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Hot corrosion behaviour of Ti–Al based intermetallics
December 2002
Materials Letters 57 (2002) 834 – 843 www.elsevier.com/locate/matlet
Hot corrosion behaviour of Ti–Al based intermetallics Kai Zhang, Zhengwei Li, Wei Gao* Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 6 March 2002; accepted 3 April 2002
Abstract Three types of hot corrosion tests were performed on TiAl-based intermetallics Ti – 48Al, Ti – Ti – 52Al and 48Al – 2Cr (at.%). The salt mixture that was used in the present hot corrosion study consists of 80 wt.% Na2SO4 + 20 wt.% NaCl, with a melting point of f 700 jC. The specimens were either suspended in the salt vapour, deposited with the mixed salt, or immersed in the molten salt, all at 800 jC. These testing methods represent different service conditions, and produced different results. However, these results are consistent in ranking the hot corrosion properties of materials. Electron microscopy and Xray diffraction (XRD) were used to study the morphology and compositions of the corrosion products. The mechanisms of hot corrosion have also been discussed based on the experimental results. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium aluminide; Molten salt corrosion; Hot corrosion experimental methods; Corrosion and oxidation; Kinetics; Metals and alloys
1. Introduction Titanium, titanium alloys and titanium intermetallic compounds have been receiving considerable attention, as they are light weighted and have excellent mechanical properties [1]. They also possess a number of excellent high temperature properties including creep resistance, fatigue resistance and static strength retention [2,3]. Therefore, they are being considered as the replacement of some Ni-based superalloys [4– 7]. The main applications of titanium materials have been in aerospace, chemical and pet-
*
Corresponding author. Tel.: +64-9-373-7599x8175; fax: +649-373-7463. E-mail address: [email protected] (W. Gao).
rochemical, metallurgical, papermaking, and medical industries. These applications would require not only excellent mechanical properties, but also good high temperature resistance in corrosive environments [8]. However, the studies on titanium materials have mainly been concentrated on the mechanical properties and ambient air oxidation behaviour. Very little work has been carried out on the hot corrosion of these materials in salt-containing environments [9]. Furthermore, the results obtained with different hot corrosion experimental methods made comparison from each other difficult [10 – 15]. The present work studies the hot corrosion behaviours of titanium – aluminium intermetallics in molten salt-containing environments with three different experimental methods and the same materials, and at the same temperature [12 –15].
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 8 2 - 0
K. Zhang et al. / Materials Letters 57 (2002) 834–843
2. Experimental procedure
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matic drawing showing the settings of these testing methods.
2.1. Materials and specimen preparation Three Ti–Al intermetallics were used in the present studies: Ti –48Al, Ti– 52Al, Ti– 48Al – 2Cr (compositions are given in at.%). Ti –48Al consists mainly of g-TiAl with a small amount of a2(Ti3Al); Ti–52Al consists of g-phase; and Ti– 48Al – 2Cr consists of near g-phase [16]. The alloys were prepared by vacuum melting and casting from pure metals. The specimens, with dimension of about 15 10 2 mm, were cut from the Ti –Al ingots with a diamond saw. All the surfaces were ground to 600 grit finish, ultrasonically degreased in ethanol, followed by blow dry. The original mass of each specimen was measured using an electronic balance with a weighing accuracy of 0.01 mg. 2.2. Hot corrosion testing methods Hot corrosion tests were carried out with 80 wt.% Na2SO4 + 20 wt.% NaCl molten salts at 800 jC for 200 h. The melting point of the mixed salts is about 700 jC. Three different testing methods were used to make salt-containing environments. Fig. 1 is a sche-
2.2.1. Suspended the specimens in salt vapour [17] The specimens were suspended with hooks in a covered crucible containing molten salts. The crucible with samples was then put into a furnace running at 800 jC (Fig. 1a). With this method, the specimens do not contact the molten salt directly. The molten salt vaporised and condensed on the surfaces of the specimens, accelerating the corrosion reactions. After a certain time of exposure, the specimens were withdrawn from the crucible, cooled in air, cleaned with a soft brush, washed in boiling water for removing the salt left on the surface, and dried in hot air. The masses were then measured to obtain the mass changes for establishing the corrosion kinetics. 2.2.2. Deposited the salt on the specimens [18] Saturated salt solution was hand-brushed onto the surfaces of the specimens that were placed on a hot plate. A layer of salt film formed on the specimen surfaces. The amount of the salt was controlled to be f 6 mg/cm2. The deposited specimens were then placed in a crucible; and the corrosion exposure was carried out in a vertical furnace at 800 jC in ambient
Fig. 1. Schematic drawings of three hot corrosion test settings: (a) specimens are suspended in salt vapour, (b) specimens are deposited with a salt film, and (c) specimens are immersed in molten salt.
