High-Temperature Sulfidation of Fe3Al Thermal Spray ... CORROSION–FEBRUARY 2000 CORROSION ENGINEERING SECTION † Trade name. (1) UNS numbers are listed in Metals and Alloys in the

CORROSION–Vol. 56, No. 2 189

CORROSION ENGINEERING SECTION

0010-9312/00/000037/$5.00+$0.50/0© 2000, NACE International

Submitted for publication March 1999; in revised form,September 1999.

* Energy Research Center, Lehigh University, Bethlehem, PA 18015.

High-Temperature Sulfidationof Fe3Al Thermal Spray Coatings at 600°C

K.R. Luer, J.N. DuPont, and A.R. Marder*

ABSTRACT

Sulfidation behavior of Fe3Al thermal spray coatings wasstudied in Ar-3.5% H2-0.1% hydrogen sulfide (H2S) at 600°Cfor 500 h. Coatings were processed from the same lot of gasatomized Fe3Al powder using a high-velocity oxygen fuel(HVOF) process and an air plasma spray (APS) process. Ingeneral, the Fe3Al-type composition displayed excellent resis-tance to sulfidation corrosion at 600°C, which correlated withthe reported literature on wrought Fe3Al alloys. However, themethod of processing affected the corrosion response. Par-ticle degradation and porosity were two important factorsthat affected corrosion resistance. HVOF processing did notdegrade significantly the composition of the powder and pro-duced coatings with low porosity, low oxide content, highsulfidation resistance, and high resistance to sulfur penetra-tion. HVOF coatings produced from finer sized powdersexhibited slightly more corrosion damage because a greaterpercentage of the consumable was degraded. In contrast,APS processing caused significant degradation to the con-sumable and created coatings with a significant quantity ofalloy-depleted regions, high oxide content, and high porosity.As a result, sulfur attacked alloy-depleted regions within the“splats” and permeated through the porous splat boundariesto the coating-substrate interface.

KEY WORDS: air plasma spray, high-velocity oxygen fuel,iron aluminide, sulfidation, thermal spray coating

INTRODUCTION

Iron aluminide alloys are attractive candidate materi-als for high-temperature corrosion applicationsbecause of their excellent resistance to oxidationand sulfidation.1 However, their use as a structuralengineering material is limited because of brittlefracture behavior, low-tensile ductility, and limitedfabricability.2 To utilize the excellent corrosion resis-tance of these alloys, iron aluminide coatings arebeing considered for protection of high-temperaturestructural components.3 Particular focus is onfossil-fired electric utility boilers where sulfidationcorrosion has caused unacceptably high wastage oflow-alloy steel boiler components.4

A viable processing method for producing ironaluminide coatings is thermal spraying.5-6 Two com-mon commercial methods include air plasmaspraying (APS) and high-velocity oxygen fuel (HVOF)spraying. For both methods, a consumable (e.g.,powder) is introduced into a high-temperature, high-velocity gas stream where it is heated and propelledat a suitably prepared substrate. The heated andpotentially molten, oxidized, or vaporized particlesstrike the substrate whereupon they deform (i.e.,“splat”) and adhere through predominantly mechani-cal mechanisms.7 Because the splat is quenchedrapidly upon impact, the deposit may consist of anamorphous, microcrystalline, or fine-grained mixtureof thermodynamically metastable and stable phases.8

Thermal spray coatings based on the same con-sumable but produced by different processes can bedistinct because of differences in gas temperatures,

190 CORROSION–FEBRUARY 2000


† Trade name.(1) UNS numbers are listed in Metals and Alloys in the Unified

Numbering System, published by the Society of AutomotiveEngineers (SAE) and cosponsored by ASTM.

particle velocities, and particle temperatures. In gen-eral, APS coatings tend to have higher internaltensile stress, higher porosity, and lower cohesion/adhesion; whereas their HVOF-sprayed counterpartstend to have desirable compressive stress, lowerporosity, and higher cohesion/adhesion.9 The high-temperature corrosion behavior also may differbetween coatings produced by different processes.Generally, high-temperature gas corrosion of conven-tional thermal spray coatings involves permeation ofcorrosive species through the coating and corrosiveattack of the susceptible substrate material.10-11

Proper surface preparation, pore-free microstruc-tures, and coating quality assurance remainimportant factors controlling performance andservice life.12 The purpose of this research was tounderstand the sulfidation behavior of two differentthermal spray coatings derived from the samepowder whose monolithic counterpart has knownsulfidation resistance.

