Electrochemical Studies on High-Temperature Corrosion of Silicon-Iron Coatings and Iron Aluminide...

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489CORROSION–Vol. 57, No. 6

Submitted for publication December 1999; in revised form,November 2000.

* Instituto Mexicano del Petróleo, Programa de Investigacion yDesarrollo de Ductos, Eje Central Lázaro Cárdenas No. 152, Col.San Bartolo Atepehuacan, C.P. 07730, México, D.F., México.

** Instituto Tecnologica de Zacatepec, Caleada Instituto TecnologicoNo. 27, 62780, Zacatepec, Morelos, Mexico.

Electrochemical Studies on High-TemperatureCorrosion of Silicon-Iron Coatings and IronAluminide Intermetallic Alloys by Molten Salts

M. Amaya,* J. Porcayo-Calderon,** and L. Martinez*

ABSTRACT

The performance of Fe-Si coatings and an iron aluminide(FeAl) intermetallic alloy (FeAl40at%+0.1at%B+10vol%Al2O3)in molten salts containing vanadium pentoxide (V2O5) andsodium sulfate (Na2SO4) is reported. Corrosion and fouling byash deposits containing V2O5 and Na2SO4 are typical corro-sion problems in fuel oil-fired electric power units. High-temperature corrosion tests were performed using bothelectrochemical polarization and immersion techniques. Thetemperature interval of this study was 600°C to 900°C, andthe molten salts were 80wt%V2O5-20wt%Na2SO4. Curves ofcorrosion current density vs temperature obtained by thepotentiodynamic studies are reported, as well as the weightloss vs temperature curves from molten salt immersion tests.Both Fe-Si coatings and FeAl40at%+0.1at%B+10vol%Al2O3

showed good behavior against molten salt corrosion. Thefinal results show the potential of these coatings and alloysto solve the high-temperature corrosion in fuel oil-fired elec-tric power units.

KEY WORDS: high-temperature corrosion, iron aluminide,molten salts, silicon-iron coatings, vanadates

INTRODUCTION

High-temperature corrosion and fouling induced bymolten salt deposits during combustion of fossil fuelsis a serious problem in the power generation indus-

try. Aggressive impurities during the combustion offossil fuels include V and S. During fuel oil combus-tion, the V is transformed into vanadium pentoxide(V2O5), and the S reacts with Na (a typical impurityof the oil) to form sodium sulfate (Na2SO4). Thesecompounds form a binary system that undergoesan eutectic reaction at relatively low temperatures(< 600°C), which causes melting and the forming of astable layer of electrolyte on the metallic surfaces.1-2

It has been shown that Fe-Si alloys have excel-lent corrosion and oxidation resistance;3-7 however,little attention has been paid to this system as aprotective coating.8-11 The favorable effect of Si in theFe-Si system is based on the absolute resistance ofsilicon dioxide (SiO2) to attack by molten V2O5.12 Also,SiO2 has a stronger acid behavior than V2O5, and it ismore stable in molten vanadates. The Fe-Si systembehavior contrasts with the performance of Cr-richalloys, which are corroded by the formation of Crvanadates and/or Cr-Fe vanadates of low meltingpoint. On the other hand, Elliott and Taylor observedthat V compounds have low adherence to the Si-based coatings.13-14 According to Shi and Rapp, SiO2

does not react with the V compounds (which must bedissolved in V molten salts by a physical dissolutionprocess without chemical reaction) in the case of thesolubility of SiO2 in fused Na2SO4.15

The iron aluminide (FeAl) alloys have drawn in-terest because they have shown excellent behavior inhigh-temperature oxidizing environments.16-19 Someworks report the behavior of FeAl alloys in moltensalts and condensed environments;20-22 however, littlehas been published about high-temperature corro-

0010-9312/01/000095/$5.00+$0.50/0© 2001, NACE International

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(1) UNS numbers are listed in Metals and Alloys in the UnifiedNumbering System, published by the Society of AutomotiveEngineers (SAE) and cosponsored by ASTM.

