Basic Understanding Of Weld Corrosion

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2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)www.asminternational.orgCHAPTER 1Basic Understandingof Weld CorrosionCORROSION FAILURES OF WELDSoccur in spite of the fact that the proper basemetal and filler metal have been selected, industry codes and standards have been followed, andwelds have been deposited that possess fullweld penetration and have proper shape andcontour. It is not unusual to find that, althoughthe wrought form of a metal or alloy is resistantto corrosion in a particular environment, thewelded counterpart is not. Further welds can bemade with the addition of filler metal or can bemade autogenously (without filler metal). However, there are also many instances in which theweld exhibits corrosion resistance superior tothat of the unwelded base metal. There also aretimes when the weld behaves in an erratic manner, displaying both resistance and susceptibility to corrosive attack. Metallurgical Factors. The cycle of heating and cooling that occurs during the weldingprocess affects the microstructure and surfacecomposition of welds and adjacent base metal.Consequently, the corrosion resistance of autogenous welds and welds made with matchingfiller metal may be inferior to that of properlyannealed base metal because of: Factors Influencing Corrosionof WeldmentsIt is sometimes difficult to determine whywelds corrode; however, one or more of the following factors often are implicated: Weldment designFabrication techniqueWelding practiceWelding sequenceMoisture contaminationOrganic or inorganic chemical speciesOxide film and scaleWeld slag and spatterIncomplete weld penetration or fusionPorosityCracks (crevices)High residual stressesImproper choice of filler metalFinal surface finishMicrosegregationPrecipitation of secondary phasesFormation of unmixed zonesRecrystallization and grain growth in theweld heat-affected zone (HAZ)Volatilization of alloying elements from themolten weld poolContamination of the solidifying weld poolCorrosion resistance can usually be maintainedin the welded condition by balancing alloy compositions to inhibit certain precipitation reactions, by shielding molten and hot metal surfacesfrom reactive gases in the weld environment,by removing chromium-enriched oxides andchromium-depleted base metal from thermallydiscolored (heat tinted) surfaces, and by choosing the proper welding parameters.Weld MicrostructuresWeldments exhibit special microstructuralfeatures that need to be recognized and understood in order to predict acceptable corrosionservice life of welded structures (Ref 1). This

2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)2 / Corrosion of Weldmentschapter describes some of the general characteristics associated with the corrosion of weldments. The role of macrocompositional andmicrocompositional variations, a feature common to weldments, is emphasized in this chapter to bring out differences that need to be realized in comparing corrosion of weldments tothat of wrought materials. More extensive presentations, with data for specific alloys, aregiven in the chapters which immediately follow.Weldments inherently possess compositionaland microstructural heterogeneities, which canbe classified by dimensional scale. On the largestscale, a weldment consists of a transition fromwrought base metal through an HAZ and intosolidified weld metal and includes five microstructurally distinct regions normally identified (Ref 2) as the fusion zone, the unmixedregion, the partially melted region, the HAZ, andthe unaffected base metal. This microstructuraltransition is illustrated in Fig. 1. The unmixedregion is part of the fusion zone, and the partiallymelted region is part of the HAZ, as describedbelow. Not all five zones are present in any givenweldment. For example, autogenous (that is, nofiller metal) welds do not have an unmixed zone.The fusion zone is the result of meltingwhich fuses the base metal and filler metal toproduce a zone with a composition that ismost often different from that of the base metal.This compositional difference produces agalvanic couple, which can influence the corrosion process in the vicinity of the weld. Thisdissimilar-metal couple can produce macroscopic galvanic corrosion.The fusion zone itself offers a microscopicgalvanic effect due to microstructural segregation resulting from solidification (Ref 3). TheFig. 1Schematic showing the regions of a heterogeneousweld. Source: Ref 2www.asminternational.orgfusion zone also has a thin region adjacent to thefusion line, known as the unmixed (chilled)region, where the base metal is melted and thenquickly solidified to produce a compositionsimilar to the base metal (Ref 4). For example,when type 304 stainless steel is welded using afiller metal with high chromium-nickel content,steep concentration gradients of chromium andnickel are found in the fusion zone, whereas theunmixed zone has a composition similar to thebase metal (Fig. 2).Heat-Affected Zone. The HAZ is the portion of the weld joint which has experiencedpeak temperatures high enough to producesolid-state microstructural changes but too lowto cause any melting. Every position in the HAZrelative to the fusion line experiences a uniquethermal experience during welding, in terms ofboth maximum temperature and cooling rate.Thus, each position has its own microstructuralfeatures and corrosion susceptibility.The partially melted region is usually one ortwo grains into the HAZ relative to the fusionline. It is characterized by grain boundary liquation, which may result in liquation cracking.These cracks, which are found in the grainboundaries one or two grains below the fusionline, have been identified as potential initiationsites for hydrogen-promoted underbead cracking in high-strength steel.Unaffected Base Metal Finally, that partof the workpiece that has not undergone anymetallurgical change is the unaffected basemetal. Although metallurgically unchanged, theunaffected base metal, as well as the entire weldjoint, is likely to be in a state of high residualtransverse and longitudinal shrinkage stress,Fig. 2Concentration profile of chromium and nickel acrossthe weld fusion boundary region of type 304 stainlesssteel. Source: Ref 4