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Table 1 Intermetallics and testing methods used in this work Methods\alloy
(1) Ti – 48Al
(2) Ti – 52Al
(3) Ti – 48Al – 2Cr
(a) Suspended (b) Deposited (c) Immersed
1a 1b 1c
2a 2b 2c
3a 3b 3c
atmosphere (Fig. 1b). The cooling and cleaning methods were the same as the first method. 2.2.3. Immersed the specimens into the molten salt [19] Specimens were completely immersed into the molten salt at 800 jC, and retrieved at regular intervals (Fig. 1c). They were then cooled in air, cleaned with a soft brush, washed in boiling water for removing the deposited salt on the surfaces and dried in hot air. The corrosion kinetics was measured as the mass changes. Table 1 lists the specimens and testing methods used in this study. These different hot corrosion testing methods represent different service conditions, and have been previously used by researchers with different materials. However, there was no comparative research on these methods with identical materials. The purpose of this work is twofold: to study the hot corrosion behaviour of several Ti– Al intermetal-
lics, and to compare the different methods of hot corrosion tests. 2.3. Specimen characterisation The corroded specimens were analysed using Xray diffraction (XRD Bruker, D8), scanning electron microscopy, and energy dispersive X-ray analysis (SEM/EDAX FEG-SEM, Philips XL 305).
3. Results Fig. 2 plots the hot corrosion kinetics of all samples at 800 jC. These curves can be classified into three groups according to the shape and developing tendency. The first group was the specimens that were suspended in the salt vapour (1a, 2a and 3a). Little spallation occurred in the first 20-h exposure. After 20-h oxidation, minor scale spallation occurred, but the oxide scale grew faster than the scale spallation, resulting in net mass gains up to 200 h of exposure. However, the mass gains and scale spallation were all very small, indicating a slow corrosion rate. The second group includes the specimens with salt deposits on the surface (1b, 2b and 3b). Scale (and salt
Fig. 2. Hot corrosion kinetics of Ti – Al intermetallics in Na2SO4 + 20 wt.% NaCl salts. Materials: (1) Ti – 48Al, (2) Ti – 52Al, (3) Ti – 48Al – 2Cr; testing methods: (a) suspended, (b) deposited, and (c) immersed.
K. Zhang et al. / Materials Letters 57 (2002) 834–843
film) spallation took place immediately with more than 5 mg/cm2-mass loss in the initial 20-h exposure. After this, the rates of scale spallation decreased sharply; and even a small amount of mass gains was recorded. This is believed due to the consumption and spallation of the salt film on the specimens. After the initial 20-h exposure, there is not much salt could be seen on the surface of the samples. The last group is the specimens entirely immersed into the molten salt (1c, 2c and 3c). The scale spallation was observed when the specimens were withdrawn from the salt and cooled in air, and were cleaned in boiling water. Sometimes, a whole layer of scale spalled away from the substrates. This method represents the severest hot corrosion of the three
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different hot corrosion methods. The kinetic curves are also not smooth with jumps up and down, evidence of fast corrosion rates and severe scale spallation. The corrosion kinetics also shows that the three Ti –Al intermetallic compounds with different compositions exhibited different hot corrosion resistance. In general, Ti – 48Al (No. 1) showed the poorest corrosion behaviour among the three, with strong tendency of scale spallation. Ti– 52Al (No. 2) exhibited better corrosion resistance than Ti– 48Al – 2Cr (No. 3) in the tests in molten salt and salt vapour. The three alloys had the similar kinetic curve shape with the salt-deposited specimens. The main difference was the amounts of scale spallation during the initial 20 h.
Fig. 3. X-ray diffraction spectra of the corrosion products formed on Ti – Al intermetallics after the 200-h exposure at 800 jC. (1) Ti – 48Al, (2) Ti – 52Al, (3) Ti – 48Al – 2Cr; (a) suspended, (b) deposited, (c) immersed.
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The salt-deposited Ti–48Cr (1b) had the highest scale spallation at the initial stage of corrosion. Fig. 3 shows the XRD spectra of the corrosion products. The main corrosion products on the suspended and deposited specimens were TiO2 and aAl2O3. The non-protective TiO2 scales, even mixed with Al2O3, are easy to be broken. The compounds formed on the immersed specimens were complex, containing several salts. They were found to be mainly Ca3Al2O6 and CaTiO3. Figs. 4– 6 show the cross-section morphologies of the specimens after 200-h hot corrosion at 800 jC. The scales formed on suspended and deposited samples were mixtures of TiO2 and a-Al2O3 according to the XRD analysis. It can be seen that the mounting epoxy has penetrated into the scale –alloy interface for the sample of Ti– 48Al suspended in the salt vapour
(Fig. 4(1)), evidence of severe scale cracking. The degradation of the alloy surfaces appears not uniform. It should be noted that the salt-deposited and saltimmersed samples all underwent severe scale spallation. The scales on the samples shown in the crosssection micrographs in Figs. 5 and 6 are not the original scales.
4. Discussion 4.1. Corrosion of suspended specimens The specimens that were suspended in the salt vapour had relatively slow corrosion rates. Fig. 7 is a re-plot of the kinetic curves of the suspended specimens with a small scale. The corrosion kinetics
Fig. 4. Cross-section SEM micrographs of Ti – Al intermetallics after the 200-h suspended tests in Na2SO4 + 20 wt.% NaCl vapour at 800 jC. (1) Ti – 48Al, (2) Ti – 52Al, (3) Ti – 48Al – 2Cr.
