Basic Understanding of Weld Corrosion

1 CHAPTER 1 Basic Understanding of weld CorrosionCORROSION FAILURES OF WELDS occur in spite of the fact that the proper basemetal and filler metal have been selected, indus-try 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.

2 Further welds can bemade with the addition of filler metal or can bemade autogenously (without filler metal). How-ever, 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 man-ner, displaying both resistance and susceptibil-ity to corrosive Influencing Corrosion of WeldmentsIt is sometimes difficult to determine whywelds corrode; however, one or more of the fol-lowing factors often are implicated.

3 Weldment design Fabrication technique Welding practice Welding sequence Moisture contamination Organic or inorganic chemical species Oxide film and scale weld slag and spatter Incomplete weld penetration or fusion Porosity Cracks (crevices) High residual stresses Improper choice of filler metal Final surface finishMetallurgical cycle of heat-ing and cooling that occurs during the weldingprocess affects the microstructure and surfacecomposition of welds and adjacent base , the Corrosion resistance of auto-genous welds and welds made with matchingfiller metal may be inferior to that of properlyannealed base metal because of.

4 Microsegregation Precipitation of secondary phases Formation of unmixed zones Recrystallization and grain growth in theweld heat-affected zone (HAZ) Volatilization of alloying elements from themolten weld pool Contamination of the solidifying weld poolCorrosion resistance can usually be maintainedin the welded condition by balancing alloy com-positions to inhibit certain precipitation reac-tions, 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)

5 Surfaces, and by choos-ing the proper welding MicrostructuresWeldments exhibit special microstructuralfeatures that need to be recognized and under-stood in order to predict acceptable corrosionservice life of welded structures (Ref 1). This 2006 ASM International. All Rights of Weldments (#05182G) of WeldmentsFig. 1 Schematic showing the regions of a heterogeneousweld. Source: Ref 2 Fig. 2 Concentration profile of chromium and nickel acrossthe weld fusion boundary region of type 304 stainless steel.

6 Source: Ref 4chapter describes some of the general character-istics associated with the Corrosion of weld -ments. The role of macrocompositional andmicrocompositional variations, a feature com-mon to weldments, is emphasized in this chap-ter to bring out differences that need to be real-ized in comparing Corrosion of weldments tothat of wrought materials. More extensive pre-sentations, with data for specific alloys, aregiven in the chapters which immediately inherently possess compositionaland microstructural heterogeneities, which canbe classified by dimensional scale.

7 On the largestscale, a weldment consists of a transition fromwrought base metal through an HAZ and intosolidified weld metal and includes five mi-crostructurally distinct regions normally identi-fied (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.

8 Not all five zones are present in any givenweldment. For example, autogenous (that is, nofiller metal) welds do not have an unmixed fusion zoneis the result of meltingwhich fuses the base metal and filler metal toproduce a zone with a composition that is most often different from that of the base compositional difference produces a galvanic couple, which can influence the corro-sion process in the vicinity of the weld . This dissimilar-metal couple can produce macro-scopic galvanic fusion zone itself offers a microscopicgalvanic effect due to microstructural segrega-tion resulting from solidification (Ref 3).

9 Thefusion 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 HAZ is the por-tion of the weld joint which has experiencedpeak temperatures high enough to producesolid-state microstructural changes but too lowto cause any melting.

10 Every position in the HAZrelative to the fusion line experiences a uniquethermal experience during welding, in terms ofboth maximum temperature and cooling , each position has its own microstructuralfeatures and Corrosion partially melted region is usually one ortwo grains into the HAZ relative to the fusionline. It is characterized by grain boundary liqua-tion, which may result in liquation cracks, which are found in the grainboundaries one or two grains below the fusionline, have been identified as potential initiationsites for hydrogen-promoted underbead crack-ing in high-strength Base MetalFinally, that partof the workpiece that has not undergone anymetallurgical change is the unaffected basemetal.