EXPERIMENTAL PROCEDURES

Thermal spray coatings were prepared from thesame lot of gas-atomized prealloyed Fe3Al powderwith a composition of 16.8 wt% Al, 2.25 wt% Cr,0.03 wt% B, 0.07 wt% O, and balance Fe. The com-position was consistent with the Fe3Al phase field ofthe binary Fe-Al phase diagram (Figure 1).13 Twoparticle size ranges were obtained by traditionalscreening methods: 25 µm to 45 µm and –25 µm.Substrates (30 mm diameter by 4 mm thickness)were grit-blasted with #30 alumina (Al2O3) grit beforethermal spraying. The coating was sprayed onto one

face of a substrate using an automated rasterdeposition scheme and air-cooled from behind withair jets. HVOF coatings were sprayed onto Fe-11% Crsubstrates using a TAFA JP-5000† HVOF system forboth particle size ranges. APS coating was sprayedonto carbon steel substrates using only the 25-µm to45-µm powder and a Miller Thermal† S-G 100 plasmatorch in air. Hot-rolled UNS G10080(1) steel (0.02 wt%Si, 0.04 wt% C, 58-62 hardness, Rockwell B [HRB])was used as the reference material becausesulfidation corrosion of carbon steel is well charac-terized.14

Corrosion tests were conducted following recentguidelines.15 Test samples were cut from the sprayedsubstrates using a precision diamond saw. Uncoatedsides of each sample were ground using a successionof silicon carbide (SiC) abrasive papers down to600 grit while the coated side was left undisturbed.Samples were cleaned using ethyl alcohol immersionfor 10 min followed by forced hot air drying and stor-age under desiccant. Ultrasonic cleaning was notused because of potential damage to the coating.Samples were tested in a sealed high-temperaturealumina furnace in an Ar-3.5% H2-0.1% hydrogensulfide (H2S) gas mixture for 500 h at 600°C. Theequilibrium sulfur partial pressure (pS2) was calcu-lated to be 10–9 atm.16-17 The oxygen partial pressure(pO2) measured 10–29 atm as determined by a solid-state zirconium oxide (ZrO2) sensor. Isothermalequilibrium thermochemistry diagrams were con-structed for the Fe-O-S and Al-O-S systems at 600°C.Assuming unit activity for iron (aFe = 1) and alumi-num (aAl = 1), the diagrams indicated that the testenvironment should thermodynamically favor theformation of fast-growing iron sulfide (Fe1-xS) andprotective Al2O3 scales.

Precorroded and postcorroded samples wereanalyzed using traditional metallographic character-ization techniques including light optical microscopy(LOM), quantitative computerized image analysis,scanning electron microscopy (SEM), secondary elec-tron imaging (SE), backscattered electron imaging(BE), and energy dispersive spectrometry (EDS,20 kV). Metallographic samples were vacuum-mounted in epoxy with fluorescent dye additions tohelp identify porosity in the sample. Samples wereground automatically using a succession of SiC pa-pers to 8 µm and polished on a low nap cloth using1-µm diamond paste. Apparent metallographic oxidecontent and porosity were measured in ten fields ofview using a LECO IA-3001† image analyzer. Electronprobe microanalysis (EPMA) was conducted on aJEOL† 733 SuperProbe equipped with four wave-length dispersive spectrometers (WDS) at 15 kV and20 nA probe current. K� x-ray counts were analyzedusing a Tracor Northern† 5600 x-ray analyzer andconverted to weight percentages and atomic percent-ages using a ZAF correction scheme. The minimum

FIGURE 1. Composition of the powder (—) is shown on the Fe-Alphase diagram.13



detectable limit for these elements was < 0.05 wt%for all of the elements. X-ray diffraction was per-formed using Cu-K� radiation at 50 kV and 100 nA.