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Fig. 5. Cross-section SEM micrographs of Ti – 48Al – 2Cr after the 200-h exposure at 800 jC with deposited Na2SO4 + 20 wt.% NaCl salt film.
of Ti– 52Al and Ti– 48Al –2Cr followed an apparently parabolic rate law. Ti – 48Al – 2Cr showed slightly lower corrosion rate than Ti – 52Al. However, the corrosion kinetics of Ti –48Al is not smooth, showing severe spallation during some cooling cycles. The poor corrosion resistance may be caused by two
reasons. Firstly, Ti–48Al contains relatively low Al content. The activities of Al in Ti – Al systems decreases sharply around 50 at.% Al, resulting in a higher oxidation rate of Ti. Secondly, as reported that Ti –48Al has a multi-phase structure but Ti– 52Al and Ti –48Al – 2Cr consists of g- or near g-phase. The
Fig. 6. Cross-section SEM micrographs of Ti – 52Al intermetallics after the 200-h exposure immersed in molten salt at 800 jC.
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Fig. 7. Hot corrosion kinetic curves for the specimens suspended in salt vapour: (1a) Ti – 48Al, (2a) Ti – 52Al, and (3a) Ti – 48Al – 2Cr.
corrosion properties are often affected by the microstructure of an alloy. When the scale spalled away, metal is directly exposed to the corrosive environments, resulting in an accelerated corrosion process. The reactions in hot corrosion often have complex mechanisms. Simplified processes, however, can be suggested below. Al2O3 and TiO2 formed on the alloy surface at the beginning in an O2-containing environment. With salt vapour in the atmosphere, it is possible that TiO2 and Al2O3 react with NaCl [20]. 2NaCl þ Al2 O3 þ 1=2O2 ! 2NaAlO2 þ Cl2
ð1Þ
NaCl þ TiO2þ O2 ! NaTiO4 þ 1=2Cl2
ð2Þ
Penetration and transportation of Cl2 (Cl , Cl), and reaction of Cl species with Al and/or Ti in the substrate surface may follow: Ti þ Cl2 ! TiCl2
ð3Þ
Al þ 3=2Cl2 ! AlCl3
ð4Þ
At the surface of scales, where the oxygen potential is high, the chlorides may re-oxidise: TiCl2 þ O2 ! TiO2 þ Cl2
ð5Þ
2AlCl3 þ 3=2O2 ! Al2 O3 þ 3Cl2
ð6Þ
This mechanism implies a self-sustaining corrosion process that requires only a very small quantity of
chloride phases for continuous reactions. The chlorides play a role of catalysts during the whole reaction. 4.2. Salt-deposited specimens Salt-deposition layer accelerated and complicated the reactions at high temperatures. However, the large mass loss in the first 15 h was believed to be caused by the spallation of the deposited salt and oxide scales. The salt film does not adhere well to the smooth specimen’s surface. The corrosion scale also formed very quickly in the initial period of exposure since the metal reacts with salt directly. When the specimens were cooled in air, some scales spalled away together with the salt film. It is difficult to analyse the kinetics because of the mixed salt –oxide layers. Since the salt films were largely broken away after the first two cycles (15 h), we can re-plot the kinetic curves from the 15-h exposure. Fig. 8 is such a plot with mass changes started from zero at 15 h of corrosion. It can be seen that Ti– 52Al and Ti–48Al– 2Cr had similar mass gains after the 200-h exposure. The kinetics, however, are different. Ti –48Al – 2Cr followed an approximately linear rate law, while Ti– 52Al showed an approximately parabolic kinetics, implying that the scales formed on Ti –52Al has better protective ability. On the other hand, Ti –48Al had a high corrosion rate, followed by severe scale spallation. In fact, the corrosion rates after the salt films were broken away were quite similar to the oxidation kinetics of these alloys in ambient atmosphere [2,16,21].
K. Zhang et al. / Materials Letters 57 (2002) 834–843
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Fig. 8. Hot corrosion kinetic curves for the specimens coated with a salt film. The alloy numbers are the same as above. The mass changes are re-plotted starting from the 15-h exposure.
