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Introduction

Many coal-fired power plant operatorshave moved toward a staged combustionprocess in order to reduce boiler emis-sions as required by recently implementedenvironmental regulations. By delayingthe mixing of fuel and oxygen, and therebycreating a reducing environment in theboiler, the amount of nitrous oxides (NOx)that are released as a byproduct of coalcombustion is reduced (Refs. 1, 2). Theuse of this staged combustion process hasbeen found by many power plant opera-tors to be the most cost- and time-effectivemethod for decreasing NOx emissions.

Prior to implementation of staged com-bustion, most boiler atmospheres were oxi-dizing, allowing for formation of protectivemetal oxides on waterwall tubes made outof carbon or low-alloy steels (Refs. 1, 3).Under those conditions, failure of water-

walls due to accelerated corrosion was gen-erally not a major problem. Staged combus-tion boilers, on the other hand, create a re-ducing atmosphere in the boiler due to thelack of oxygen. Sulfur compounds from thecoal are transformed into highly corrosiveH2S gas (Ref. 4). Subsequent reaction withthe steel waterwall tubes leads to the for-mation of metal sulfides or mixed sulfidesand oxides on the tube surfaces. Addition-ally, corrosive deposits may form on the wa-terwall tubes due to the accumulation ofsolid particles in the combustion environ-ment, such as ash and unburnt coal. As a re-sult of these changes, the low-alloy steeltubes are often susceptible to acceleratedcorrosion and unsatisfactory service life-times (Refs. 1, 4).

The current industry solution to accel-erated waterwall corrosion is to deposit aweld cladding of a more corrosion-resis-tant alloy on the tube. Commercially avail-able nickel-based alloys have been usedfor weld claddings (Refs. 5–7). These al-loys generally provide good resistance togeneral corrosion for this application.However, weld claddings have recentlybeen shown to be susceptible to corrosion-fatigue cracking in many boiler environ-ments (Ref. 6). The primary features as-sociated with corrosion-fatigue crackingare summarized in Fig. 1 (Ref. 6). Figure1A is a photograph of a weld cladding withextensive corrosion-fatigue cracks thatwere observed after approximately 18months of service (Ref. 6). Figure 1Bshows a scanning electron photomicro-graph of several small cracks that were ex-amined early in the cracking stage, andFig. 1C shows the distribution of alloyingelements across the dendritic substructureof the overlay. Figure 1D provides a lower-magnification view that demonstrates thecracks initiate at the valley of the weld rip-ple. The dendrite cores in the cladding ex-hibit a minimum in alloy concentrationdue to the relatively rapid solidificationconditions associated with welding (Ref.7). As a result, the corrosion rate is accel-erated in these regions and localized at-tack occurs at the dendrite cores. These lo-calized penetrations form stressconcentrations that eventually grow intofull-size corrosion-fatigue cracks underthe influence of service-applied stresses.As shown in Fig. 1D, most cracks initiatein the valley of surface weld ripples wherean additional stress concentration exists.The high residual stress that results fromwelding also probably contributes to thecracking problem. In addition, dilutionfrom the underlying tube substrate, whichresults in reduced alloy content of thecladding, compromises the corrosion re-sistance of the cladding.

It is important to note that the primaryfactors that contribute to corrosion-fatigue cracking (weld ripple, microsegre-gation, high residual stresses, and dilu-tion) are all associated with the localized

High-Temperature Corrosion Behavior ofAlloy 600 and 622 Weld Claddings

and Coextruded Coatings

Thermogravimetric and solid-state corrosion testing techniques were used to evaluate the corrosion behavior of nickel-based alloys