RESULTS

UNS G10080 Carbon SteelThe corroded sample is shown in Figure 2. SEM,

EPMA, and XRD indicated that the scale consisted ofan outer columnar and inner porous Fe1-xS scale. Theformation of this scale was consistent with the iso-thermal thermodynamic phase stability diagram forthe Fe-O-S system at 600°C and the reportedsulfidation behavior of carbon steel in H2S-bearinggas mixtures.14

HVOF Fe3Al Coating (25-µm to 45-µm Powder)The morphology of the as-sprayed coating is

shown in Figure 3. The microstructure was typical ofHVOF Fe3Al coatings produced in combustion gasjets where particle temperatures are relatively cool(1,200°C to 1,700°C) and particle velocities are rela-tively high (500 m/s to 600 m/s).6 The coatingconsisted of partially deformed particles, fully meltedregions, oxidation products, and pores. The apparentmetallographic oxide content measured 4.1 vol% ±0.7 vol%, while the apparent metallographic porositymeasured 0.4 vol% ± 0.2 vol%. XRD and EPMA deter-mined that the primary metallic phase (light gray)was Fe3Al. The unmelted and fully melted regionsexhibited similar atomic number contrast, indicatingthat the regions had similar chemical compositions.EPMA further showed that the Al content in theunmelted regions (15.4 ± 17.3 wt%) was similar tothe melted regions (15.3 wt%). Thus, it did not ap-pear that the powder was degraded significantly bythe HVOF processing parameters. Unmelted and par-tially melted particles often were encased in a thinoxide shell, whereas fully melted regions often con-sisted of a mixture of a Fe3Al matrix with dispersedoxides. Oxidation products, caused by the spray pro-cess, were thin (< 1 µm) and not detectable by XRD.

(a)

(b)

(c)

FIGURE 2. Morphology of the UNS G10080 steel sample after500 h exposure.

FIGURE 3. Characteristics of the as-sprayed HVOF Fe3Al (25 µm to45 µm) coating: (a) surface: SE image and (b and c) cross section:BE image.



They appeared to contain Fe and Al, but their exactcomposition could not be determined reliably byEPMA because the electron probe interaction volumeexceeded their physical size.

The corroded sample is shown in Figure 4. Thecoating was relatively unattacked since many as-sprayed surface features (e.g., partially deformed

particles) were still visible on exposed surfaces. Cor-rosion resistance may be attributed to a thinprotective aluminum oxide scale that has beenshown to protect monolithic alloys of similar compo-sition.1 Small nuclei of metal sulfides were detectedin isolated areas on the surface of the coating. EDSand EPMA spot analysis of these sulfur-rich corro-

(a) (b)

(a) (b)

FIGURE 4. Small nuclei of metal sulfides were found on the surface of the coating: (a) surface: SE image and (b) crosssection: BE image.

FIGURE 5. The K� x-ray map for sulfur showed negligible sulfur penetration into the HVOF Fe3Al (25 µm to 45 µm) coating:(a) BE image: etched and (b) sulfur K� x-ray map.



sion products indicated that they contained eitherFe+S or Fe+Al+S, consistent with Fe1-xS, Al-enrichedFe1-xS, and/or FeAl2S4 corrosion products reported inthe literature.18 An attempt was made to determinethe composition of these thin sulfide particles byEPMA, but the data was unreliable because of prob-lems with charging, surface roughness, andinteraction volume effects. Similar problems havebeen reported in the literature.19 LOM examinationand EDS x-ray mapping indicated that sulfur did notgrossly permeate the splat boundaries of the coating

(Figure 5). Sulfur was not detected at the coating/substrate interface.

HVOF Fe3Al Coating (–25-µm Powder)The microstructure of the as-sprayed –25-µm

coating is shown in Figures 6(a) and (b). The appar-ent metallographic oxide content measured 6.4 vol%± 0.5 vol%, while the apparent metallographic poros-ity measured 0.2 vol% ± 0.1 vol%. Generally, thecoating was similar to the 25-µm to 45-µm coatingexcept that the finer powder size produced a finer

(a) (b)

(c) (d)FIGURE 6. Characteristics of the as-sprayed HVOF Fe3Al (–25 µm) coating: (a) cross section: BE image, etched; (b) crosssection: BE image, etched; (c) BE image: alloy-depleted regions; and (d) EDS spectra from alloy-depleted region.



overall coating structure; a slightly lower porosity; agreater degree of particle melting; a slightly higheroxide content; and a few small, isolated, alloy-depleted regions (Figure 6[c]). XRD and EPMA deter-mined that the primary phase was Fe3Al. Unmeltedparticles contained 14.7 wt% to 15.5 wt% Al,whereas fully melted regions contained 14.7 wt% to

15.2 wt%, indicating that the majority of the powderdid not degrade significantly during HVOF spraying.In the few alloy-depleted regions revealed by atomicnumber contrast, EDS analysis indicated that onlyFe and Cr were present (Figure 6[d]). Oxidation prod-ucts, caused by the spray process, were thin (< 1 µm)and not detectable by XRD. Again, they appeared to

(a) (b)

(a) (b)

FIGURE 7. Metal sulfides formed across the entire surface of the coating and began to penetrate 20 µm to 30 µm into thesplat boundaries: (a) surface: SE image and (b) cross section: BE image.