EDS was performed on the cross-section microstructure shown in Fig. 5, exhibiting that the scale was a mixture of TiO2 and Al2O3. Along the interface between the scale and the substrate of Ti– 48Al – 2Cr, a thin layer of light contrast could be observed. This is the ‘‘Z – Ti 50Al 30O 20 phase’’ (labelled) that was reported in the literature [22]. EDS quantitative analysis showed that this phase has a composition of O = 11.6, Al = 31.7, Ti = 51.3, Cr = 2.8 and N = 2.6, in at.%. The formation mechanism of Z-phase have been suggested: c Ti50 Al48 þ 20O ! Z Ti50 Al30 O20 þ 18Al ð7Þ The aluminium released in this reaction may diffuse outward to form an Al2O3 scale. However, it was also found that the Z-phase was metastable and eventually decomposed to a2-Ti3Al(O) + Al2O3 [22]. 4.3. Immersed specimens The specimens that were immersed in the molten salt suffered the severest hot corrosion among the three methods. The mass loss of Ti– 48Al was quite high ( f 14 mg/cm2) as shown in Fig. 9. Complex corrosion products including CaTiO3 and Ca3Al2O6 were identified by XRD (Fig. 3). It is believed that the reactions were occurred between the salts, specimens and ceramic crucible. CaO, which was from the ceramic crucibles, was dissolved into the molten salts,
and reacted with the TiAl alloys. The reaction mechanisms are complex and have not been well understood. Fig. 6 shows that the scales are porous and nonuniform, which are of non-protective nature, and are easy to break away from the substrate. 4.4. Overview Among the three Ti– Al intermetallics, Ti– 48Al showed the highest corrosion rates and strongest tendency of scale spallation. It was believed that the relatively low Al content might reduce the formation of Al oxides; and the two-phase structure may play a detrimental role to the oxide scales. Ti– 52Al has a microstructure of single g-phase with relatively high Al content, resulting in an Al-rich scale. The XRD spectra shown in Fig. 3 support these mechanisms as the peaks of a-Al2O3 from Ti– 52Al (2a and 2b) are stronger than those from Ti– 48Al (1a and 1b). Therefore, Ti– 52Al showed lower corrosion rates and scale spallation tendency than Ti– 48Al. Ti –48Al – 2Cr also has relatively good hot corrosion behaviour, especially for the suspended samples, implying that Cr can improve the high temperature corrosion resistance of Ti –Al intermetallics. According to Brady et al [23,24], the substitution of Cr for Ti in sufficient quantities forms Laves phase Ti(Cr,Al)2, which exhibits a low oxygen permeability and is capable to form a protective Al2O3 scale. This is probably the reason that Ti– 48Cr – 2Cr has better corrosion resistance than Ti –48Al.
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Fig. 9. Hot corrosion kinetic curves for the specimens immersed in the molten salt. The alloy numbers are the same as above.
With respect to the different hot corrosion methods, the samples that were immersed in the molten salts showed the highest corrosion rates. The scales were porous; and the direct reactions between the scale and molten salts were severe. The specimens that were suspended in the salt vapour had the lowest corrosion rates, with mass gains smaller than 2 mg/cm2 after the 200-h exposure. The reaction kinetics measured by this method was also more reproducible as the corrosion conditions were easy to control. The hot corrosion with deposited salts represents corrosion caused by one-time severe contamination. It had a reaction rate between the other two methods. The amounts of the deposited and spalled salts have a strong influence on the measured kinetics. However, the corrosion behaviour of materials can be characterised with a relatively long-time exposure, in which the influence of salt-film spallation at the initial stage is less significant.
5. Conclusions (1) Three Ti – Al intermetallics, Ti – 48Al, Ti – 48Al – 2Cr, Ti – 52Al, were exposed in 80 wt.% Na2SO4 + 20 wt.% NaCl containing environments at 800 jC. Hot corrosion and scale spallation took place during the 200-h tests. Ti – 52Al showed the best corrosion resistance among the three alloys. It is believed that the high Al content and single g-phase played a role, forming an Al-rich oxide scale with relatively good protective ability. Ti –48Al – 2Cr also
showed better corrosion resistance than Ti– 48Al. The latter suffered high corrosion rate and severe scale spallation. (2) Three different exposure methods were used to conduct hot corrosion in salt-containing environment. They represent different service conditions. Among these methods, suspending the specimens in salt vapour appeared to be the most reliable method for kinetic studies. The influential factors are relatively easy to be controlled. When use salt-deposition method, the spallation of the salt film at the early stage should be considered. The immersion method showed the severest hot corrosion. Crucible materials and corrosion products dissolve into the molten salts, further complicating the reactions.
Acknowledgements The authors would like to thank Dr. Quadakkers for providing Ti– 48Al –2Cr samples, and all technical staff at the Department and Research Centre for Surface and Materials Science for their assistances.
References [1] F.H. Fores, C. Suryanarayana, D. Eliezer, J. Mater. Sci. 27 (1992) 5113. [2] C.M. Austin, T.J. Kelly, in: R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Mirade, M.V. Nathal (Eds.), Structural Intermetallics, TMS, Warrendale, PA, USA, 1993, p. 143.
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[10] [11]
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S.C. Huang, E.L. Hall, Metall. Trans. 22A (1991) 427. J. Stringer, Mater. Sci. Technol. 3 (1987) 483. R.A. Rapp, Corrosion 42 (10) (1986) 568. Y.W. Kim, JOM 42 (1989) 24. Y.W. Kim, JOM 47 (1995) 38. B.D. Craig, Handbook of Corrosion Data, ASM International, Materials Park, OH, 1989. Z. Li, W. Gao, Corrosion, oxidation and mechanical properties of titanium, titanium alloys and titanium composites, An Internal Report, The University of Auckland (2000). N. Birks, G.H. Meier, Introduction to High Temperature Oxidation of Metals, Edward Arnold, London, 1983. R. Zhu, Y. He, H. Qi, High-Temperature Corrosion and HighTemperature Oxidation Resistant Materials, Shanghai Science and Technology Publisher, Shanghai, 1995. Z. Tang, F. Wang, W. Wu, Mater. Sci. Eng. 276 (2000) 70. Z. Tang, F. Wang, W. Wu, Intermetallics 7 (1999) 1271. Z. Tang, F. Wang, W. Wu, Chin. Soc. Corros. Prot. 17 (1997) 233.