BY J. N. DUPONT, A. W. STOCKDALE, A. CAIZZA, AND A. ESPOSITO

ABSTRACT

Weld claddings are often used for corrosion protection for waterwalls in coal-firedpower plants. Although these coatings provide good resistance to general corrosion,recent industry experience has shown they are susceptible to premature failure due tocorrosion-fatigue cracking. The failure has been attributed, in part, to microsegrega-tion and dilution of the weld cladding that compromise the corrosion resistance. Co-extruded coatings may provide improved resistance to this type of failure due to elim-ination of microsegregation and dilution. In this work, the high-temperature gaseousand solid-state corrosion behavior of Alloys 600 and 622 weld claddings, and coex-truded coatings were evaluated using thermogravimetric and solid-state corrosiontesting techniques. The results demonstrate that Alloy 622 exhibits better corrosionresistance than Alloy 600 under the simulated combustion gases of interest, and co-extruded coatings provide corrosion resistance that is significantly better than theweld claddings. The improved corrosion resistance of Alloy 622 is attributed to thehigher Cr and Mo concentrations, while the better corrosion resistance of the coex-truded coatings is attributed to elimination of dilution and microsegregation. Addi-tional benefits of the coextruded coatings in terms of service performance are alsolikely, and include better control over coating thickness and surface finish and reducedresidual stresses.

J. N. DUPONT and A. W. STOCKDALE([email protected]) are with the Department ofMaterials Science and Engineering, Lehigh Uni-versity, Bethlehem, Pa. A. CAIZZA and A. ES-POSITO are with Plymouth Engineered Shapes,Hopkinsville, Ky.

KEYWORDS

CorrosionWeld Overlay CoatingCoextruded CoatingNi-Based AlloysDilutionMicrosegregation

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heating, melting, and solidification of thewelding process. As such, use of a coatingthat can be applied uniformly on the sub-strate surface in the solid state (i.e., with-out the need for localized heating) shouldhelp mitigate these problems and improvethe cracking resistance of the coating.Thus, there is a need to develop alterna-tive coating technologies that avoid thesedrawbacks. Coextruded coatings provide apotential alternative because they are pro-duced completely in the solid state andtherefore require no melting and resolidi-fication. In this work, the high-tempera-ture corrosion resistance of two nickel-based alloys (600 and 622) wereinvestigated in the form of both coex-truded coatings and weld claddings. Thecounterpart wrought product form wasalso tested for Alloy 600 for comparison.

Experimental Procedure

Three types of samples were corrosiontested: coextruded coating, wrought alloy(for Alloy 600 only), and weld cladding.Coextruded tubes were manufactured atPlymouth Engineered Shapes using anouter layer of either Alloy 600 or 622 and

a 1.25Cr-0.5Mo (SA213-T11) steel sub-strate. The composition of the Ni-basedalloys and the steel are provided in Table1. The steel substrate and nickel alloyouter layer were joined by an explosionwelding process prior to coextrusion. Asshown in Fig. 2, the substrate and outerlayer had a starting diameter of 6 in. andlength of about 2 ft. The bimetallic billetwas heated to 1040°C prior to coextrusion,and the coextrusion occurred in approxi-mately 5 s. Figure 3 shows an example ofthe final bimetallic tube produced aftercoextrusion that has an outside diameterof 2.5 in. with a 0.250-in. wall thicknessand a coating thickness of 0.085 in. Thefinal tube length was approximately 20 ft.Simulated weld claddings were fabricatedby mixing (by weight) 10% of an Alloy 285Grade C steel substrate (this alloy is simi-lar to those typically used for waterwalltubes) with 90% of Alloy 600 or 622. The10% steel was added to simulate a typicaldilution level of a commercial weldcladding. (It is recognized that Alloy 600 isnot available in wire form for use as a weldcladding. However, the weld claddingsamples were prepared and tested here toprovide a direct comparison to the coex-

truded coating of the same composition.)The mixture was then melted and resolid-ified in an arc button melter, which essen-tially duplicates the chemical compositionand thermal conditions used to make weldcladdings. This process has been used ex-tensively for preparing and corrosion test-ing weld cladding samples (Ref. 8).Gaseous corrosion testing was carried outat 600°C for 100 h in a Netzsch thermo-gravimetric balance. The gas used for thecorrosion tests was modeled after a typicallow-NOx environment and consisted ofthe following mixture (Ref. 8): 10%CO-5%CO2-2%H2O-0.12%H2S-N2 (vol-%).Corrosion samples were acquired fromthe coating of the bimetallic tube by com-pletely machining away the underlyingsteel substrate.