FIGURE 8. The K� x-ray map for sulfur revealed a semi-continuous, sulfur-bearing scale on the surface of the HVOF Fe3Al(–25 µm) coating: (a) cross section: BE image boxed region is Figure 7(b) and (b) sulfur K� x-ray map.



contain Fe and Al, but their exact composition couldnot be determined reliably by EPMA because theelectron probe interaction volume exceeded theirphysical size.

The corroded sample is shown in Figure 7. Agreater concentration of sulfur was detected on thesurface of the –25-µm coating than on the 25-µm to45-µm coating. EDS spot analysis of these sulfur-rich corrosion products indicated that they containedeither Fe+S or Fe+Al+S and that they slightly pen-etrated into the coating along splat boundaries. LOMexamination and EDS x-ray mapping indicated thatsulfur did not penetrate more than 20 µm to 30 µminto the coating (Figure 8). The composition of thesecorrosion products could not be quantified reliablyby EPMA and WDS. Sulfur was not detected at thecoating/substrate interface.

APS Fe3Al Coating (25-µm to 45-µm Powder)The morphology of the as-sprayed coating was

complex (Figure 9). The layered microstructure wastypical of APS Fe3Al coatings produced in air plasmajets where particles are hotter (2,000°C to 2,300°C)and slower (125 m/s to 175m/s) than those in anHVOF combustion jet.6 The coating deposit exhibiteda high degree of melting and particle degradation.The microstructure consisted of a few unmelted par-ticles, fully melted splats, oxidation products, andpores. The apparent metallographic oxide contentmeasured 30.8 vol% ± 2.9 vol%, while the apparentmetallographic porosity measured 2.0 vol% ±1.7 vol%. XRD and EPMA identified at least two me-tallic phases with widely varying compositions: Fe3Al(gray) and �–Fe (lightest gray). The Fe3Al phase in thesplats was Al-depleted (11.8 wt% to 15.6 wt% Al).The �–Fe phase often encased the Fe3Al phase withina splat suggesting that processing caused selectiveoxidation or vaporization of the Al from the matrix.EPMA further showed that the �-Fe phase contained< 10 wt% Al, while some areas, often encased in athick oxide shell, were depleted completely in Al andCr. Oxidation products were thick (2 µm to 10 µm)and complex. Two general types of oxides were foundthat had different atomic number contrast: Al-rich(dark gray) and Fe-rich (medium gray). Al2O3 andFeAl2O4-type oxidation products have been found inreaction plasma-sprayed FeAl coatings.20 EPMA datais summarized in Table 1.

The morphology of the corroded coating wasequally complex (Figure 10). Large nodules (100 µmto 300 µm) of metal sulfides grew out from thesurface of the coating and completely masked ordestroyed the original surface features. EPMA deter-mined that the composition of these nodules was45 at% to 46 at% Fe and 54 at% to 55 at% S, whichwas consistent with the Fe1-xS phase formed on theUNS G10080 sample. The two morphologies of thenodules (platelike and cuboid) appeared to be more

(a)

(b)

(c)FIGURE 9. Characteristics of the as-sprayed APS Fe3Al (25 µm to45 µm) coating: (a) surface of coating: SE image and (b and c) crosssection: BE, high atomic number contrast.



developed versions of the small nuclei of iron sulfidesfound on the surface of the HVOF coatings. In Figure11, EDS x-ray mapping showed that sulfur attackedlocal areas of the coating surface; completely pen-etrated the splat boundaries of the coating; andattacked the underlying substrate. Corrosion prod-ucts in the locally attacked areas (Arrow A) consistedof a mixture of Al, S, Fe, Cr, and O compounds thatcould not be quantified reliably by WDS analysis. Inadjacent areas (Arrow B), the coating was protectedby a thin Al-bearing oxide scale presumed to be Al2O3.EPMA further showed that Fe, Al, Cr, O, and S alsowere present in the splat boundaries of the coatingand at the coating-substrate interface, but the x-raydata collected in these regions could not be quanti-fied reliably because of concerns with interactionvolume effects, surface roughness, fine porosity, etc.21

