Corrosion of Weldments

Corrosion of Weldments A. Wahid, D.L. Olson, and D.K. Matlock, Colorado School of Mines C.E. Cross, Martin Marietta Astronautics Group WELDMENTS exhibit special microstruc- tural features that need to be recognized and understood in order to predict acceptable corro- sion service life of welded structures (Ref 1). This article describes some of the general char- acteristics associated with the corrosion of weld- ments. The role of macrocompositional and mi- croeompositional variations, a feature common to weldments, is emphasized in this article to bring out differences that need to be realized in comparing corrosion of weldments to that of wrought materials. A more extensive presenta- tion, with data for specific alloys, is given in Volume 13 of the ASM Handbook (Ref 2). Weldments inherently possess compositional and microstructural heterogeneities, which can be classified by dimensional scale. On the larg- est scale, a weldment consists of a transition from wrought base metal through a heat-affected zone and into solidified weld metal and includes five microstructurally distinct regions normally identified (Ref 3) as the fusion zone, the un- mixed region, the partially melted region, the heat-affected zone, and the base metal. This mi- crostructural transition is illustrated in Fig. 1. The unmixed region is part of the fusion zone, and the partially melted region is part of the heat-affected zone, as described below. Not all five zones are present in any given weldment. For example, autogenous (that is, no filler metal) welds do not have an unmixed zone. The fusion zone is the result of melting which fuses the base metal and filler metal to produce a zone with a composition that is most Weld nugget Weld interface / Unmixed region P a r t i a l l y melted ~ / region Unaffected base metal Schematic showing the regions of a heteroge- Fig. 1 neousweld. Source: Ref3 often different from that of the base metal. This compositional difference produces a galvanic couple, which can influence the corrosion proc- ess in the vicinity of the weld. This dissimilar- metal couple can produce macroscopic galvanic corrosion. The fusion zone itself offers a microscopic galvanic effect due to microstructural segrega- tion resulting from solidification (Ref 4). The fusion zone also has a thin region adjacent to the fusion line, known as the unmixed (chilled) re- gion, where the base metal is melted and then quickly solidified to produce a composition sim- ilar to the base metal (Ref 5). For example, when type 304 stainless steel is welded using a filler metal with high chromium-nickel content, steep concentration gradients of chromium and nickel are found in the fusion zone, whereas the unmixed zone has a composition similar to the base metal (Fig. 2). Heat-Affected Zone. Every position in the heat-affected zone relative to the fusion line ex- periences a unique thermal experience during welding, in terms of both maximum temperature and cooling rate. Thus, each position has its own microstructural features and corrosion suscepti- bility. The partially melted region is usually one or two grains into the heat-affected zone relative to the fusion line. It is characterized by grain boundary liquation, which may result in liqua- tion cracking. These cracks, which are found in the grain boundaries one or two grains below the 30 D 28 2 4 0 22 2O 16 . . . . . . +.". +oo i=:01 ,:z HAz°°°I . . . . . . . . . CHROMIUM I I 20 18 ' 16 ~ 14 z 12 o NICKEL 10 m I 8 0 500 1000 1500 DISTANCE(MICRONS) Concentration profile of chromium and F ig . 2 nickel across the weld fusion boundary re- gion of type 304 stainless steel. Source: Ref 5 fusion line, have been identified as potential ini- tiation sites for hydrogen-promoted underbead cracking in high-strength steel. Microstructural Gradients. On a fine scale, microstructural gradients exist within the heat- affected zone due to different time-temperature cycles experienced by each element of material. Gradients on a similar scale exist within solidi- fied multi-pass weld metal due to bead-to-bead variations in thermal experience. Compositional gradients on the scale of a few microns, referred to as microsegregation, exist within individual weld beads due to segregation of major and trace elements during solidification (Ref 4). Forms of Weld Corrosion Weldments can experience all the classical forms of corrosion, but they are particularly sus- ceptible to those affected by variations in micro- structure and composition. Specifically, gal- vanic corrosion, pitting, stress corrosion, intergranular corrosion, and hydrogen cracking must be considered when designing welded structures. Galvanic Couples. Although some alloys can be autogenously welded, filler metals are more commonly used. The use of filler