The Alloy 622 weld cladding and coex-truded samples were also tested undersolid-state corrosion conditions. (Alloy600 was not evaluated under solid-stateconditions, since the gaseous corrosion re-sults demonstrated that Alloy 622 had su-perior corrosion resistance.) Samples thatwere ¾ × ¾ × 5⁄16 in. were machined fromthe coextruded tube and the weldcladding. A quartz ring was placed on top

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Fig. 1 — A — Photograph of an IN625 weld cladding with extensive circumferential cracks; B — cross-sectional scanning electron photomicrograph of several small cracks early in the cracking stage; C — dis-tribution of alloying elements across the dendritic substructure of the IN625 weld cladding; D — photo-graph showing crack initiation at the valley of the weld ripple.

Fig. 2 — Photograph of starting bimetallic billetshowing the inner steel substrate and outer nickelalloy layer prior to coextrusion. The starting billet hada diameter of approximately 6 in. and a length ofabout 2 ft.

A B

DC

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of the samples, and 1680 mg of FeS2 pow-der was poured into the quartz ring. TheFeS2 powder simulates the iron sulfidethat is often deposited on waterwall sur-faces in the form of coal particles that arenot completely combusted. The iron sul-fide will oxidize at high temperature andsubsequently release sulfur gas that cor-rodes the underlying coating (Refs. 9, 10).The samples were placed in a furnace andheated to 600°C for 50, 150, and 300 h(separate samples were used for each ex-posure time). The samples were then ex-amined in cross section to reveal the depthof attack and corrosion morphology aftereach exposure time. This test has beenshown (Ref. 8) to simulate the solid-statecorrosion that occurs when deposits formon the waterwall tubes in service. Corro-sion test coupons from the gaseous andsolid-state tests were mounted under vac-uum in cold setting epoxy and groundthrough 600 grit with a SiC abrasive. Thesamples were then polished to a 0.05-μmsurface finish. Post-test imaging of corro-sion scales was conducted via light opticalmicroscopy and scanning electron mi-croscopy on a Hitachi 4300 scanning elec-tron microscope (SEM) equipped with anenergy-dispersive spectrometer.

Results

Figure 4 shows photographs that com-pare the coating surface finish and thicknessuniformity of the coextruded coating (Fig.4A, B) and a weld cladding typically used forthis application (Fig. 4C, D). The weldcladding exhibits the typical surface ripplesassociated with solidification and a rela-tively uneven coating thickness. The coex-trusion process provides a relatively smoothsurface finish and more uniform coatingthickness. Elimination of the weld ripple issignificant, since the valleys of the weld rip-ple present stress concentrations that exac-erbate corrosion-fatigue crack initiation(Ref. 6). The more uniform coating thick-ness and improved surface finish associated

with the coextruded coating eliminates thisform of stress concentration and shouldtherefore be more resistant to initiation ofcorrosion-fatigue cracks.

Figure 5 shows the thermogravimetricanalysis results from the gaseous corro-sion testing. These results compare thenormalized weight gain of the weldcladding and coextruded coating. Corro-sion results from the alloy in the wroughtcondition are also shown for Alloy 600 forcomparison. Good corrosion results areindicated by relatively low weight gains,and the slopes of the lines are an indica-tion of the corrosion rates. The coex-truded coating clearly shows improvedcorrosion resistance over the weldcladding, and the corrosion resistance ofthe wrought alloy and coextruded coatingfor Alloy 600 is comparable. Also notethat Alloy 622 demonstrates better corro-sion resistance (i.e., lower weight gains)than Alloy 600.

