitiate corrosion fatigue.

Figure 7 demonstrates that a low boiler water pH de -creases the number of cycles to initiation of corrosionfatigue cracks.

Not only the pH depression but also a high level of oxygenduring inadequate boiler layup may contribute to aggrava-tion of corrosion fatigue damage since oxygenated stag-nant water promotes pitting and the formation of corrosionfatigue initiation centers.

590 PowerPlant Chemistry 2009, 11(10)

PPChem PPChem 101 – Boiler and HRSG Tube Failures

Figure 7:

The influence of pH depression on the initiation of corrosionfatigue cracks in boiler water with a very low level of dissolvedoxygen (< 5 µg · kg–1 oxygen).

Dooley, B. R., Paul, L., Proc., International Water Conference,1995 (Pittsburgh. PA, U.S.A.). Engineers' Society of WesternPennsylvania, Pittsburgh, PA, U.S.A., 56, 146-151 (Paper #95-17).

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PPChemPPChem 101 – Boiler and HRSG Tube Failures

Chemical Cleaning

The use of inhibited hydrochloric acid as a chemical cleansolvent may aggravate corrosion fatigue in comparisonwith other solvents such as ammoniated citric or ethylene-diaminetetraacetic acids or hydroxyacetic-formic acid.

PROBABILITY OF CORROSION FATIGUEFAILURES

For estimating the probability of corrosion fatigue failures,the stress level, environmental conditions, and the influ-ence of operation mode have to be evaluated.

Two of these factors of influence, the stress level and theoperation mode, are typically a standard part of any analy-sis of root causes. Their influence on the probability of cor-rosion fatigue failures is most important and should not bedisregarded in the root cause analysis. Corrosion fatigue,however, occurs by the combined synergistic actions ofcyclic loading and an adverse environment. Note that theenvironment or cycle chemistry is only of a minor influencecompared to the stress or strain. For this reason, it is hardlypossible to numerically express the probability of corrosionfatigue as a function of improper environmental conditions.

However, some of the risk-aggravating factors are:

– phosphate hideout under phosphate boiler water alka-linity control

– chemistry excursions resulting in hydrogen damage orcaustic gouging

– boiler water pH < 8 (at 25 °C) during startup at the pointof reaching operation pressure (sampling point: blow-down or downcomer)

Any of these factors are detrimental only if they occur atthe same time that the strain is highest during whicheveroperating space is applicable. For this reason, theapproach is to conduct a root cause analysis, whichinvolves monitoring typical locations across a range ofoperating spaces as already discussed.

Chemical cleans with inhibited hydrochloric acid andimproper or no corrosion protection during boiler shut-down promote pitting and in this way the formation of cor-rosion fatigue initiation centers.

Figures 1–6 courtesy of Structural Integrity Associates, Inc.