- Email: [email protected]
High-temperature electrochemical testing of spray atomized and deposited iron aluminides alloyed with boron and reinforced with alumina particulate
Materials Science and Engineering A258 Ž1998. 306]312
High-temperature electrochemical testing of spray atomized and deposited iron aluminides alloyed with boron and reinforced with alumina particulate L. Martinez a,U , M. Amayaa , J. Porcayo-Calderonb , E.J. Lavernia c a Instituto de Fisica, UNAM, A.P. 48-3, 62251, Cuerna¨ aca, Morelos, Mexico Instituto de In¨ estigaciones Electricas, A¨ . Reforma No. 113, 62490, Temixco, Morelos, Mexico c Chemical and Biochemical Engineering and Materials Science Department, Uni¨ ersity of California, Ir¨ ine, CA 92697-2575, USA b
Abstract The corrosion behavior of FeAl40 at.%, FeAl40q 0.1 at.% B and FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 in a mixture of molten salts of 80% V2 O5 q 20% Na 2 SO4 Žwt.%. from 6008C to 9008C was studied using a potentiodynamic polarization technique. Experiments were conducted in a typical three-electrode cell immersed in the fused salt. Curves of corrosion current density Ž Icorr . as a function of molten salt temperature were obtained and discussed in terms of the passive layer and corrosion products formed during the electrochemical tests. The corrosion resistance of the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 samples was superior at all temperatures in the interval between 6008C and 9008C. It is apparent that the alumina is playing an important role in corrosion protection. The log i vs. potential polarization curves indicate that corrosion in the FeAl40 at.% and FeAl40q 0.1 at.% B samples may be associated with the formation and spalling of oxide layers rich in vanadium. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: Iron aluminides; Corrosion current density; Molten salt temperature; Electrochemical testing
1. Introduction The iron aluminide-based FeAl with ordered B2 crystal structure have good yield strength from intermediate temperatures up to 10008C, especially when they are reinforced with low expansion fibers or small inert particles such as Al 2 O 3 w1]3x. Also, these alloys have low density compared to iron and some nickelbase alloys w3]5x, and low raw materials costs w1x. Poor room-temperature ductility has been a limiting factor for processing and applications w6x. During the last few years, novel processing routes w7]9x, as well as microalloying w9]12x, microstructure control w13,14x and particulate or fiber reinforcing, have been ex-
U
Corresponding author. Tel.: q52 73 138615; fax: q52 73 153077; e-mail: [email protected]
plored in the direction of improving the ductility of iron aluminides. Several studies have reported that iron aluminides have good corrosion resistance at elevated temperatures under oxidizing and sulfidizing environments, and in molten salts w6,10,14]16x. Iron aluminides are competitive in this respect to stainless steels of the series 300 and 400, which are now used in high-temperature and high-corrosive environments w6,15x. Molten salt corrosion is typical in fossil fuel fired steam generators. Combustion products contain aggressive inorganic impurities, such as oxidizing gases containing some aggressive inorganic impurities, such as vanadium, sodium and sulfur rich salts w17,18x. Ash deposits such as sodium sulfate ŽNa 2 SO4 ., sodium metavanadate ŽNaVO3 . and vanadium pentoxide ŽV2 O5 ., form a stable electrolyte layer on materials surfaces causing severe corrosion w19]21x.
0921-5093r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093Ž98.00949-6
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
The high-temperature corrosion of materials can be measured by the total weight loss method and by electrochemical techniques w15,22]26x. Electrochemical techniques may provide a more complete picture of the corrosion process rather than just the total weight loss w27,28x. We are aware, however, that the use of electrochemical techniques does not fully comply with the premises of the standard techniques employed at room temperature, because the corrosion processes at high temperatures are complicated. In this work we compare the high-temperature corrosion of FeAl40 at.% iron aluminides alloyed with boron and reinforced with alumina particulate obtained by atomization-deposition, when immersed in a mixture of molten salts of V2 O5 and Na 2 SO4 , by means of a potentiodynamic polarization electrochemical technique.
307
is described elsewhere w8,29]32x. Heats of Gaussian shaped cones of the alloys were produced by the atomization and deposition procedure. The heats were sectioned in a spark cutting machine in the form of specimens of 5 = 5 = 5 mm. 2.2. Salts A synthetic mixture of 80% V2 O5 q 20% Na 2 SO4 Žwt.%., was used as the molten electrolyte. The composition of the mixture has been reported as most corrosive w33x. This mixture forms complex vanadates of type Na 2 OV2 O4 5V2 O5 and 5Na 2 OV2 O4 11V2 O5 with relatively low molten points Ž625 and 5358C, respectively. w34,35x. These vanadium compounds have high capacity for oxygen absorption. Other authors have reported a strong correlation between oxygen absorption capability and corrosivity w33,36x.