Figure 6 shows SEM cross-sectionalphotomicrographs of the corrosioncoupons from the gaseous corrosion tests.These samples reveal an outer scale in ad-dition to an inner corrosion scale thatformed adjacent to the coating surface dur-ing corrosion testing. It is important to notethat, due to the large differences in corro-sion scale thickness, the photomicrographs

acquired from the coextruded coating (Fig.6A, B) are generally taken at a higher mag-nification than those of the weld cladding(Fig. 6C, D). The inner corrosion scale thatformed on the coextruded sample is signifi-

JULY 2013, VOL. 92

Fig. 4 — Comparison of coating surface finish and thickness uniformity of the following: A, B — Coex-truded coating; C, D — a weld cladding typically used for this application.

A B

C D

Fig. 3 — A section of the final bimetallic tube pro-duced after coextrusion. The tube has an outside di-ameter of 2.5 in. with a 0.250-in. wall thickness anda coating thickness of 0.085 in. The final tubelength was approximately 20 ft.

Table 1 — Composition of the Ni-Based Alloysand Steel Used to Make the Coextruded Tubes(all values are given in wt-%)

622 600 T11C 0.002 0.06 0.12

Co 0.81 0.06 —Cr 21.3 16 1.22Fe 3.7 7.47 —Mn 0.25 0.36 0.52Mo 13.1 — 0.52Ni Bal Bal 0.02P 0.012 — 0.009S 0.002 0 0.026Si 0.03 0.34 0.62V 0.02 0.04 0.006W 2.8 — —Nb — 0.01 —Ta — 0.01 —Ti — 0.22 —Al — 0.2 0.029Cu — 0.03 0.02Cs 0.002N 0.005Sn 0.002


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cantly thinner than the inner scale thatformed on the weld cladding. Figure 6A,which was acquired from the coextrudedsample, was taken at the same magnifica-tion as Fig. 6D acquired from the weldcladding, and the differences in scale thick-ness are readily apparent when these twoimages are compared. Also note that corro-sion has occurred more uniformly on the co-

extruded coating compared to the weldcladding. The differences in scale thicknessare consistent with the differences in theweight gain results shown in Fig. 5, wherethe weld cladding exhibited a higher weightgain (due to the higher corrosion rate andconcomitantly larger scale thickness). Athinner inner corrosion scale is preferred, asthis indicates that the scale provides better

protection between the corrosion environ-ment and underlying coating surface.

Figure 7 shows EDS spectra collectedfrom the corrosion scales that formed onthe gaseous corrosion samples. The loca-tions that the EDS scans were acquiredfrom are shown as white boxes in Fig. 6B,D. (In each case, EDS spectra acquiredfrom the area between the inner and outer

Fig. 6 — SEM cross-sectional photomicrographs of the corrosion coupons from the gaseous corrosion tests of Alloy 600 for the coextruded coating (Fig. 6A, B)and weld cladding (Fig. 6C, D).

A B

C D

Fig. 5 — Thermogravimetric results from the gaseous corrosion testing. A — Alloy 600; B — Alloy 622.

A B


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scales were observed to reveal the pres-ence of carbon and oxygen, indicating thatit is merely the mounting material used toprepare the samples. This occurs when theinner and outer scales separate duringpreparation.) For each coating type, theouter scales are rich in nickel and sulfur.The inner scales of each sample are alsosimilar and reveal the presence of achromium-rich mixed oxygen-sulfur scale.

Figure 8 shows the extent of corrosionthat occurred during the solid-state corro-sion testing for the Alloy 622 weldcladding and coextruded coatings. A sig-nificant amount of corrosion scale can beobserved on the surface of each sample.The amount of scale on the surface is in-dicative of the severity of the corrosive at-tack. The corrosion resistance of the weldcladding and coextruded coatings aresomewhat similar up to 50 h of exposure.However, at 150 and 300 h, the depth ofattack is greater on the weld cladding.Also note that the weld cladding exhibitslocalized corrosion penetrations (arrowsin Fig. 8F) while corrosion on the coextruded coating is uniform.