sion in molten salt mixtures of V2O5-Na2SO4.Gesmundo, et al., studied the corrosion behavior ofFeAl alloys (Fe3Al [27at%Al-2.2at%Cr-0.1at%B] andFeAl [40at%Al-0.05at%Zr-0.06at%B-0.085at%C)by coating them with a molten salt of Na2SO4 and15 wt% V2O5 and exposing them to N2, 1 vol% O2, and0.5 vol% sulfur dioxide (SO2)-simulated combustiongases at 600°C. They found that, after 96 h, theFeAl alloy showed better corrosion resistance (massgain of 8 mg/cm2) than the Fe3Al alloy (mass gainof 14 mg/cm2).23

In the present study, the corrosion resistance ofFe-Si coatings on the substrate of Type 304 stainlesssteel (SS) (UNS S30400)(1) and an FeAl alloy(FeAl40at%+0.1at%B+10vol%Al2O3) was evaluatedduring exposure to a synthetic molten salt mixture of

80wt%V2O5-20wt%Na2SO4 in the temperature intervalof 600°C to 900°C.

EXPERIMENTAL PROCEDURES

MaterialsThe substrates for the Fe-Si coatings were cylin-

drical specimens of 25.4 mm by 3.17 mm in the caseof electrochemical tests and rectangular parallelepi-peds of 12 by 8 by 3 mm for the immersion tests ofType 304 SS. All specimens were precoated with a200-mm thick coating of Ni20Cr to give a better ad-herence to the final Fe-Si coating. The Fe-Si coatingswere applied using a powder flame spraying gun bya mixture of O2 and acetylene (C2H2) as the heatsource. The final thickness of the coatings was~ 400 mm. On the other hand, FeAl40at%+0.1at%B+10vol%Al2O3 cubic specimens of 5 by 5 by 5 mm wereused for electrochemical tests, and rectangular paral-lelepiped specimens of 10 by 5 by 5 mm for immer-

FIGURE 1. Potentiodynamic polar ization curves of theFeAl40at%+0.1at%B+10vol%Al2O3 intermetallic alloy.

FIGURE 2. Potentiodynamic polarization curves of the coating ofFe17Si.

FIGURE 3. Potentiodynamic polarization curves of the coating ofFe35Si.

FIGURE 4. Potentiodynamic polarization curves of the coating ofFe75Si.

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sion tests. The FeAl alloy coupons were fabricated byspray-atomization and deposition. The synthesis andexperimental variables are described elsewhere.24 Ithas been shown that the technical structural appli-cation of FeAl alloys is restricted by their poor ductil-ity at room temperature and low fracture toughness.The B addition25-26 and reinforcement with discon-tinuous ceramic particulate27-29 have been proposedto improve the mechanical behavior of FeAl alloys. Inthe present study, Al2O3 particulate and B wereadded to improve the ductility and fracture tough-ness of the FeAl alloy only.

Finally, a synthetic mixture of 80wt%V2O5+20wt%Na2SO4 was used as the melting electrolyteand corrosion media.

Corrosion TestsThree-electrode cells were constructed and used

for the electrochemical tests. The specimens wereused as the working electrode and two Pt wires of0.5-mm diameter were used as the counter electrodeand reference electrode. All electrochemical experi-ments were performed in a vertical furnace in staticair at 600, 700, 800, and 900°C. According to a pre-vious study, at the last condition of partial pressureof O2 and temperatures, the melt basicities were20.6, 19.4, 17.5, and 15.9, respectively.30 The inter-pretation of these conditions corresponds to regionsof high melt basicity, where the corrosivity of theformed species is higher. A potentiostat operated by apersonal computer was used to obtain potentiody-namic polarization curves at a scan rate of 1 mV/s.Immersion corrosion tests also were performed toevaluate the Fe-Si coatings and the FeAl-based alloy.The amount of synthetic mixture of 80%V2O5+20%Na2SO4 was calculated to provide 500 mg/cm2 ofmetal surface exposed to corrosion. The specimens ofFe-Si coatings and the intermetallic alloy FeAl40at%+0.1at%B+10vol%Al2O3 were immersed in the corro-

FIGURE 5. Microphotographs of: (a) Fe17Si, (b) Fe35Si, and (c) Fe75Si coatings after electrochemical studies in80%V2O5+20%Na2SO4 at 600°C.