2. Experimental 2.3. Electrochemical cells 2.1. Materials A master alloy of FeAl40 at.% was fabricated employing iron and aluminum both 99.99% of purity. The alloy was cast in an induction furnace under a protective atmosphere of argon and was poured by gravity in a cylindrical graphite mold of 4-cm diameter. Small pieces were cut from the master alloy bar, and fed to spray atomization and deposition system. Also 99.99% purity boron in the form of Ni 2 B was used as microalloying constituent. The reinforcement phase used was commercially pure Ž99.99%. single crystal a-alumina platelets with an average diameter of 3 m m. The synthesis of FeAl alloys and experimental variables used by spray atomization and deposition
Fig. 1 shows a scheme of the three-electrode cell of the experimental setup for the present study. The working electrode ŽWE. consists of the FeAl specimen. Each FeAl specimen was drilled to insert and clinch a conductive platinum wire. The specimen was inserted in a quartz tube, where the gap between specimen and quartz tube was sealed with a ceramic cement and cured at 1108C for 12 h. The face of the working electrode to be exposed to the molten salt was ground employing normal 240, 320, 400 and 600 grits silicon carbide sand paper and ultrasonically cleaned in ethyl alcohol. Two Pt wires of 0.5-mm diameter were used as the counter electrode ŽCE. and reference electrode ŽRE.. Each one was inserted in a
Fig. 1. Schematic diagram of the electrochemical three-electrode cell.
308
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
mullite tube for electrical isolation. Prior to experiments, all of the components of the corrosion cell were cleaned with acetone and dried. 2.4. Electrochemical testing The quartz crucible with salt mixture was introduced in a vertical furnace and heated up to the working temperature. All electrochemical experiments were conducted in a AUTOTAFEL Potentiostat ACM instrument interfaced to a personal computer. In the polarization experiments the corrosion potential of the working electrode was continuously recorded starting immediately after immersion until a constant potential was obtained. Potentiodynamic polarization studies were conducted immediately after. The tests ranged between y2000 and 2000 mV with respect to corrosion potential. The corrosion current density Icorr was calculated from the intersection of the extrapolated straight line sections of cathodic and anodic Tafel lines within an interval of "300 mV with respect to potential corrosion, as indicated in Fig. 5. The fundamentals of this procedure are described elsewhere w25,40x. After the corrosion tests, the specimens were examined by scanning electron microscopy aided with energy dispersive spectroscopy ŽEDS., to analyze the corrosion surfaces and characterize the corrosion products. 3. Results and discussion 3.1. Cathodic beha¨ ior The polarization potentiodynamic curves of the al-
loys FeAl40 at.%, FeAl40q 0.1 at.% B and FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 are shown in Figs. 2]4, respectively. In the cathodic region Žbelow the corrosion potential. all three samples exhibited similar behavior, mainly because the cathodic region is basically related to transformations in the molten salts which are the same in the three systems. The curves in the cathodic region also have little influence from the test temperature, since no systematic trend could be identified. This behavior has been typically observed in other electrochemical polarization studies at high temperature in molten salt mixtures of V2 O5 q Na 2 SO4 . Some authors have speculated about the Z shape trend for the cathodic curve. The phenomena has been related to reduced ions of vanadium along the test w37]39x. The first change in trend is presented at approximately y400 to 1800 mV where the current density decrease has been related to a reduced step ions from V 5q to V 4q. The reduction in the current density is apparently related to depletion of V 5q ions. Beyond y1800 mV another reduction is triggered where V 4q begins to reduce to V 3q and the current density increases again. 3.2. Anodic beha¨ ior As shown in Figs. 2]4 all the curves above the corrosion potential exhibit a clear Tafel regime where log i increases steadily with voltage. Several authors have reported that this regime is difficult to find in polarization studies involving molten salts at high temperatures. After the Tafel regime several samples
Fig. 2. Potentiodynamic polarization curves of the FeAl40 at.%.