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[15] C. Guan, Z. Tang, F. Wang, W. Wu, Corros. Sci. Prot. Technol. 12 (2000) 134. [16] V.A.C. Haanappel, R. Hofman, J.D. Sunderkotter, W. Glatz, H. Clemens, M.F. Stroosnijder, Oxid. Met. 48 (1997) 263. [17] J. Takahashi, Y. Kawai, S. Shimada, J. Eur. Ceram. Soc. 18 (1998) 1121. [18] Z. Yao, M. Marek, Mater. Sci. Eng. 192/193 (1995) 994. [19] M. Hara, D. Hieda, Y. Hinata, J. Jpn. Inst. Met. 63 (1999) 1238. [20] Z. Tang, F. Wang, W. Wu, Oxid. Met. 51 (1999) 235. [21] G. Welsch, P.D. Desal, Oxidation and Corrosion of Intermetallic Alloys, Purdue Research Foundation, West Lafayatte, IN, 1996, p. 295. [22] E.H. Copland, B. Gleeson, D.J. Young, Acta Mater. 47 (1999) 2937. [23] M.P. Brady, J.L. Smialek, J. Smith, D.L. Humphrey, Acta Mater. 45 (6) (1997) 2357. [24] M.P. Brady, J.L. Smialek, D.L. Humphrey, J. Smith, Acta Mater. 45 (6) (1997) 2371.
Materials Letters 57 (2002) 834 – 843 www.elsevier.com/locate/matlet
Hot corrosion behaviour of Ti–Al based intermetallics Kai Zhang, Zhengwei Li, Wei Gao* Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 6 March 2002; accepted 3 April 2002
Abstract Three types of hot corrosion tests were performed on TiAl-based intermetallics Ti – 48Al, Ti – Ti – 52Al and 48Al – 2Cr (at.%). The salt mixture that was used in the present hot corrosion study consists of 80 wt.% Na2SO4 + 20 wt.% NaCl, with a melting point of f 700 jC. The specimens were either suspended in the salt vapour, deposited with the mixed salt, or immersed in the molten salt, all at 800 jC. These testing methods represent different service conditions, and produced different results. However, these results are consistent in ranking the hot corrosion properties of materials. Electron microscopy and Xray diffraction (XRD) were used to study the morphology and compositions of the corrosion products. The mechanisms of hot corrosion have also been discussed based on the experimental results. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium aluminide; Molten salt corrosion; Hot corrosion experimental methods; Corrosion and oxidation; Kinetics; Metals and alloys
1. Introduction Titanium, titanium alloys and titanium intermetallic compounds have been receiving considerable attention, as they are light weighted and have excellent mechanical properties [1]. They also possess a number of excellent high temperature properties including creep resistance, fatigue resistance and static strength retention [2,3]. Therefore, they are being considered as the replacement of some Ni-based superalloys [4– 7]. The main applications of titanium materials have been in aerospace, chemical and pet-
*
Corresponding author. Tel.: +64-9-373-7599x8175; fax: +649-373-7463. E-mail address: [email protected] (W. Gao).
rochemical, metallurgical, papermaking, and medical industries. These applications would require not only excellent mechanical properties, but also good high temperature resistance in corrosive environments [8]. However, the studies on titanium materials have mainly been concentrated on the mechanical properties and ambient air oxidation behaviour. Very little work has been carried out on the hot corrosion of these materials in salt-containing environments [9]. Furthermore, the results obtained with different hot corrosion experimental methods made comparison from each other difficult [10 – 15]. The present work studies the hot corrosion behaviours of titanium – aluminium intermetallics in molten salt-containing environments with three different experimental methods and the same materials, and at the same temperature [12 –15].
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 8 2 - 0
K. Zhang et al. / Materials Letters 57 (2002) 834–843
2. Experimental procedure
835
matic drawing showing the settings of these testing methods.
2.1. Materials and specimen preparation Three Ti–Al intermetallics were used in the present studies: Ti –48Al, Ti– 52Al, Ti– 48Al – 2Cr (compositions are given in at.%). Ti –48Al consists mainly of g-TiAl with a small amount of a2(Ti3Al); Ti–52Al consists of g-phase; and Ti– 48Al – 2Cr consists of near g-phase [16]. The alloys were prepared by vacuum melting and casting from pure metals. The specimens, with dimension of about 15 10 2 mm, were cut from the Ti –Al ingots with a diamond saw. All the surfaces were ground to 600 grit finish, ultrasonically degreased in ethanol, followed by blow dry. The original mass of each specimen was measured using an electronic balance with a weighing accuracy of 0.01 mg. 2.2. Hot corrosion testing methods Hot corrosion tests were carried out with 80 wt.% Na2SO4 + 20 wt.% NaCl molten salts at 800 jC for 200 h. The melting point of the mixed salts is about 700 jC. Three different testing methods were used to make salt-containing environments. Fig. 1 is a sche-
2.2.1. Suspended the specimens in salt vapour [17] The specimens were suspended with hooks in a covered crucible containing molten salts. The crucible with samples was then put into a furnace running at 800 jC (Fig. 1a). With this method, the specimens do not contact the molten salt directly. The molten salt vaporised and condensed on the surfaces of the specimens, accelerating the corrosion reactions. After a certain time of exposure, the specimens were withdrawn from the crucible, cooled in air, cleaned with a soft brush, washed in boiling water for removing the salt left on the surface, and dried in hot air. The masses were then measured to obtain the mass changes for establishing the corrosion kinetics. 2.2.2. Deposited the salt on the specimens [18] Saturated salt solution was hand-brushed onto the surfaces of the specimens that were placed on a hot plate. A layer of salt film formed on the specimen surfaces. The amount of the salt was controlled to be f 6 mg/cm2. The deposited specimens were then placed in a crucible; and the corrosion exposure was carried out in a vertical furnace at 800 jC in ambient
Fig. 1. Schematic drawings of three hot corrosion test settings: (a) specimens are suspended in salt vapour, (b) specimens are deposited with a salt film, and (c) specimens are immersed in molten salt.