(a) (b) (c)

FIGURE 6. Temperature effect on Icorr in the alloys studied.

FIGURE 7. Penetration depth of V compounds into the coatings atdifferent temperatures.

sive agent in porcelain crucibles and heated in staticair for 500 h and 200 h, respectively. The test tem-peratures were also 600, 700, 800, and 900°C.

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the polarization curves at an interval of 300 mVbelow and above the corrosion potential. In Figure 6,Icorr is plotted as a function of the temperature, in-cluding data for Type 304 SS and the Ni20Cr coatingfor comparison. Icorr increased with temperature inboth coated and uncoated Type 304 SS, while Icorr inthe FeAl40at%+0.1at%B+10vol%Al2O3 alloy decreasedas the temperature increased. However, the Icorr

values in the FeAl40at%+0.1at%B+10vol%Al2O3 alloywere above the values shown for Fe-Si coatings. Inthe case of the coatings of stainless steel studiedhere, in general terms, it can be said that the worstcorrosion resistance was exhibited by the Ni20Crcoating, and that the Fe-Si coatings exhibited thebest corrosion resistance under the conditions ofthese experiments. The coating of higher Si contentdeveloped better corrosion resistance. The finalresults suggest that the Fe-Si coatings could beused to protect the Type 304 SS components usedin boilers of electric power units exposed to moltensalts.

Figure 7 shows the depth of penetration duringexposure to the molten salt compounds. The penetra-tion depth of V compounds in the Ni20Cr was higherat 800°C, and in this case the specimens were com-pletely corroded. In the case of the Fe-Si coatings,

RESULTS AND DISCUSSION

Figures 1 through 4 show the potentiodynamicpolarization curves of the FeAl40at%+0.1at%B+10vol%Al2O3 intermetallic alloy and the Fe-Si coat-ings. All cathodic regions of the polarization curvesexhibit similar behavior, mainly because the cathodicregion is related to transformation in the moltensalts, which were identical in both tests. On the otherhand, the anodic regions show some differencesamong the different samples. In Figure 1, theFeAl40at%+0.1at%B+10vol%Al2O3 intermetallic alloyshows passivation at higher temperatures. But, inthe other systems, the Fe17Si, Fe35Si, and Fe75Sicoatings shown in Figures 2 through 4 exhibit passi-vation at the lower temperatures. The higher Si con-tent seems to have enhanced the passive region.Transversal sections of Fe17Si, Fe35Si, and Fe75Sicoatings obtained by scanning electron microscopy(SEM) after electrochemical tests at 600°C are pre-sented in Figure 5. These observations confirm thatat this temperature the Fe-Si coatings prevent corro-sive attack. It can be seen that the V compounds donot penetrate into the coatings.

The corrosion current densities (Icorr) were calcu-lated by using Tafel extrapolation from the data of

FIGURE 8. Corrosion morphology and x-ray mapping obtained after corrosion tests of Fe17Si at 700°C.

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the maximum penetration depth occurred at 700°C,and then at 800°C it dropped dramatically. Figures 8and 9 show the corrosion morphology and x-raymapping obtained in transversal sections after thecorrosion tests of Fe17Si at 700°C and 800°C, re-spectively. The x-ray maps show that V detected inthe coating at 700°C was higher than at 800°C. Thisbehavior can be attributed to the different formationkinetics of protective SiO2 (SiO2 was detected by aquantitative analysis of the Si detected in the x-raymaps), which obstructed the flux of corrosive V com-pounds into the coatings. A similar mechanism oc-curred in the Fe35Si coating. The Fe17Si, Fe35Si,and Ni20Cr coatings showed severe corrosion at tem-peratures > 800°C. However, coatings made of Fe75Sishowed relatively slower corrosion in the moltensalts, at all temperature intervals, than other coat-ings. Figure 10 shows the aspect of a protective layerof SiO2 compound that prevents the corrosion attackin the Fe75Si coating.