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
309
Fig. 3. Potentiodynamic polarization curves of the FeAl40q 0.1 at.% B.
exhibited instabilities as those marked in Figs. 2 and 3. The instabilities are due to the formation of oxide layers, which eventually spall from the metal surface. The final stage of the polarization experiment, at potential above 400 mV, the material is fully passivated as indicated by the nil increase in current density as potential increases. 3.3. Effect of temperature on the corrosion current density The calculation of the corrosion current density was performed employing the data of the polarization curves in an interval of 300 mV below and above the corrosion potential as shown in Fig. 5. In this interval the intersection of the tangent lines to the anodic and
cathodic curves is found at the corrosion potential as described elsewhere w40x. The calculations are summarized in Fig. 6. The systems FeAl40 at.% and FeAl40q 0.1 at.% B corrode faster at temperatures 6008C and 7008C and the corrosion rate reduces steadily at temperatures of 8008C and 9008C. The role of boron in the corrosion rate apparently is not important. On the other hand the role of alumina particulate seems to be important since the corrosion rate is considerably lower in the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 sample. The alumina particulate may have two different roles in reducing the corrosion rate. Clearly the alumina itself contributes to the passivation of the metal surface. The other may be
Fig. 4. Potentiodynamic polarization curves of the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 .
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
310
Fig. 5. Schematic procedure for calculate the corrosion current density Icorr , Ž"300 mV with respect to potential corrosion..
grain size, since it is considerably smaller in this sample. Figs. 7 and 8 exhibit the corroded surfaces of the FeAl40q 0.1 at.% B and FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 samples. The oxide formations on the FeAl40q 0.1 at.% B sample are very rich in vanadium and exhibit spalling. On the other hand, the microprobe analysis on the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 sample indicates the presence of a layer rich in aluminum oxide.
temperatures in the interval between 6008C and 9008C. It is apparent that the alumina is playing an important role in corrosion protection. The log i vs. potential polarization curves indicate that corrosion in the FeAl40 at.% and the FeAl40q 0.1 at.% B samples may be associated to the formation and spalling of oxide layers rich in vanadium. The temperature interval of a maximum corrosion rate is between 6008C and 7008C, at higher temperatures up to 9008C the corrosion rate diminishes.
4. Conclusions
Acknowledgements
The corrosion resistance of the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 samples was superior in all the
This work has been supported by ARO ŽDAAH0495-1-0424. and CONACYT grant 400363-5-3756PA.
Fig. 6. Temperature effect on the corrosion current density of the FeAl alloys in a molten salt of 80% V2 O5 q 20% Na 2 SO4 .
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312 w8 x w9x w10x w11x w12x w13x w14x w15x Fig. 7. Typical corrosion surface on the FeAl40q 0.1 at.% B at 6008C. w16x
w17x w18x w19x w20x w21x w22x Fig. 8. Typical corrosion surface on the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 at 6008C. w23x
The authors acknowledge the valuable technical support of Anselmo Gonzales, ´ Jose L. Albarran, Osvaldo Flores and Ivonne Espinoza. EJL also wishes to acknowledge NSF ŽINT-9600517. for financial support.
w24x
w25x
References w1x G. Webb, A. Lefort, in: W.O. Soboyejo, T.S. Srivatsan, D.L. Davidson ŽEds.., Symposium Proceedings on Fatigue and Fracture of Ordered Intermetallics Materials: I. TMS, Warrendale, PA, 1994, p. 103. w2x J.L. Smialek, J. Doychak, D.J. Gaydosh, in: T. Grobstein, J. Doychak ŽEds.., Symposium Proceedings on Oxidation of High-Temperature Intermetallics. TMS, Warrendale, PA, 1989, p. 83. w3x S.L. Draper, D.J. Gaydosh, M.V. Nathal, J. Mater. Res. 5 Ž1990. 1976. w4x I. Baker, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 101. w5x J.H. Schneibel, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 329. w6x V.K. Sikka, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 3. w7x X. Zeng, S.R. Nutt, E.J. Lavernia, Metall. Mater. Trans. A 26 Ž1995. 817.