2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)www.asminternational.orgChapter 1: Basic Understanding of Weld Corrosion / 3depending on the degree of restraint imposed onthe weld.Microstructural Gradients. On a finescale, microstructural gradients exist within theHAZ due to different time-temperature cyclesexperienced by each element of material. Gradients on a similar scale exist within solidifiedmulti-pass weld metal due to bead-to-bead variations in thermal experience. Compositionalgradients on the scale of a few microns, referredto as microsegregation, exist within individualweld beads due to segregation of major andtrace elements during solidification (Ref 3).maintaining a high hydrogen content withoutcracking. However, the cathodic behavior of theaustenitic weld deposit may increase the susceptibility for stress-corrosion cracking (SCC)in the HAZ of the high-strength steel. A 40%thermal expansion mismatch between theaustenitic stainless steel and ferritic base metalproduces a significant residual stress field in theweldment; this residual stress field also con-Forms of Weld CorrosionWeldments can experience all the classicalforms of corrosion, but they are particularly susceptible to those affected by variations inmicrostructure and composition. Specifically,galvanic corrosion, pitting, stress corrosion,intergranular corrosion, hydrogen cracking, andmicrobiologically influenced corrosion must beconsidered when designing welded structures.Galvanic Couples. Although some alloyscan be autogenously welded, filler metals aremore commonly used. The use of filler metalswith compositions different from the base material may produce an electrochemical potentialdifference that makes some regions of the weldment more active. For example, Fig. 3 depictsweld metal deposits that have different corrosion behavior from the base metal in three aluminum alloys (Ref 5).For the majority of aluminum alloys, theweld metal and the HAZ become more noblerelative to the base metal, as demonstrated inFig. 3(a) and (b) for a saltwater environment(Ref 5). Certain aluminum alloys, however,form narrow anodic regions in the HAZ and areprone to localized attack. Alloys 7005 and 7039are particularly susceptible to this problem (Fig.3c).There are a number of other common welddeposit/base metal combinations that are knownto form galvanic couples. It is common practiceto use austenitic stainless steel welding consumables for field repair of heavy machinery, particularly those fabricated from high-strengthlow-alloy steel. This practice leaves a cathodicstainless steel weld deposit in electrical contactwith the steel. In the presence of corrosive environments, hydrogen is generated at the austenitic weld metal cathode, which is capable of(a)(b)(c)Fig. 3Effect of welding heat on microstructure, hardness,and corrosion potential of three aluminum alloywelded assemblies. (a) Alloy 5456-H321 base metal with alloy5556 filler. (b) Alloy 2219-T87 base metal with alloy 2319 filler.(c) Alloy 7039-T651 base metal with alloy 5183 filler. Source:Ref 5