On the other hand, the effect of temperature onweight loss in the FeAl40at%+0.1at%B+10vol%Al2O3

intermetallic alloy is presented in Figure 11. The in-termetallic alloy suffered a maximum corrosion rateat 700°C, similar to Fe17Si and Fe35Si coatings. Aprevious work reported the results of a heat treat-ment study conducted at a range of 600°C to 1,000°Con a synthetic mixture of 80%V2O5+20%Na2SO4 in astatic air atmosphere after 5 h.31 Compounds of thestoichiometry of NV6- and NV3-type (the compoundsthat represent these better are Na2OV2O45V2O5 and5Na2OV2O411V2O5, respectively, where N = Na2O andV = V2O5) were identified by x-ray analysis. The studyrevealed that the signal intensity of NV3 compoundsincreases from 700°C while the NV6 signal diminishesfrom this temperature. The corrosivity of differentkinds of vanadates is known to be a function of O2

absorption capacity.1,32 This suggests that, at thistemperature, the excessively high weight loss of theFeAl alloy is likely related to high O2 solubility andtherefore high corrosivity of the melt at this tempera-ture. In the case of the Fe-Si coatings, this mecha-nism could control the high corrosion rate observedat 700°C.

FIGURE 9. Corrosion morphology and x-ray mapping obtained after corrosion tests of Fe17Si at 800°C.

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Figure 12 shows the corrosion morphology andthe x-ray mapping obtained in transversal sectionsafter the corrosion tests from 600°C to 900°C in theFeAl alloy. After high-temperature corrosion tests, itwas confirmed that the salts were still present andnot totally consumed. The Al detected by x-ray map-

ping revealed traces of Al2O3 oxides after a quantita-tive analysis. Also, Figure 12 shows that at 700°C theV penetrated and dissolved the Al2O3 protective layercorroding the metal surface, while at 800°C and900°C the Al2O3 layer seems to have obstructed the Vflux into the metal surface. Above 700°C, the de-creased corrosion rate can be related to the increasein the formation kinetics of protective layers or to theformation of the most protective type of Al2O3 scales,which prevent the corrosion attack of the vanadates.

Figure 13 compares the corrosion rate of theFeAl40at%+0.1at%B+10vol%Al2O3 alloy (200 h of ex-posure and a corrosivity index of 3.18) with the cor-rosion rates of MA956 (UNS S67956) and RA446(UNS S44600) high-temperature alloys tested in theinterval of 750°C to 900°C for 250 h in ashes fromboilers of Mexican power units with a corrosivity in-dex (V/Na+S) of 1.96.33 Actually, the MA956 andRA446 alloys have been considered for application inhigh-temperature conditions in the presence of highlycorrosive molten salts. However, considering that thehigher corrosivity index in the FeAl alloy compen-sates for the shorter exposure compared to the othertwo alloys, the FeAl alloy showed remarkable behav-ior against corrosion > 800°C in the molten salts.

FIGURE 10. Aspect of the protective layer of SiO2 that prevents the corrosion attack in the Fe75Si coating.

FIGURE 11. Temperature effect on the weight loss in theFeAl40at%+0.1at%B+10vol%Al2O3 intermetallic alloy.

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CONCLUSIONS

❖ The FeAl40at%+0.1at%B+10vol%Al2O3 intermetallicalloy showed a better corrosion resistance in moltensalts compared to MA956 and RA446 alloys, especiallyat 800°C and 900°C. The FeAl alloy may be an alter-native for use in components exposed to the environ-ments of electric power plants operated at high tem-peratures in the presence of mixtures of V2O5+Na2SO4.On the other hand, in the case of stainless steel coat-

ings made of Fe-Si alloys, the best corrosion resistancewas exhibited by the sample with high Si content. TheFe-Si coatings also may be an option to protect com-ponents of Type 304 SS used in fuel oil-fired boilers.

ACKNOWLEDGMENTS

This work was partially supported by CONACYTGrant 400363-5-3756PA. The technical support ofA. Gonzalez and O. Flores is acknowledged.

FIGURE 12. Aspect of the corrosion morphology and x-ray mapping obtained in transversal sections after the corrosiontests from 600°C to 900°C in the FeAl alloy.

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FIGURE 13. Corrosion rates of the FeAl40at%+0.1at%B+10vol%Al2O3, MA956, and RA446 high-temperature alloys.