w26x w27x w28x w29x
w30x w31x w32x w33x w34x
311
L. Martinez, O. Flores, M. Amaya, A. Duncan, S. Viswanathan, D. Lawrynowics, E.J. Lavernia, J. Mater. Synth. Processing 5 Ž1997. 65. R.G. Baligidad, U. Prakash, A. Radha Krishna, Mater. Sci. Eng. A230 Ž1997. 188. J.H. Schneibel, Mater. Sci. Eng. A153 Ž1992. 684. D.J. Gaydosh, S.L. Draper, R.D. Noebe, M.V. Nathal, Mater. Sci. Eng. A150 Ž1992. 7. E.P. George, C.L. White, J.A. Horton, Scr. Metall. Mater. 25 Ž1991. 1259. I. Baker, P. Nagpal, F. Liu, P.R. Munroe, Acta Metall. Mater. 39 Ž1991. 1637. R. Subramanian, J.H. Schneibel, K.B. Alexander, K.P. Plucknett, Scr. Mater. 35 Ž1996. 583. P.F. Tortorelli, J.H. DeVan, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 257. D. Pocci, O. Tassa, C. Testani, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 19. S. Hwang, R.A. Rapp, Corrosion Sci. 45 Ž1989. 993. R.A. Rapp, in: O. Johannesen, A.G. Anderson ŽEds.., Selected Topics in High Temperature Chemistry. Elsevier, Amsterdam, Oxford, New York, Tokyo, 1989, p. 291. E. Otero, A. Pardo, J. Hernaez, Corrosion Sci. 33 ´ F.J. Perez, ´ Ž1992. 1747. A. Pardo, E. Otero, F.J. Perez, J.F. Alvarez, M.V. Utrilla, ´ Rev. Metal. Madrid 29 Ž1993. 300. Y. Longa-Nava, Y.S. Zhang, M. Takemoto, R.A. Rapp, Corrosion Sci. 52 Ž1996. 680. R.A. Corbett, in: R. Baboian, S.W. Dean Jr., H.P. Hack, G.S. Haynes, J.R. Scully, D.O. Sprowls ŽEds.., Corrosion Tests and Standars: Application and Interpretation. ASTM Philadelphia, USA, 1995, p. 98. ASTM Designation: G 1-72, American National Standards, USA, 1973, p. 626. G.D. Smith, in: R. Baboian, S.W. Dean Jr., H.P. Hack, G.S. Haynes, J.R. Scully, D.O. Sprowls ŽEds.., Corrosion Tests and Standards: Application and Interpretation. ASTM Philadelphia, USA, 1995, p. 149. J.R. Scully, in: R. Baboian, S.W. Dean Jr., H.P. Hack, G.S. Haynes, J.R. Scully, D.O. Sprowls ŽEds.., Corrosion Tests and Standards: Application and Interpretation. ASTM Philadelphia, USA, 1995, p. 75. F. Mansfeld, Corrosion Sci. 44 Ž1988. 856. B.W. Madsen, in: A.A. Sagues, E.Y. Meletis ŽEds.., Symposium Proceedings on Wear-Corrosion Interactions in Liquid Media. TMS, Warrendale, PA Ž1991., p. 49. Z. Nagy, in: J.O’M. Bockris et al. ŽEds.., Modern Aspects of Electrochemistry. Plenum Press, New York, 1993, p.135. L. Martinez, M. Amaya, O. Flores, D. Lawrynowics, E. Lavernia, in: J.C. Earthman, F.A. Mohamed ŽEds.., Symposium Proceedings on The Seventh International Conference on Creep and Fracture of Engineering Materials and Structures. University of California, Irvine, August 10]15, A publication of TMS, Warrendale, PA, 1997, p. 395. X. Liang, E.J. Lavernia, Metall. Mater. Trans. A 25A Ž1994. 2341. J. Zhang, R.J. Perez, E.J. Lavernia, Acta Metall. Mater. 42 Ž1994. 395. B. Li, N. Nordstrom, E.J. Lavernia, Mater. Sci. Eng. A237 Ž1997. 207. G.W. Cunningham, A.d.S. Brasunas, Corrosion 12 Ž1956. 389t. N.J.H. Small, H. Strawson, A.L. Lewis, in: L.M. Wyatt, G.J.
312
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
Evans ŽEds.., International Conference on the Mechanism of Corrosion by Fuel Impurities. Butterworths, Marchwood, May 1963, p. 238. w35x J. Porcayo-Calderon, PHD Thesis. UNAM-Mexico, 1998, 121. w36x W.J. Greenert, Corrosion 18 Ž1962. 57t. w37x Mittelstadt, K. Schwerdfeger, Metall. Trans. B 21B Ž1990. 111.
w38x D.Z. Shi, J.C. Nava, R.A. Rapp, High Temperature Materials Chemistry IV, Electrochemical Soc., USA, 1987, p. 1. w39x Zheng, R.A. Rapp, J. Electrochem. Soc. 142 Ž1995. 142. w40x H.J. Ratzer-Scheibe, 4th International Symposium on High ¨ Temperature Corrosion and Protection of Materials, Les EMBIEZ ŽVar., France, 20]24 May 1996.