Figure 9 shows the 300-h corrosionsample of the weld cladding after it wasetched to reveal the dendritic substruc-ture. Note that preferential corrosion hasoccurred at the dendrite cores (arrows).Figure 10 provides an EDS line scan thatwas acquired across the dendritic sub-structure of the weld cladding. As ex-pected (Refs. 6, 7), the dendrite cores are

depleted in Mo, with Mo concentrationlevels down to ~ 11 wt-% (the nominalMo concentration of the filler metal is ~13 wt-%). Note that the Ni segregates inthe opposite direction compared to Mo(i.e., to the dendrite cores). The particu-larly high Mo level of ~ 38 wt-% (at a po-sition of ~ 3 μm) is coincident with Ni de-pletion down to ~ 28 wt-% and isassociated with the electron beam inter-acting with Mo-rich interdendritic phase.

Figure 11 shows the microstructure ofthe coextruded coating, and an EDS linescan acquired across several grains of thecoating is shown in Fig. 12. The coex-truded coating exhibits a uniform,equiaxed grain structure and a uniformdistribution of alloying elements.

Discussion

The corrosion results demonstrate theeffect of the coating process on the result-ant corrosion resistance. For each alloy,the coextruded coatings provide signifi-cantly better corrosion resistance than theweld claddings. Since the alloy is the samein each case, the reduced corrosion resist-ance of the weld cladding must be attrib-uted to differences in processing that af-fect the microstructure. This is confirmedby the results shown for Alloy 600 in whicha wrought alloy was also tested for com-parison — Fig. 5A. Note that the wroughtalloy and coextruded coating exhibit es-sentially identical corrosion rates. This in-

dicates the coextrusion process has nodetrimental effect on the inherent corro-sion resistance of the wrought alloy. Thisis consistent with the observed mi-crostructure (Fig. 11) and distribution ofalloying elements (Fig. 12) observed forthe coextruded coating. The equiaxedgrain structure and uniform distributionof alloying elements is similar to that ob-served for a wrought alloy, so the corro-sion resistance is also expected to be sim-ilar, as observed in Fig. 5A.

The differences in corrosion resistanceamong the two alloys and coating typesevaluated here can be understood by con-sidering differences in their composition.It is well known that Cr and Mo additionssignificantly improve the sulfidation re-sistance of Ni-based alloys (Refs. 11, 12).For example, Chen and Douglass (Refs.11, 13) evaluated the effect of Mo addi-tions on the sulfidation resistance of Ni-Mo alloys at 600°C at a sulfur partial pres-sure of 0.01 atm PS2. Five alloys wereevaluated, including Ni, Ni-10wt-%Mo,Ni-20wt-%Mo, Ni-30wt-%Mo, and Ni-40wt-%Mo. The parabolic rate constantdecreased by four orders of magnitude asthe Mo content was increased to 40 wt-%Mo. Similar reductions in the parabolicrate constant were also observed for Ni-Cralloys tested by Mrowec et al. (Refs. 12,14). The sulfidation behavior of Ni with upto 82 at-% Cr was evaluated in a sulfurpartial pressure of 1 atm PS2 at 600°C. Theparabolic rate constant decreased by threeorders of magnitude as the chromium con-tent was increased up to 82 at.-% Cr.These results demonstrate that the corro-sion resistance of Ni-based materials is im-proved by alloying additions of Cr and Mo.Thus, the improved corrosion resistanceof Alloy 622 over Alloy 600 is attributed tothe higher Cr and Mo concentration ofAlloy 622.