2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)4 / Corrosion of Weldmentstributes to cracking susceptibility. A similar, butmore localized, behavior may explain the correlation between SCC susceptibility and the presence of retained austenite in high-strength steelweld deposits.Another common dissimilar metal combination involves the use of high-nickel alloys forweld repair of cast iron. Fe-55Ni welding electrodes are used to make weld deposits that canhold in solid solution many of the alloying elements common to cast iron. Furthermore, welddeposits made with Fe-55Ni welding consumables have an acceptable thermal expansionmatch to the cast iron. Because cast iron isanodic to the high-nickel weld deposit, corrosive attack occurs in the cast iron adjacent to theweld deposit. It is suggested that cast iron weldsmade with high-nickel deposits be coated(painted) to reduce the susceptibility to selective corrosion attack.Plain carbon steel weldments can also exhibitgalvanic attack. For example, the E6013 welding electrode is known to be highly anodic toA285 base metal in a seawater environment(Ref 6). It is important to select a suitable fillermetal when an application involves a harshenvironment.Weld Decay of Stainless Steel. Duringwelding of stainless steels, local sensitizedzones (i.e., regions susceptible to corrosion)often develop. Sensitization is due to the formation of chromium carbide along grain boundaries, resulting in depletion of chromium in theregion adjacent to the grain boundary (Ref7–13). This chromium depletion produces verywww.asminternational.orglocalized galvanic cells (Fig. 4). If this depletiondrops the chromium content below the necessary 12 wt% that is required to maintain a protective passive film, the region will become sensitized to corrosion, resulting in intergranularattack. This type of corrosion most often occursin the HAZ. Intergranular corrosion causes aloss of metal in a region that parallels the welddeposit (Fig. 5). This corrosion behavior iscalled weld decay (Ref 12).The formation of sufficient chromium carbide to cause sensitization can be described bythe C-shaped curves on the continuous coolingdiagram illustrated in Fig. 6. The figure showssusceptibility to sensitization as a function oftemperature, time, and carbon content (Ref 14).If the cooling rate is sufficiently great (curve Ain Fig. 6), the cooling curve will not intersectthe given C-shaped curve for chromium carbideand the stainless steel will not be sensitized. Bydecreasing the cooling rate, the cooling curve(curve B) eventually intersects the C-shapeFig. 5Fig. 4Depleted regions adjacent to precipitates. Theseregions cause an electrochemical potential (E) difference that can promote localized corrosion at the microstructural level.Fig. 6Intergranular corrosion (weld decay) of stainless steelweldmentsTime-temperature-sensitization curves for type 304stainless steel in a mixture of CuSO4 and HSO4 containing copper. Source: Ref 14. Curves A and B indicate high andmedium cooling rates, respectively.

2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)www.asminternational.orgChapter 1: Basic Understanding of Weld Corrosion / 5nucleation curve, indicating that sensitizationmay occur. At very low cooling rates, the formation of chromium carbide occurs at highertemperature and allows for more nucleation andgrowth, resulting in a more extensive chromium-depleted region.The minimum time required for sensitizationas a function of carbon content in a typical stainless steel alloy is depicted in Fig. 7. Because thenormal welding thermal cycle is completed inapproximately two minutes, for this examplethe carbon content must not exceed 0.07 wt% toavoid sensitization. Notice that the carbidenucleation curves of Fig. 6 move down and tolonger times with decreasing carbon content,making it more difficult to form carbides for agiven cooling rate.The control of stainless steel sensitizationmay be achieved by using: A postweld high-temperature anneal andquench to redissolve the chromium at grainboundaries, and hinder chromium carbideformation on coolingA low-carbon grade of stainless steel (e.g.,304L or 316L) to avoid carbide formationA stabilized grade of stainless steel containing titanium (alloy 321) or niobium (alloy327), which preferentially form carbides andleave chromium in solution. (There is thepossibility of knife-line attack in stabilizedgrades of stainless steel.)A high-chromium alloy (e.g., alloy 310)Role of Delta Ferrite in Stainless SteelWeld Deposits. Austenitic weld deposits areFig. 7Minimum sensitization time from a time-temperaturesensitization diagram as a function of carbon contentfor a typical 300-series stainless steel alloy. Source: Ref 14frequently used to join various ferrous alloys. Ithas been well established that it is necessary tohave austenitic weld deposits solidify as primary ferrite, also known as a δ ferrite, if hotcracking is to be minimized (Ref 15, 16). Theamount and form of ferrite in the weld metal canbe controlled by selecting a filler metal with theappropriate chromium and nickel equivalent. Ahigh chromium-to-nickel ratio favors primaryferrite formation, whereas a low ratio promotesprimary austenite (Fig. 8). An optimum condition can be attained for ferrite contents between3 and 8 vol% in the weld deposit. Ferrite contents above 3 vol% usually guarantee primaryferrite formation and thus reduce hot crackingsusceptibility. However, ferrite above 10 vol%can degrade mechanical properties at low- orhigh-temperature service. At low temperatures,excess ferrite can promote crack paths when thetemperature is below the ductile-brittle transition temperature. At high temperatures, continuous brittle σ phase may form at the interfacebetween the austenite and the ferrite. The ferritecontent can be confirmed using magnetic measuring equipment (Ref 15, 16).Figure 8 can be used to predict the type of ferrite (primary or eutectic) and the ferrite contentwhen a difference exists between the stainlesssteels being joined, such as when welding type304 to type 310 stainless steel (Ref 17). This diagram shows the compositional range for thedesirable primary solidification mode. The dotted lines on the diagram indicate the varioustransitions in the primary solidification phase.Because not all ferrite is primary ferrite (i.e.,some is a phase component of a ferrite-austeniteeutectic), this diagram can be used to ensure thatferrite is the first solid (primary) phase to form.This condition occurs when the weld deposit hasa composition in the range labeled FA in Fig. 8.Because primary ferrite is the preferable microstructure, use of this diagram should reduceproblems of hot cracking during welding. Also,the corrosion behavior of stainless steel welddeposits and castings is measurably differentdepending on whether the stainless steel has amicrostructure generated with primary ferrite orprimary austenite (Ref 18–24). Thus, knowledgeof the weld metal ferrite content and form is necessary in order to be able to properly characterizeand predict corrosion behavior.Pitting is a form of localized attack causedby a breakdown in the thin passive oxide filmthat protects material from the corrosionprocess. Pits are commonly the result of a con-