metals with compositions different from the base material may produce an electrochemical potential differ- ence that makes some regions of the weldment more active. For example, Fig. 3 depicts weld metal deposits that have different corrosion be- havior from the base metal in three aluminum alloys (Ref 6). For the majority of aluminum alloys, the weld metal and the heat-affected zone become more noble relative to the base metal, as demonstrated in Fig. 3(a) and (b) for a saltwater environment (Ref 6). Certain aluminum alloys, however, form narrow anodic regions in the heat-affected zone and are prone to localized attack. Alloys 7005 and 7039 are particularly susceptible to this problem (Fig. 3c). There are a number of other common weld deposit/base metal combinations that are known to form galvanic couples. It is common practice to use austenitic stainless steel welding consum- ables for field repair of heavy machinery, partic- ularly those fabricated from high-strength low- ASM Handbook, Volume 6: Welding, Brazing, and Soldering D.L. Olson, T.A. Siewert, S. Liu, and G.R. Edwards, editors, p 1065-1069 Copyright © 1993 ASM International® All rights reserved. www.asminternational.org 1 0 6 6 / Special W e l d i n g and Joining Topics Distance from weld centerline, in. o ~ 1 2 3 4 o 900' I Edg~ of wel~l bead ' = , I Hardness i=-f - - " ~-E > 750 ~I~ I ~Co~oslon potential 70C I I 0 25 50 50 1 O0 Distance from weld centerllne, mm 65 = 55 -r 45 ,~ 2 5 Distance from weld centertine, in. O 1 2 3 4 800 =-~mm o 750 llLII [Edgel of weld = bead ' l 1 65 m e= (n Hardness := ¢~ 600 If t I I I I 25 0 25 50 50 1 O0 Distance from weld centerline, mm Distance from weld centerllne, in. 1 2 3 4 uo 10500 , , [ IEdoe°'w'd'"ad' '1 '° w 1 DO0 ~ Hardness m~T: / , . . . . . . . . . . . 80 n- o = 950 70 == i T 900 Corrosion potential 5O o 8 5 0 I J I 0 25 50 50 100 Distance from weld centerline, mm (a) (b) (c) Effect of welding heat on microstructure, hardness, and corrosion potential of three aluminum alloy welded assemblies. (a) Alloy 5456-H321 base metal with alloy Fig. 3 5556 filler. (b) Alloy 2219-T87 base metal with alloy 2319 filler. (c) Alloy 7039-T651 base metal with alloy 5183 filler. Source: Ref 6 i n t r ag ranu la r~ grain boundary ~ E 2 , , ~ ~ d e p l e t i o n ~3EI ~ El: potential for ppt. boundary E2: potential for depleted z o n e precipitate E3: potential for matrix Depleted regions adjacent to precipitates. r l a 4 ='~" These regions cause an electrochemical po- tential (E) difference that can promote localized corro- sion at the microstructural level. alloy steel. This practice leaves a cathodic stainless steel weld deposit in electrical contact with the steel. In the presence of corrosive envi- ronments, hydrogen is generated at the austen- itic weld metal cathode, which is capable of maintaining a high hydrogen content without cracking. However, the cathodic behavior of the austenitic weld deposit may increase the suscep- tibility for stress-corrosion cracking in the heat- affected zone of the high-strength steel. A 40% thermal expansion mismatch between the auste- nitic stainless steel and ferritic base metal pro- duces a significant residual stress field in the weldment; this residual stress field also contrib- utes to cracking susceptibility. A similar, but more localized, behavior may explain the corre- lation between stress-corrosion cracking suscep- tibility and the presence of retained austenite in high-strength steel weld deposits. Another common dissimilar metal combina- tion involves the use of high-nickel alloys for weld repair of cast iron. Fe-55Ni welding elec- trodes are used to make weld deposits that can hold in solid solution many of the alloying ele- ments common to cast iron. Furthermore, weld deposits made with Fe-55Ni welding consum- ables have an acceptable thermal expansion match to the cast iron. Because cast iron is an- odic to the high-nickel weld deposit, corrosive attack occurs in the cast iron adjacent to the weld deposit. It is suggested that cast iron welds made with high-nickel deposits be coated (painted) to reduce the susceptibility to selective corrosion attack. Plain carbon steel weldments can also exhibit galvanic attack. For example, the E6013 weld- ing electrode is known to be highly anodic to A285 base metal in a seawater environment (Ref 7). It is important to select a suitable filler metal when an application involves a harsh environ- ment. Weld Decay of Stainless Steel. During welding of stainless steels, local sensitized zones (i.e., regions susceptible to corrosion) of- ten develop. Sensitization is due to the forma- tion of chromium carbide along grain bound- aries, resulting in depletion of chromium in the region adjacent to the grain boundary (Ref 8-14). This chromium depletion produces