The improved corrosion resistance ofthe coextruded coatings can be attributedto two factors. First, the weld cladding ex-hibits a 10% reduction in key alloying ele-ments (e.g., Cr and Mo) due to 10% dilu-tion with the steel substrate, and areduction in the concentration of these el-ements will produce an increase in the cor-rosion rate. The 10% dilution value usedfor these tests represents a lower limit onthe dilution level for commercially appliedweld claddings. The dilution level in field-applied weld claddings can often be higherthan this, and the corrosion resistance canbe reduced even further as a result. Suchdilution effects do not occur with the co-extruded coating because there is no melt-ing and mixing associated with thisprocess. Although there is localized solid-state diffusion across the coating/sub-strate interface during processing, there isno bulk change in coating composition.Second, the weld cladding exhibits mi-

Fig. 7 — EDS spectra acquired from gaseous corrosion samples of Alloy 600. A — Top surface scale ofthe coextruded coating; B — inner surface scale of coextruded coating; C — top surface scale of the weldcladding; D — inner surface scale of weld cladding.

A

C D

B


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crosegregation in which the dendrite coresare depleted in alloying elements (partic-ularly of Mo) that are important for cor-rosion protection (Ref. 15). As a result,corrosion occurs more rapidly at the alloy-depleted cores, thus leading to the prefer-ential corrosive attack at the dendritecores observed in Figs. 6C, 6D, and 9.

It is worth noting that the coextruded(and wrought) Alloy 600 provides corro-

sion resistance that is nearly comparableto the Alloy 622 weld cladding, suggestingthat Alloy 600 may be useful as a coex-truded coating. However, the objectivehere is to develop a coating/process com-bination that provides performance betterthan the current industry standard (622weld cladding). Thus, the use of Alloy 600as a coextruded coating does not appearwarranted based on this consideration.

These results indicate that coextrudedcoatings should provide significant benefitsover weld claddings for corrosion protec-tion in fossil-fired boilers. (It should be rec-ognized that weld claddings can be appliedin the field or the shop, while coextrudedcoatings can only be applied in the shop.)Reduction or elimination of failures due tocorrosion-fatigue cracking will require thedevelopment of coatings with improved re-sistance to both general corrosion and lo-calized corrosion that occurs due to mi-crosegregation. Other factors that promotecorrosion-fatigue crack initiation should

also be avoided, such as surface irregulari-ties and high residual stresses. Coextrudedcoatings provide several advantages overthe weld claddings in these regards. First,the coextruded coatings will not exhibit di-lution and microsegregation that compro-mise corrosion resistance. The weldcladdings also exhibit surface ripples asso-ciated with the solidification process, andthe valleys of these ripples are sources ofstress concentration that can contribute tocorrosion-fatigue cracking (Ref. 6). In con-trast, the coextruded coatings have a uni-form coating thickness and smooth surfacefinish that should help eliminate localizedstress concentrations that initiate corro-sion-fatigue cracks. Weld claddings also de-velop very high levels of residual stress thatare associated with localized heating andcooling. The residual stress level is gener-ally on the order of the yield strength of thealloy (Ref. 16), and this may also be a con-tributing factor to the corrosion-fatigueproblem. In contrast, the heating and cool-

WELDING JOURNAL

Fig. 8 — Light optical photomicrographs showing the extent of corrosion that occurred during solid-state corrosion testing for the following: A, B, C — Alloy622 coextruded; D, E, F — weld cladding coatings.

Fig. 9 — Light optical photomicrographs of the300-h corrosion sample of the weld claddingafter it was etched to reveal the dendritic sub-structure. Note that preferential corrosion hasoccurred at the dendrite cores (arrows).

Fig. 10 — A — EDS line scan acquired across the dendritic substructure of the weld cladding showing thecomposition profiles for Fe, Ni, and Cr; B — EDS line scan acquired across the dendritic substructure ofthe weld cladding showing Mo depletion at the dendrite cores.