High-temperature electrochemical testing of spray atomized and deposited iron aluminides alloyed with boron and reinforced with alumina particulate L. Martinez a,U , M. Amayaa , J. Porcayo-Calderonb , E.J. Lavernia c a Instituto de Fisica, UNAM, A.P. 48-3, 62251, Cuerna¨ aca, Morelos, Mexico Instituto de In¨ estigaciones Electricas, A¨ . Reforma No. 113, 62490, Temixco, Morelos, Mexico c Chemical and Biochemical Engineering and Materials Science Department, Uni¨ ersity of California, Ir¨ ine, CA 92697-2575, USA b
Abstract The corrosion behavior of FeAl40 at.%, FeAl40q 0.1 at.% B and FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 in a mixture of molten salts of 80% V2 O5 q 20% Na 2 SO4 Žwt.%. from 6008C to 9008C was studied using a potentiodynamic polarization technique. Experiments were conducted in a typical three-electrode cell immersed in the fused salt. Curves of corrosion current density Ž Icorr . as a function of molten salt temperature were obtained and discussed in terms of the passive layer and corrosion products formed during the electrochemical tests. The corrosion resistance of the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 samples was superior at all temperatures in the interval between 6008C and 9008C. It is apparent that the alumina is playing an important role in corrosion protection. The log i vs. potential polarization curves indicate that corrosion in the FeAl40 at.% and FeAl40q 0.1 at.% B samples may be associated with the formation and spalling of oxide layers rich in vanadium. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: Iron aluminides; Corrosion current density; Molten salt temperature; Electrochemical testing
1. Introduction The iron aluminide-based FeAl with ordered B2 crystal structure have good yield strength from intermediate temperatures up to 10008C, especially when they are reinforced with low expansion fibers or small inert particles such as Al 2 O 3 w1]3x. Also, these alloys have low density compared to iron and some nickelbase alloys w3]5x, and low raw materials costs w1x. Poor room-temperature ductility has been a limiting factor for processing and applications w6x. During the last few years, novel processing routes w7]9x, as well as microalloying w9]12x, microstructure control w13,14x and particulate or fiber reinforcing, have been ex-
U
Corresponding author. Tel.: q52 73 138615; fax: q52 73 153077; e-mail: [email protected]
plored in the direction of improving the ductility of iron aluminides. Several studies have reported that iron aluminides have good corrosion resistance at elevated temperatures under oxidizing and sulfidizing environments, and in molten salts w6,10,14]16x. Iron aluminides are competitive in this respect to stainless steels of the series 300 and 400, which are now used in high-temperature and high-corrosive environments w6,15x. Molten salt corrosion is typical in fossil fuel fired steam generators. Combustion products contain aggressive inorganic impurities, such as oxidizing gases containing some aggressive inorganic impurities, such as vanadium, sodium and sulfur rich salts w17,18x. Ash deposits such as sodium sulfate ŽNa 2 SO4 ., sodium metavanadate ŽNaVO3 . and vanadium pentoxide ŽV2 O5 ., form a stable electrolyte layer on materials surfaces causing severe corrosion w19]21x.
0921-5093r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093Ž98.00949-6
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
The high-temperature corrosion of materials can be measured by the total weight loss method and by electrochemical techniques w15,22]26x. Electrochemical techniques may provide a more complete picture of the corrosion process rather than just the total weight loss w27,28x. We are aware, however, that the use of electrochemical techniques does not fully comply with the premises of the standard techniques employed at room temperature, because the corrosion processes at high temperatures are complicated. In this work we compare the high-temperature corrosion of FeAl40 at.% iron aluminides alloyed with boron and reinforced with alumina particulate obtained by atomization-deposition, when immersed in a mixture of molten salts of V2 O5 and Na 2 SO4 , by means of a potentiodynamic polarization electrochemical technique.
307
is described elsewhere w8,29]32x. Heats of Gaussian shaped cones of the alloys were produced by the atomization and deposition procedure. The heats were sectioned in a spark cutting machine in the form of specimens of 5 = 5 = 5 mm. 2.2. Salts A synthetic mixture of 80% V2 O5 q 20% Na 2 SO4 Žwt.%., was used as the molten electrolyte. The composition of the mixture has been reported as most corrosive w33x. This mixture forms complex vanadates of type Na 2 OV2 O4 5V2 O5 and 5Na 2 OV2 O4 11V2 O5 with relatively low molten points Ž625 and 5358C, respectively. w34,35x. These vanadium compounds have high capacity for oxygen absorption. Other authors have reported a strong correlation between oxygen absorption capability and corrosivity w33,36x.