very localized galvanic cells (Fig. 4). If this depletion drops 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 intergranular attack. This type of corrosion most often occurs in the heat-affected zone. Intergranular corrosion causes a loss of metal in a region that parallels the weld deposit (Fig. 5). This corrosion behav- ior is called weld decay (Ref 13). The formation of sufficient chromium carbide to cause sensitization can be described by the C-shaped curves on the continuous cooling dia- gram illustrated in Fig. 6. The figure shows sus- ceptibility to sensitization as a function of tem- perature, time, and carbon content (Ref 15). If the cooling rate is sufficiently great (curve A in Fig. 6), the cooling curve will not intersect the given C-shaped curve for chromium carbide and the stainless steel will not be sensitized. By de- creasing the cooling rate, the cooling curve (curve B) eventually intersects the C-shape nu- cleation curve, indicating that sensitization may occur. At very low cooling rates, the formation of chromium carbide occurs at higher tempera- ture and allows for more nucleation and growth, resulting in a more extensive chromium-de- pleted region. The minimum time required for sensitization as a function of carbon content in a typical stain- less steel alloy is depicted in Fig. 7. Because the WELD DECAY Intergranular corrosion (weld decay) of stain- Fig. 5 less steel weldments. FZ, fusion zone normal welding thermal cycle is completed in approximately two minutes, for this example the carbon content must not exceed 0.07 wt% to avoid sensitization. Notice that the carbide nu- cleation curves of Fig. 6 move down and to longer times with decreasing carbon content, making it more difficult to form carbides for a given cooling rate. The control of stainless steel sensitization may be achieved by using: • A postweld high-temperature anneal and quench to redissolve the chromium at grain boundaries, and hinder chromium carbide for- mation on cooling • A low-carbon grade of stainless steel (e.g., 304L or 316L) to avoid carbide formation • A stabilized grade of stainless steel containing titanium (alloy 321) or niobium (alloy 327), which preferentially form carbides and leave chromium in solution. (There is the possibil- ity of knife-line attack in stabilized grades of stainless steel.) • A high-chromium alloy (e.g., alloy 310) Role of Delta Ferrite in Stainless Steel Weld Deposits. Austenitic weld deposits are frequently used to join various ferrous alloys. It has been well established that it is necessary to have austenitic weld deposits solidify as primary ferrite, also known as a ~ ferrite, if hot cracking is to be minimized (Ref 16, 17). The amount and C o r r o s i o n of W e l d m e n t s / 1 0 6 7 900 8O0 P 700 o . E 600 500 400 10 ~"o.oso ,t" \ ( o0 0( 1 min 10 min 1 h 10 h Time to sensitization - 16oo 1400 1200 ~. E 19% C - lOOO ~ 800 100 h 1000 h Time-temperature-sensitization curves for type 304 stainless steel in a mixture of CuSO4 and HSO 4 Fig. 6 containing copper. Source: Ref 15. Curves A and B indicate high and medium cooling rates, respec- tively. 104 "~ 10 3 d E 10 2 p, . - 10 i - \ 0.1 0.01 0.03 0.05 0.07 0.09 0.11 Carbon content, wt% Minimum sensitization time from a time-tem- =::nr,~. 7 perature-sensitization diagram as a function of carbon content for a typical 300-series stainless steel alloy. Source: Ref 15 form of ferrite in the weld metal can be con- trolled by selecting a filler metal with the appro- priate chromium and nickel equivalent. A high chromium/nickel ratio favors primary ferrite for- mation, whereas a low ratio promotes primary austenite (Fig. 8). An optimum condition can be attained for ferrite contents between 3 and 8 vol% in the weld deposit. Ferrite contents above 3 vol% usually guarantee primary ferrite forma- tion and thus reduce hot cracking susceptibility. However, ferrite above 10 vol% can degrade mechanical properties at low- or high-tempera- ture service. At low temperatures, excess ferrite can promote crack paths when the temperature is below the ductile-brittle transition temperature. At high temperatures, continuous brittle sigma phase may form at the interface between the austenite and the ferrite. The ferrite content can be confirmed using magnetic measuring equip- ment (Ref 16, 17). Figure 8 can be used to predict the type of ferrite (primary or eutectic) and the ferrite con- tent when a difference exists between the stain- less steels being joined, such as when welding type 304 to type 310 stainless steel (Ref 18). This diagram shows the compositional range for the desirable primary solidification mode. The dotted lines on the diagram indicate the various transitions in the primary solidification phase. Because not all ferrite is primary ferrite (i.e., some