A

D E F

B C

A B


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ing cycles experienced during coextrusionare less severe and more uniform. As a re-sult, the residual stresses should be signifi-cantly reduced. Corrosion-fatigue testingand field tests are currently in progress toverify this expected level of improvementand will be reported in the future.

Summary

The high-temperature corrosion resist-ance of Alloys 600 and 622 weld claddingsand coextruded coatings was evaluated inthis work. A wrought sample of Alloy 600was also corrosion tested for comparison.The results demonstrate: 1) Alloy 622 ex-hibits better corrosion resistance thanAlloy 600; and 2) coextruded coatings pro-vide corrosion resistance that is signifi-cantly better than the weld claddings. Theimproved corrosion resistance of Alloy622 is attributed to the higher Cr and Moconcentrations. The improved corrosionresistance of the coextruded coatings rel-ative to the weld cladding is attributed toelimination of dilution and microsegrega-tion in the coextruded coating. Additionalbenefits of the coextruded coating interms of service performance are alsolikely, and include better control over

coating thickness/surface finish and re-duced residual stresses.

Acknowledgments

The authors gratefully acknowledge fi-nancial support through the National Sci-ence Foundation Center for Integrated Ma-terials Joining Science for EnergyApplications, Grant IIP-1034703, and PPLCorp., Contract 00474836. Useful technicaldiscussions with Ruben Choug and RobertSchneider of PPL Corp. are also gratefullyappreciated.

References

1. Jones, C. 1997. Power January/February,pp. 54–60.

2. Whitaker, R. 1982. EPRI Journal, pp.18–25.

3. Urich, J. A., and Kramer, E. 1996. FACT(American Society of Mechanical Engineers),Vol. 21, pp. 25–29.

4. Kung, S. C., and Bakker, W. T. 1997.Mater. High Temp. 14: 175–182.

5. Smith, G. D., and Tassen, C. S. 1989.Mater. Perf. 28: 41–43.

6. Luer, K., DuPont, J. N., Marder, A. R., andSkelonis, C. 2001. Mater. High Temp. 18: 11–19.

7. DuPont, J., Lippold, J., and Kiser, S. 2009.Welding Metallurgy and Weldability of Nickel-

base Alloys. p. 440, Hoboken, N.J.: John Wiley& Sons.

8. Regina, J. R., DuPont, J. N., and Marder,A. R. 2004. Corrosion behavior of Fe-Al-Cr al-loys in sulfur- and oxygen-rich environments inthe presence of pyrite. Corrosion. pp. 501–509.

9. Bakker, W. 1998. Waterwall Wastage inLow Nox Boilers: Root Cause and Remedies. TR-111155.

10. Kung, S., and Bakker, W. 2000. Water-wall corrosion in coal-fired boilers a new cul-prit: FeS. Corrosion 2000. pp. 26–31. NACE International.

11. Chen, M. F., and Douglass, D. L. 1989.The effect of molybdenum on the high-temper-ature sulfidation of nickel. Oxid. Met. 32:185–206.

12. Mrowec, S., Werber, T., and Zastawnik,M. 1966. The mechanism of high temperaturesulfur corrosion of nickel-chromium alloys.Corros. Sci. 6: 47–68.

13. Gleeson, B., Douglass, D. L., and Ges-mundo, F. Effect of niobium on the high-tem-perature sulfidation behavior of cobalt. Oxid.Met. 31: 209–236.

14. Czerski, L., Mrowec, S., and Werber, T.1962. Kinetics and mechanism of nickel-sulfurreaction. Journal of the Electrochemical Society109: 273–278.

15. Deacon, R. M., DuPont, J. N., andMarder, A. R. 2007. Materials Science & Engi-neering A Vol. 460–461, pp. 392–402.

16. Kou, S. 2002. Welding Metallurgy. p. 461,Hoboken, N.J.: John Wiley & Sons.

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Fig. 11 — Light optical photomicrograph showing the microstructure ofthe coextruded coating.

12 — EDS line scan acquired across several grains of the coating.

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