2. Experimental 2.3. Electrochemical cells 2.1. Materials A master alloy of FeAl40 at.% was fabricated employing iron and aluminum both 99.99% of purity. The alloy was cast in an induction furnace under a protective atmosphere of argon and was poured by gravity in a cylindrical graphite mold of 4-cm diameter. Small pieces were cut from the master alloy bar, and fed to spray atomization and deposition system. Also 99.99% purity boron in the form of Ni 2 B was used as microalloying constituent. The reinforcement phase used was commercially pure Ž99.99%. single crystal a-alumina platelets with an average diameter of 3 m m. The synthesis of FeAl alloys and experimental variables used by spray atomization and deposition
Fig. 1 shows a scheme of the three-electrode cell of the experimental setup for the present study. The working electrode ŽWE. consists of the FeAl specimen. Each FeAl specimen was drilled to insert and clinch a conductive platinum wire. The specimen was inserted in a quartz tube, where the gap between specimen and quartz tube was sealed with a ceramic cement and cured at 1108C for 12 h. The face of the working electrode to be exposed to the molten salt was ground employing normal 240, 320, 400 and 600 grits silicon carbide sand paper and ultrasonically cleaned in ethyl alcohol. Two Pt wires of 0.5-mm diameter were used as the counter electrode ŽCE. and reference electrode ŽRE.. Each one was inserted in a
Fig. 1. Schematic diagram of the electrochemical three-electrode cell.
308
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
mullite tube for electrical isolation. Prior to experiments, all of the components of the corrosion cell were cleaned with acetone and dried. 2.4. Electrochemical testing The quartz crucible with salt mixture was introduced in a vertical furnace and heated up to the working temperature. All electrochemical experiments were conducted in a AUTOTAFEL Potentiostat ACM instrument interfaced to a personal computer. In the polarization experiments the corrosion potential of the working electrode was continuously recorded starting immediately after immersion until a constant potential was obtained. Potentiodynamic polarization studies were conducted immediately after. The tests ranged between y2000 and 2000 mV with respect to corrosion potential. The corrosion current density Icorr was calculated from the intersection of the extrapolated straight line sections of cathodic and anodic Tafel lines within an interval of "300 mV with respect to potential corrosion, as indicated in Fig. 5. The fundamentals of this procedure are described elsewhere w25,40x. After the corrosion tests, the specimens were examined by scanning electron microscopy aided with energy dispersive spectroscopy ŽEDS., to analyze the corrosion surfaces and characterize the corrosion products. 3. Results and discussion 3.1. Cathodic beha¨ ior The polarization potentiodynamic curves of the al-
loys FeAl40 at.%, FeAl40q 0.1 at.% B and FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 are shown in Figs. 2]4, respectively. In the cathodic region Žbelow the corrosion potential. all three samples exhibited similar behavior, mainly because the cathodic region is basically related to transformations in the molten salts which are the same in the three systems. The curves in the cathodic region also have little influence from the test temperature, since no systematic trend could be identified. This behavior has been typically observed in other electrochemical polarization studies at high temperature in molten salt mixtures of V2 O5 q Na 2 SO4 . Some authors have speculated about the Z shape trend for the cathodic curve. The phenomena has been related to reduced ions of vanadium along the test w37]39x. The first change in trend is presented at approximately y400 to 1800 mV where the current density decrease has been related to a reduced step ions from V 5q to V 4q. The reduction in the current density is apparently related to depletion of V 5q ions. Beyond y1800 mV another reduction is triggered where V 4q begins to reduce to V 3q and the current density increases again. 3.2. Anodic beha¨ ior As shown in Figs. 2]4 all the curves above the corrosion potential exhibit a clear Tafel regime where log i increases steadily with voltage. Several authors have reported that this regime is difficult to find in polarization studies involving molten salts at high temperatures. After the Tafel regime several samples
Fig. 2. Potentiodynamic polarization curves of the FeAl40 at.%.