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Table 1 Intermetallics and testing methods used in this work Methods\alloy
(1) Ti – 48Al
(2) Ti – 52Al
(3) Ti – 48Al – 2Cr
(a) Suspended (b) Deposited (c) Immersed
1a 1b 1c
2a 2b 2c
3a 3b 3c
atmosphere (Fig. 1b). The cooling and cleaning methods were the same as the first method. 2.2.3. Immersed the specimens into the molten salt [19] Specimens were completely immersed into the molten salt at 800 jC, and retrieved at regular intervals (Fig. 1c). They were then cooled in air, cleaned with a soft brush, washed in boiling water for removing the deposited salt on the surfaces and dried in hot air. The corrosion kinetics was measured as the mass changes. Table 1 lists the specimens and testing methods used in this study. These different hot corrosion testing methods represent different service conditions, and have been previously used by researchers with different materials. However, there was no comparative research on these methods with identical materials. The purpose of this work is twofold: to study the hot corrosion behaviour of several Ti– Al intermetal-
lics, and to compare the different methods of hot corrosion tests. 2.3. Specimen characterisation The corroded specimens were analysed using Xray diffraction (XRD Bruker, D8), scanning electron microscopy, and energy dispersive X-ray analysis (SEM/EDAX FEG-SEM, Philips XL 305).
3. Results Fig. 2 plots the hot corrosion kinetics of all samples at 800 jC. These curves can be classified into three groups according to the shape and developing tendency. The first group was the specimens that were suspended in the salt vapour (1a, 2a and 3a). Little spallation occurred in the first 20-h exposure. After 20-h oxidation, minor scale spallation occurred, but the oxide scale grew faster than the scale spallation, resulting in net mass gains up to 200 h of exposure. However, the mass gains and scale spallation were all very small, indicating a slow corrosion rate. The second group includes the specimens with salt deposits on the surface (1b, 2b and 3b). Scale (and salt
Fig. 2. Hot corrosion kinetics of Ti – Al intermetallics in Na2SO4 + 20 wt.% NaCl salts. Materials: (1) Ti – 48Al, (2) Ti – 52Al, (3) Ti – 48Al – 2Cr; testing methods: (a) suspended, (b) deposited, and (c) immersed.
K. Zhang et al. / Materials Letters 57 (2002) 834–843
film) spallation took place immediately with more than 5 mg/cm2-mass loss in the initial 20-h exposure. After this, the rates of scale spallation decreased sharply; and even a small amount of mass gains was recorded. This is believed due to the consumption and spallation of the salt film on the specimens. After the initial 20-h exposure, there is not much salt could be seen on the surface of the samples. The last group is the specimens entirely immersed into the molten salt (1c, 2c and 3c). The scale spallation was observed when the specimens were withdrawn from the salt and cooled in air, and were cleaned in boiling water. Sometimes, a whole layer of scale spalled away from the substrates. This method represents the severest hot corrosion of the three
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different hot corrosion methods. The kinetic curves are also not smooth with jumps up and down, evidence of fast corrosion rates and severe scale spallation. The corrosion kinetics also shows that the three Ti –Al intermetallic compounds with different compositions exhibited different hot corrosion resistance. In general, Ti – 48Al (No. 1) showed the poorest corrosion behaviour among the three, with strong tendency of scale spallation. Ti– 52Al (No. 2) exhibited better corrosion resistance than Ti– 48Al – 2Cr (No. 3) in the tests in molten salt and salt vapour. The three alloys had the similar kinetic curve shape with the salt-deposited specimens. The main difference was the amounts of scale spallation during the initial 20 h.
Fig. 3. X-ray diffraction spectra of the corrosion products formed on Ti – Al intermetallics after the 200-h exposure at 800 jC. (1) Ti – 48Al, (2) Ti – 52Al, (3) Ti – 48Al – 2Cr; (a) suspended, (b) deposited, (c) immersed.