is a phase component of a ferrite-austenite eutectic), this diagram can be used to ensure that ferrite is the first solid (primary) phase to form. This condition occurs when the weld deposit has a composition in the range labeled FA in Fig. 8. Because primary ferrite is the preferable mi- crostructure, use of this diagram should re- duce problems of hot cracking during welding. Also, the corrosion behavior of stainless steel weld deposits and castings is measurably dif- ferent depending on whether the stainless steel has a microstructure generated with primary ferrite or primary austenite (Ref 19-25). Thus, knowledge of the weld metal ferrite content and form is necessary in order to be able to properly characterize and predict corrosion be- havior. Pitting is a form of localized attack caused by a breakdown in the thin passive oxide film that protects material from the corrosion process. Pits are commonly the result of a concentration cell established by a variation in solution com- position that is in contact with the alloy material. Such compositional variations result when the solution at a surface irregularity is different from that of the bulk solution composition. Once a pit has formed, it acts as an anode supported by relatively large cathodic regions. Pitting has a delay time prior to nucleation and growth, and nucleation is very site-selective and microstruc- ture-dependent. Pits are often initiated at spe- cific microstructural features in the weld deposit (Ref 26). Pitting occurs when the material/ solution combination achieves a potential that exceeds a critical value, known as the pitting potential. The tendency for a given alloy/ solution combination to pit can often be charac- terized by critical potentials for pitting and repassivation determined by a cyclic potentiody- namic polarization technique. Pits develop more readily in metallurgically heterogeneous materials. For example, when austenitic stainless steel is heated to tempera- tures where sensitization takes place (Ref 26, 27), the resulting chromium-depleted region is subject to pitting. Pits may also initiate at the austenite-ferrite interfaces in stainless steel weld metal. Although weld metal has a higher probability of being locally attacked because of microsegre- gation in the dendritic structure, filler metals are now available that have better pitting resistance than their respective base metals; information about these filler metals can be obtained from consumable suppliers. However, even when the proper filler metal is used, pitting may still occur in the unmixed zone. Duplex stainless steels, with ferrite contents in the range of 40 to 50 vol%, are often used to decrease the tendency of stress-corrosion crack- ing in chromium-nickel high-alloy steels. The welding practice for duplex stainless steels must be given special attention (Ref 20-22, 24, 25, 27) to avoid reduction in corrosion resistance. The combination of a low carbon content and a carefully specified nitrogen addition have been reported to improve resistance to pitting corro- sion, stress-corrosion cracking, and intergranu- lar corrosion in the as-welded condition. The low carbon content helps avoid sensitization, while the addition of nitrogen slows the precipi- tation kinetics associated with the segregation of chromium and molybdenum during the welding process (Ref 1). On rapid cooling from high temperature, nitrogen also has been reported to form deleterious precipitates (for example, Cr2N) in the ferrite, thus reducing the corrosion resistance (Ref 28). Nitrogen also increases the formation of austenite in the heat-affected zone and weld metal during cooling. A minimum pit- ting corrosion rate is achieved at a ferrite content of about 50 vol%. Stress-Corrosion Cracking. Weldments can be susceptible to stress-corrosion cracking under specific environmental conditions. This crack- ing requires the proper combination of corrosive media, susceptible microstructure, and tensile stress. Welds are often loaded in tension (due to residual stress) to a level approaching the yield strength of the base metal. A weld, with its var- ious heterogeneous microstructural features, thus becomes an excellent candidate for stress- corrosion cracking. Stress-corrosion cracks have an anodic crack tip and often leave apparent corrosion products along the fracture. Cracking is often character- ized by crack branching and usually has a delay time prior to crack initiation, with initiation oc- curring at corrosion pits. Increasing the ferrite content in stainless steel weld metal reduces stress-corrosion cracking susceptibility. Ap- proximately 50 vol% ferrite gives optimum stress-corrosion cracking resistance. Welding parameters influence the amount and distribution of residual stress, because the extent o