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
309
Fig. 3. Potentiodynamic polarization curves of the FeAl40q 0.1 at.% B.
exhibited instabilities as those marked in Figs. 2 and 3. The instabilities are due to the formation of oxide layers, which eventually spall from the metal surface. The final stage of the polarization experiment, at potential above 400 mV, the material is fully passivated as indicated by the nil increase in current density as potential increases. 3.3. Effect of temperature on the corrosion current density The calculation of the corrosion current density was performed employing the data of the polarization curves in an interval of 300 mV below and above the corrosion potential as shown in Fig. 5. In this interval the intersection of the tangent lines to the anodic and
cathodic curves is found at the corrosion potential as described elsewhere w40x. The calculations are summarized in Fig. 6. The systems FeAl40 at.% and FeAl40q 0.1 at.% B corrode faster at temperatures 6008C and 7008C and the corrosion rate reduces steadily at temperatures of 8008C and 9008C. The role of boron in the corrosion rate apparently is not important. On the other hand the role of alumina particulate seems to be important since the corrosion rate is considerably lower in the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 sample. The alumina particulate may have two different roles in reducing the corrosion rate. Clearly the alumina itself contributes to the passivation of the metal surface. The other may be
Fig. 4. Potentiodynamic polarization curves of the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 .
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
310
Fig. 5. Schematic procedure for calculate the corrosion current density Icorr , Ž"300 mV with respect to potential corrosion..
grain size, since it is considerably smaller in this sample. Figs. 7 and 8 exhibit the corroded surfaces of the FeAl40q 0.1 at.% B and FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 samples. The oxide formations on the FeAl40q 0.1 at.% B sample are very rich in vanadium and exhibit spalling. On the other hand, the microprobe analysis on the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 sample indicates the presence of a layer rich in aluminum oxide.
temperatures in the interval between 6008C and 9008C. It is apparent that the alumina is playing an important role in corrosion protection. The log i vs. potential polarization curves indicate that corrosion in the FeAl40 at.% and the FeAl40q 0.1 at.% B samples may be associated to the formation and spalling of oxide layers rich in vanadium. The temperature interval of a maximum corrosion rate is between 6008C and 7008C, at higher temperatures up to 9008C the corrosion rate diminishes.
4. Conclusions
Acknowledgements
The corrosion resistance of the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 samples was superior in all the
This work has been supported by ARO ŽDAAH0495-1-0424. and CONACYT grant 400363-5-3756PA.
Fig. 6. Temperature effect on the corrosion current density of the FeAl alloys in a molten salt of 80% V2 O5 q 20% Na 2 SO4 .
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312 w8 x w9x w10x w11x w12x w13x w14x w15x Fig. 7. Typical corrosion surface on the FeAl40q 0.1 at.% B at 6008C. w16x
w17x w18x w19x w20x w21x w22x Fig. 8. Typical corrosion surface on the FeAl40q 0.1 at.% B q 10 at.% Al 2 O 3 at 6008C. w23x
The authors acknowledge the valuable technical support of Anselmo Gonzales, ´ Jose L. Albarran, Osvaldo Flores and Ivonne Espinoza. EJL also wishes to acknowledge NSF ŽINT-9600517. for financial support.
w24x
w25x
References w1x G. Webb, A. Lefort, in: W.O. Soboyejo, T.S. Srivatsan, D.L. Davidson ŽEds.., Symposium Proceedings on Fatigue and Fracture of Ordered Intermetallics Materials: I. TMS, Warrendale, PA, 1994, p. 103. w2x J.L. Smialek, J. Doychak, D.J. Gaydosh, in: T. Grobstein, J. Doychak ŽEds.., Symposium Proceedings on Oxidation of High-Temperature Intermetallics. TMS, Warrendale, PA, 1989, p. 83. w3x S.L. Draper, D.J. Gaydosh, M.V. Nathal, J. Mater. Res. 5 Ž1990. 1976. w4x I. Baker, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 101. w5x J.H. Schneibel, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 329. w6x V.K. Sikka, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 3. w7x X. Zeng, S.R. Nutt, E.J. Lavernia, Metall. Mater. Trans. A 26 Ž1995. 817.