2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)6 / Corrosion of Weldmentscentration cell established by a variation in solution composition that is in contact with the alloymaterial. Such compositional variations resultwhen the solution at a surface irregularity is different from that of the bulk solution composition. Once a pit has formed, it acts as an anodesupported by relatively large cathodic regions.Pitting has a delay time prior to nucleation andgrowth, and nucleation is very site-selective andmicrostructure-dependent. Pits are often initiated at specific microstructural features in theweld deposit (Ref 25). Pitting occurs when thematerial/solution combination achieves a potential that exceeds a critical value, known as thepitting potential. The tendency for a givenalloy/solution combination to pit can often becharacterized by critical potentials for pittingand repassivation determined by a cyclic potentiodynamic polarization technique.Pits develop more readily in metallurgicallyheterogeneous materials. For example, whenaustenitic stainless steel is heated to temperatures where sensitization takes place (Ref 25,26), the resulting chromium-depleted region issubject to pitting. Pits may also initiate at theaustenite-ferrite interfaces in stainless steelweld metal.Fig. 8www.asminternational.orgAlthough weld metal has a higher probabilityof being locally attacked because of microsegregation in the dendritic structure, filler metalsare now available that have better pitting resistance than their respective base metals; information about these filler metals can be obtainedfrom consumable suppliers. However, evenwhen the proper filler metal is used, pitting maystill occur in the unmixed zone.Duplex stainless steels, with ferrite contentsin the range of 40 to 50 vol%, are often used todecrease the tendency of stress-corrosion cracking in chromium-nickel high-alloy steels. Thewelding practice for duplex stainless steels mustbe given special attention (Ref 19, 21, 23, 24,26) to avoid reduction in corrosion resistance.The combination of a low carbon content and acarefully specified nitrogen addition have beenreported to improve resistance to pitting corrosion, SCC, and intergranular corrosion in the aswelded condition. The low carbon content helpsavoid sensitization, while the addition of nitrogen slows the precipitation kinetics associatedwith the segregation of chromium and molybdenum during the welding process (Ref 1). Onrapid cooling from high temperature, nitrogenalso has been reported to form deleterious pre-Welding Research Council (WRC-1988) diagram used to predict weld metal ferrite content. Source: Ref 17

2006 ASM International. All Rights Reserved.Corrosion of Weldments (#05182G)www.asminternational.orgChapter 1: Basic Understanding of Weld Corrosion / 7cipitates (for example, Cr2N) in the ferrite, thusreducing the corrosion resistance (Ref 27).Nitrogen also increases the formation of austenite in the HAZ and weld metal during cooling. Aminimum pitting corrosion rate is achieved at aferrite content of about 50 vol%.Stress-Corrosion Cracking. Weldments canbe susceptible to SCC under specific environmental conditions. This cracking requires theproper combination of corrosive media, suscept

Chapter 1: Basic Understanding of Weld Corrosion / 3 Fig. 3 Effect of welding heat on microstructure, hardness, and corrosion potential of three aluminum alloy welded assemblies. (a) Alloy 5456-H321 base metal with alloy 5556 filler. (b) Alloy 2219-T87 base metal with alloy 2319 filler.

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