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The salt-deposited Ti–48Cr (1b) had the highest scale spallation at the initial stage of corrosion. Fig. 3 shows the XRD spectra of the corrosion products. The main corrosion products on the suspended and deposited specimens were TiO2 and aAl2O3. The non-protective TiO2 scales, even mixed with Al2O3, are easy to be broken. The compounds formed on the immersed specimens were complex, containing several salts. They were found to be mainly Ca3Al2O6 and CaTiO3. Figs. 4– 6 show the cross-section morphologies of the specimens after 200-h hot corrosion at 800 jC. The scales formed on suspended and deposited samples were mixtures of TiO2 and a-Al2O3 according to the XRD analysis. It can be seen that the mounting epoxy has penetrated into the scale –alloy interface for the sample of Ti– 48Al suspended in the salt vapour
(Fig. 4(1)), evidence of severe scale cracking. The degradation of the alloy surfaces appears not uniform. It should be noted that the salt-deposited and saltimmersed samples all underwent severe scale spallation. The scales on the samples shown in the crosssection micrographs in Figs. 5 and 6 are not the original scales.
4. Discussion 4.1. Corrosion of suspended specimens The specimens that were suspended in the salt vapour had relatively slow corrosion rates. Fig. 7 is a re-plot of the kinetic curves of the suspended specimens with a small scale. The corrosion kinetics
Fig. 4. Cross-section SEM micrographs of Ti – Al intermetallics after the 200-h suspended tests in Na2SO4 + 20 wt.% NaCl vapour at 800 jC. (1) Ti – 48Al, (2) Ti – 52Al, (3) Ti – 48Al – 2Cr.
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Fig. 5. Cross-section SEM micrographs of Ti – 48Al – 2Cr after the 200-h exposure at 800 jC with deposited Na2SO4 + 20 wt.% NaCl salt film.
of Ti– 52Al and Ti– 48Al –2Cr followed an apparently parabolic rate law. Ti – 48Al – 2Cr showed slightly lower corrosion rate than Ti – 52Al. However, the corrosion kinetics of Ti –48Al is not smooth, showing severe spallation during some cooling cycles. The poor corrosion resistance may be caused by two
reasons. Firstly, Ti–48Al contains relatively low Al content. The activities of Al in Ti – Al systems decreases sharply around 50 at.% Al, resulting in a higher oxidation rate of Ti. Secondly, as reported that Ti –48Al has a multi-phase structure but Ti– 52Al and Ti –48Al – 2Cr consists of g- or near g-phase. The
Fig. 6. Cross-section SEM micrographs of Ti – 52Al intermetallics after the 200-h exposure immersed in molten salt at 800 jC.
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Fig. 7. Hot corrosion kinetic curves for the specimens suspended in salt vapour: (1a) Ti – 48Al, (2a) Ti – 52Al, and (3a) Ti – 48Al – 2Cr.
corrosion properties are often affected by the microstructure of an alloy. When the scale spalled away, metal is directly exposed to the corrosive environments, resulting in an accelerated corrosion process. The reactions in hot corrosion often have complex mechanisms. Simplified processes, however, can be suggested below. Al2O3 and TiO2 formed on the alloy surface at the beginning in an O2-containing environment. With salt vapour in the atmosphere, it is possible that TiO2 and Al2O3 react with NaCl [20]. 2NaCl þ Al2 O3 þ 1=2O2 ! 2NaAlO2 þ Cl2
ð1Þ
NaCl þ TiO2þ O2 ! NaTiO4 þ 1=2Cl2
ð2Þ
Penetration and transportation of Cl2 (Cl , Cl), and reaction of Cl species with Al and/or Ti in the substrate surface may follow: Ti þ Cl2 ! TiCl2
ð3Þ
Al þ 3=2Cl2 ! AlCl3
ð4Þ
At the surface of scales, where the oxygen potential is high, the chlorides may re-oxidise: TiCl2 þ O2 ! TiO2 þ Cl2
ð5Þ
2AlCl3 þ 3=2O2 ! Al2 O3 þ 3Cl2
ð6Þ
This mechanism implies a self-sustaining corrosion process that requires only a very small quantity of
chloride phases for continuous reactions. The chlorides play a role of catalysts during the whole reaction. 4.2. Salt-deposited specimens Salt-deposition layer accelerated and complicated the reactions at high temperatures. However, the large mass loss in the first 15 h was believed to be caused by the spallation of the deposited salt and oxide scales. The salt film does not adhere well to the smooth specimen’s surface. The corrosion scale also formed very quickly in the initial period of exposure since the metal reacts with salt directly. When the specimens were cooled in air, some scales spalled away together with the salt film. It is difficult to analyse the kinetics because of the mixed salt –oxide layers. Since the salt films were largely broken away after the first two cycles (15 h), we can re-plot the kinetic curves from the 15-h exposure. Fig. 8 is such a plot with mass changes started from zero at 15 h of corrosion. It can be seen that Ti– 52Al and Ti–48Al– 2Cr had similar mass gains after the 200-h exposure. The kinetics, however, are different. Ti –48Al – 2Cr followed an approximately linear rate law, while Ti– 52Al showed an approximately parabolic kinetics, implying that the scales formed on Ti –52Al has better protective ability. On the other hand, Ti –48Al had a high corrosion rate, followed by severe scale spallation. In fact, the corrosion rates after the salt films were broken away were quite similar to the oxidation kinetics of these alloys in ambient atmosphere [2,16,21].