w26x w27x w28x w29x
w30x w31x w32x w33x w34x
311
L. Martinez, O. Flores, M. Amaya, A. Duncan, S. Viswanathan, D. Lawrynowics, E.J. Lavernia, J. Mater. Synth. Processing 5 Ž1997. 65. R.G. Baligidad, U. Prakash, A. Radha Krishna, Mater. Sci. Eng. A230 Ž1997. 188. J.H. Schneibel, Mater. Sci. Eng. A153 Ž1992. 684. D.J. Gaydosh, S.L. Draper, R.D. Noebe, M.V. Nathal, Mater. Sci. Eng. A150 Ž1992. 7. E.P. George, C.L. White, J.A. Horton, Scr. Metall. Mater. 25 Ž1991. 1259. I. Baker, P. Nagpal, F. Liu, P.R. Munroe, Acta Metall. Mater. 39 Ž1991. 1637. R. Subramanian, J.H. Schneibel, K.B. Alexander, K.P. Plucknett, Scr. Mater. 35 Ž1996. 583. P.F. Tortorelli, J.H. DeVan, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 257. D. Pocci, O. Tassa, C. Testani, in: J.H. Schneibel, M.A. Crimp ŽEds.., Symposium Proceedings on Processing, Properties, and Applications of Iron Aluminides. TMS, Warrendale, PA, 1994, p. 19. S. Hwang, R.A. Rapp, Corrosion Sci. 45 Ž1989. 993. R.A. Rapp, in: O. Johannesen, A.G. Anderson ŽEds.., Selected Topics in High Temperature Chemistry. Elsevier, Amsterdam, Oxford, New York, Tokyo, 1989, p. 291. E. Otero, A. Pardo, J. Hernaez, Corrosion Sci. 33 ´ F.J. Perez, ´ Ž1992. 1747. A. Pardo, E. Otero, F.J. Perez, J.F. Alvarez, M.V. Utrilla, ´ Rev. Metal. Madrid 29 Ž1993. 300. Y. Longa-Nava, Y.S. Zhang, M. Takemoto, R.A. Rapp, Corrosion Sci. 52 Ž1996. 680. R.A. Corbett, in: R. Baboian, S.W. Dean Jr., H.P. Hack, G.S. Haynes, J.R. Scully, D.O. Sprowls ŽEds.., Corrosion Tests and Standars: Application and Interpretation. ASTM Philadelphia, USA, 1995, p. 98. ASTM Designation: G 1-72, American National Standards, USA, 1973, p. 626. G.D. Smith, in: R. Baboian, S.W. Dean Jr., H.P. Hack, G.S. Haynes, J.R. Scully, D.O. Sprowls ŽEds.., Corrosion Tests and Standards: Application and Interpretation. ASTM Philadelphia, USA, 1995, p. 149. J.R. Scully, in: R. Baboian, S.W. Dean Jr., H.P. Hack, G.S. Haynes, J.R. Scully, D.O. Sprowls ŽEds.., Corrosion Tests and Standards: Application and Interpretation. ASTM Philadelphia, USA, 1995, p. 75. F. Mansfeld, Corrosion Sci. 44 Ž1988. 856. B.W. Madsen, in: A.A. Sagues, E.Y. Meletis ŽEds.., Symposium Proceedings on Wear-Corrosion Interactions in Liquid Media. TMS, Warrendale, PA Ž1991., p. 49. Z. Nagy, in: J.O’M. Bockris et al. ŽEds.., Modern Aspects of Electrochemistry. Plenum Press, New York, 1993, p.135. L. Martinez, M. Amaya, O. Flores, D. Lawrynowics, E. Lavernia, in: J.C. Earthman, F.A. Mohamed ŽEds.., Symposium Proceedings on The Seventh International Conference on Creep and Fracture of Engineering Materials and Structures. University of California, Irvine, August 10]15, A publication of TMS, Warrendale, PA, 1997, p. 395. X. Liang, E.J. Lavernia, Metall. Mater. Trans. A 25A Ž1994. 2341. J. Zhang, R.J. Perez, E.J. Lavernia, Acta Metall. Mater. 42 Ž1994. 395. B. Li, N. Nordstrom, E.J. Lavernia, Mater. Sci. Eng. A237 Ž1997. 207. G.W. Cunningham, A.d.S. Brasunas, Corrosion 12 Ž1956. 389t. N.J.H. Small, H. Strawson, A.L. Lewis, in: L.M. Wyatt, G.J.
312
L. Martinez et al. r Materials Science and Engineering A258 (1998) 306]312
Evans ŽEds.., International Conference on the Mechanism of Corrosion by Fuel Impurities. Butterworths, Marchwood, May 1963, p. 238. w35x J. Porcayo-Calderon, PHD Thesis. UNAM-Mexico, 1998, 121. w36x W.J. Greenert, Corrosion 18 Ž1962. 57t. w37x Mittelstadt, K. Schwerdfeger, Metall. Trans. B 21B Ž1990. 111.
w38x D.Z. Shi, J.C. Nava, R.A. Rapp, High Temperature Materials Chemistry IV, Electrochemical Soc., USA, 1987, p. 1. w39x Zheng, R.A. Rapp, J. Electrochem. Soc. 142 Ž1995. 142. w40x H.J. Ratzer-Scheibe, 4th International Symposium on High ¨ Temperature Corrosion and Protection of Materials, Les EMBIEZ ŽVar., France, 20]24 May 1996.