DISCUSSION

As compared to carbon steel, the Fe3Al-typecoating composition displayed excellent resistance tosulfidation corrosion at 600°C. However, the methodof processing significantly affected the corrosionresponse. In binary Fe-Al alloys, it is known thatsulfidation resistance decreases with decreasing Alcontent.1 Degradation of the Fe3Al powder consum-able during APS spray processing (e.g., melting,vaporization, and/or oxidation) created numerousAl-depleted regions in the splats (Arrows B and C,Figure 9) that were susceptible to localized corrosionattack (Region A, Figure 11). In contrast, HVOF pro-cessing did not degrade significantly the compositionof the Fe3Al powder and produced coatings with low-oxide content and high resistance to local sulfidationattack. However, HVOF coatings produced from finersized powders (Figure 7) exhibited slightly more local-ized corrosion damage because a greater percentageof feedstock was degraded. Similar regions have beenfound in as-sprayed HVOF Ni-based coatings.22

The method of processing also affected porosityand permeability. HVOF-processed coatings con-tained less porosity than their APS counterparts.Consequently, the more porous APS coating pro-moted sulfur permeation along splat boundaries(Figure 11), whereas the denser HVOF coatings re-sisted sulfur permeation. Their corrosion responsesuggested that the HVOF-processed coatings wereless permeable than the APS-processed coatings. Thepores in thermal spray coatings have been consid-

TABLE 1Summary of WDS Spot Analysis

Fe Al Cr O

Bulk powder(A) Bal. 16.8 2.25 0.07Fe3Al phase(A)

(Arrow 1) Bal. 11.8 to 15.6 1.1 to 2.0 Trace�-Fe phase(A)

(Arrows 2 and 3) Bal. 0 to 5.9 0 to 1.5 TraceOxide - type 1(B)

(Arrow 4) 4 to 9 31 to 35 1 to 2 57 to 60Oxide - type 2(B)

(Arrow 5) 32 7 0 60

(A) in wt%. (B) in at%.

FIGURE 10. Morphology of the corrosion products on the APS coating: (a) surface: SE image and (b) cross section: BEimage.

(a) (b)



ered to be open and interconnected, requiring seal-ants for severe corrosion service.23 Gas permeabilitymeasurements on a variety of coatings have shownthem to be highly gas permeable with the permeabil-ity constant dependant upon thermal spray processtype, material, etc.24-26 Since interconnected porosityhas been related to aqueous corrosion behavior inAPS ceramic coatings, it reasonably could be arguedthat gas permeability is a primary factor controllinghigh-temperature gaseous corrosion behavior.27-28

Preliminary research on Ni-Cr thermal spray coatingshas shown that high-temperature corrosion behaviorcan be related to gas permeability measurements.29

(a) (b)

(c) (d)

Recent investigations of HVOF-type metallic andceramic-metal coatings in aqueous environmentsalso suggests that HVOF coatings can be imperme-able to fluids.30-31 Unfortunately, little research hassystematically correlated porosity, gas permeability,and corrosion behavior.

CONCLUSIONS

❖ The Fe3Al-type coating composition displayed ex-cellent resistance to sulfidation corrosion at 600°C.However, the method of processing significantlyaffected the corrosion behavior.

FIGURE 11. EDS x-ray mapping showed that sulfur completely permeated the splat boundaries of the APS coating.



❖ Particle degradation and open porosity were twoimportant factors that affected corrosion resistance.HVOF processing did not degrade significantly thecomposition of the consumable and producedcoatings with low porosity, low oxide content, highsulfidation resistance, and high resistance to sulfurpenetration. In contrast, APS processing caused sig-nificant degradation to the consumable and createdcoatings with a significant quantity of alloy-depletedregions, high oxide content, and high porosity. As aresult, sulfur attacked alloy-depleted regions withinthe splats and permeated through porous splatboundaries to the coating-substrate interface.

ACKNOWLEDGMENTS

The authors acknowledge Ametek SpecialtyPowders for supplying the powder and D. Sordelet atAmes National Engineering Laboratory for sprayingthe coatings.

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High-Temperature Sulfidation of Fe3Al Thermal Spray ... CORROSION–FEBRUARY 2000 CORROSION ENGINEERING SECTION † Trade name. (1) UNS numbers are listed in Metals and Alloys in the