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Fig. 8. Hot corrosion kinetic curves for the specimens coated with a salt film. The alloy numbers are the same as above. The mass changes are re-plotted starting from the 15-h exposure.
EDS was performed on the cross-section microstructure shown in Fig. 5, exhibiting that the scale was a mixture of TiO2 and Al2O3. Along the interface between the scale and the substrate of Ti– 48Al – 2Cr, a thin layer of light contrast could be observed. This is the ‘‘Z – Ti 50Al 30O 20 phase’’ (labelled) that was reported in the literature [22]. EDS quantitative analysis showed that this phase has a composition of O = 11.6, Al = 31.7, Ti = 51.3, Cr = 2.8 and N = 2.6, in at.%. The formation mechanism of Z-phase have been suggested: c Ti50 Al48 þ 20O ! Z Ti50 Al30 O20 þ 18Al ð7Þ The aluminium released in this reaction may diffuse outward to form an Al2O3 scale. However, it was also found that the Z-phase was metastable and eventually decomposed to a2-Ti3Al(O) + Al2O3 [22]. 4.3. Immersed specimens The specimens that were immersed in the molten salt suffered the severest hot corrosion among the three methods. The mass loss of Ti– 48Al was quite high ( f 14 mg/cm2) as shown in Fig. 9. Complex corrosion products including CaTiO3 and Ca3Al2O6 were identified by XRD (Fig. 3). It is believed that the reactions were occurred between the salts, specimens and ceramic crucible. CaO, which was from the ceramic crucibles, was dissolved into the molten salts,
and reacted with the TiAl alloys. The reaction mechanisms are complex and have not been well understood. Fig. 6 shows that the scales are porous and nonuniform, which are of non-protective nature, and are easy to break away from the substrate. 4.4. Overview Among the three Ti– Al intermetallics, Ti– 48Al showed the highest corrosion rates and strongest tendency of scale spallation. It was believed that the relatively low Al content might reduce the formation of Al oxides; and the two-phase structure may play a detrimental role to the oxide scales. Ti– 52Al has a microstructure of single g-phase with relatively high Al content, resulting in an Al-rich scale. The XRD spectra shown in Fig. 3 support these mechanisms as the peaks of a-Al2O3 from Ti– 52Al (2a and 2b) are stronger than those from Ti– 48Al (1a and 1b). Therefore, Ti– 52Al showed lower corrosion rates and scale spallation tendency than Ti– 48Al. Ti –48Al – 2Cr also has relatively good hot corrosion behaviour, especially for the suspended samples, implying that Cr can improve the high temperature corrosion resistance of Ti –Al intermetallics. According to Brady et al [23,24], the substitution of Cr for Ti in sufficient quantities forms Laves phase Ti(Cr,Al)2, which exhibits a low oxygen permeability and is capable to form a protective Al2O3 scale. This is probably the reason that Ti– 48Cr – 2Cr has better corrosion resistance than Ti –48Al.
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Fig. 9. Hot corrosion kinetic curves for the specimens immersed in the molten salt. The alloy numbers are the same as above.
With respect to the different hot corrosion methods, the samples that were immersed in the molten salts showed the highest corrosion rates. The scales were porous; and the direct reactions between the scale and molten salts were severe. The specimens that were suspended in the salt vapour had the lowest corrosion rates, with mass gains smaller than 2 mg/cm2 after the 200-h exposure. The reaction kinetics measured by this method was also more reproducible as the corrosion conditions were easy to control. The hot corrosion with deposited salts represents corrosion caused by one-time severe contamination. It had a reaction rate between the other two methods. The amounts of the deposited and spalled salts have a strong influence on the measured kinetics. However, the corrosion behaviour of materials can be characterised with a relatively long-time exposure, in which the influence of salt-film spallation at the initial stage is less significant.
5. Conclusions (1) Three Ti – Al intermetallics, Ti – 48Al, Ti – 48Al – 2Cr, Ti – 52Al, were exposed in 80 wt.% Na2SO4 + 20 wt.% NaCl containing environments at 800 jC. Hot corrosion and scale spallation took place during the 200-h tests. Ti – 52Al showed the best corrosion resistance among the three alloys. It is believed that the high Al content and single g-phase played a role, forming an Al-rich oxide scale with relatively good protective ability. Ti –48Al – 2Cr also
showed better corrosion resistance than Ti– 48Al. The latter suffered high corrosion rate and severe scale spallation. (2) Three different exposure methods were used to conduct hot corrosion in salt-containing environment. They represent different service conditions. Among these methods, suspending the specimens in salt vapour appeared to be the most reliable method for kinetic studies. The influential factors are relatively easy to be controlled. When use salt-deposition method, the spallation of the salt film at the early stage should be considered. The immersion method showed the severest hot corrosion. Crucible materials and corrosion products dissolve into the molten salts, further complicating the reactions.
Acknowledgements The authors would like to thank Dr. Quadakkers for providing Ti– 48Al –2Cr samples, and all technical staff at the Department and Research Centre for Surface and Materials Science for their assistances.
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