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Hot corrosion resistance of electrospark-deposited Al and Ni Cr coatings containing dispersed Y2O3 particles
Materials Letters 58 (2004) 3280 – 3284 www.elsevier.com/locate/matlet
Hot corrosion resistance of electrospark-deposited Al and Ni Cr coatings containing dispersed Y2O3 particles J. Lianga, W. Gaoa,*, Z. Lia, Y. Heb a
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Aukland, New Zealand b The University of Science and Technology Beijing, China Received 14 June 2004; accepted 20 June 2004 Available online 20 July 2004
Abstract A novel electrospark deposition technique has been developed to produce high-temperature coatings. Al and Ni–20Cr were used as coating materials. Dispersed Y2O3 particles were added into the coating layer. Hot corrosion tests at 900 8C in a vapour of mixed salts of Na2SO4+10% NaCl indicated that both NiCr and Al coatings improved the corrosion resistance of the substrates. Scanning electron microscopy (SEM) observation, combined with energy-dispersive X-ray (EDX) analysis, indicated that a thick layer of Al2O3 still existed after hot corrosion exposure. Elemental mapping analysis showed that the dispersed Y2O3 particles form a layer inside the Al2O3 scale, which retards the outward diffusion of the substrate elements and promotes the adhesion of Al2O3 layer onto the coating surface. D 2004 Elsevier B.V. All rights reserved. Keywords: Electrospark deposition; Molten salt vapour corrosion; Oxide dispersive alloy coating; Dispersive Y2O3
1. Introduction It is known that high-temperature corrosion failure of an alloy often results from failure of its protective oxide scale. Yttria (Y2O3) phase, when being finely dispersed in an alloy, can promote the formation of protective oxide scale and increase its adhesion effectively [1,2]. Oxide dispersionstrengthened (ODS) alloys have been developed success-
fully in the last 20 years. Studies have also been conducted to develop ODS alloy coatings by employing various coating techniques. For instance, coatings with rare earth oxide dispersion were produced by electroplating followed by aluminising [3]. NiCrAl coatings containing various dispersed oxides (MgO, La2O3, and Y2O3) were produced by low-pressure plasma spraying (LPPS) [1]. Y–Al oxides coatings were made by EB-PVD [4]. An overlay alloy
Table 1 Nominal composition of the materials used (wt.%) Materials
Fe
Cr
Ni
Si
Mn
AISI 304 AISI 310 AISI 430 Inconel 600 NiCr (NICHROME 80) Al
Bal Bal Bal 8.0 V5.00 –
18.0–20.0 24.0–26.0 16.0–18.0 15.5 18–21 –
8–10 19–22 – Bal Bal –
1.0 1.5 1.0 – V1.00 –
2.0 2.0 1.0 –
* Corresponding author. Tel.: +64 9 3737599; fax: +64 9 3737463. E-mail address: [email protected] (W. Gao). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.06.018
–
Al
– 0.5–1.8 99.9
J. Liang et al. / Materials Letters 58 (2004) 3280–3284
3281
coating with dispersed Y2O3 phase was achieved through processes combined with electrophoretic deposition and pack cementation [5]. We have developed two novel electrospark deposition (EPD) techniques to produce Ni–Cr-based ODS coatings [6,7]. In both cases, the mechanically alloyed MGH754 (Ni–20Cr–Y2O3) was used as the depositing electrode, and microcrystalline structure was obtained in the coatings. The high-temperature oxidation tests showed that the formation of Cr2O3 scale has been promoted, and the scale spallation resistance was greatly improved. In the present paper, an in situ formation of alloy coatings with Y2O3 dispersion produced by EPD is studied. The hot corrosion resistance of the coating is evaluated by comparing with coatings without Y2O3 dispersion.
2. Experimental Fig. 1. AFM micrograph showing that the grain size of Al coating is ~50 nm.
Commercial AISI 304, 310, and 430 stainless steel and Inconel 600 alloy were selected as the substrate materials. The commercial pure Al plate and Ni20Cr alloy were used
Fig. 2. SEM image of a cross-section of Al–Y2O3 coating on AISI 304 stainless steel after etching with Marble’s reagent (10 g of CuSO4+50 ml of HCl+50 ml of water). (a) Coating morphology; (b) high-magnification morphology of area A in the cross-section; and (c) high-magnification morphology of area B.
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as coating materials. The nominal compositions of all materials used are listed in Table 1. To produce coatings containing dispersed Y2O3 particles, fine Y2O3 (99.9%) powder with particle size ~100 nm was spread onto on the surface of the specimen to be coated. Powder spreading was carried out for four times during the five-run coverage deposition on each side of the specimen. Surface morphologies of the coatings were observed using a high-resolution scanning electron microscopy (SEM) (Philips XL30 FEG) and an atomic force microscope (Digital Instruments Nano Scope IIIa). The phases of the coatings were identified using X-ray diffraction (XRD; Phillips PW1050/25 with Co Ka radiation). The thickness of the coatings was measured using an optical microscope (Olympus BX 60M). Isothermal hot corrosion tests at 900 8C, in the presence of Na2SO4/NaCl deposits, were performed on all coatings. The specimens were preheated at about 90 8C on a hot plate. Na2SO4+10% NaCl were applied to the specimens by brushing an aqueous solution of the salts onto the hot samples. A layer of salt, 2.5–3.0 mg/cm2, was deposited on the specimen surface. The specimens were held in alumina crucibles individually during the tests. The exposure time
was selected as 5, 10, 20, 30, and 50 h, after which the specimens were withdrawn and weighed together with the crucible. Mass gains of the specimens were measured using an electronic balance with an accuracy of 0.01 mg. The morphology and composition of the sample surfaces and cross-sections after hot corrosion were examined using SEM and energy-dispersive X-ray (EDX) elemental mapping. The corrosion products were also characterised by using XRD.
3. Results Observation of the coating gain size using atomic force microscopy (AFM) is shown in Fig. 1, indicating that the grain size is around 50 nm. The interface between Al coating and substrate was not clear when observed directly from a polished cross-section. Metallographic etching with Marble’s reagent (10 g of CuSO4+50 ml of HCl+50 ml of H2O) was therefore employed to reveal the boundary between the coating and the base metal. The SEM observation of etched Al coating provided more detailed information about the coating structure, as shown in Fig. 2. The coating consists two sublayers. The outside layer
Fig. 3. Comparison of hot corrosion kinetics for the coatings with and without Y2O3 addition on different alloys after exposure at 900 8C for 50 h: (a) NiCr coatings on AISI 310; (b) Al coatings on AISI 304; (c) Al coatings on AISI 310; and (d) Al coatings on Inconel 600.
J. Liang et al. / Materials Letters 58 (2004) 3280–3284
consists of mainly Al2O3, which maintains a smooth surface due to its good resistance to the corrosive Marble’s reagent. Underneath the Al2O3 is a layer of mixed FeAl, Fe3Al, NiAl, and Ni3Al alloys, which was confirmed by XRD analysis. These intermetallic alloys are vulnerable to the Marble’s reagent, therefore exhibiting a typical corroded morphology. The gaps and holes between the two layers are believed to be due to the spallation of the brittle phases when the surrounding phases are etched away. Interestingly, Al coating with Y2O3 addition showed that, among the Al2O3 phase, there is an evenly dispersed particle phase, which was identified as Y2O3 by EDX and shown in Fig. 2(b). On the contrary, no Y2O3 particle was found in the rest of the coating zone (Fig. 2(c)). To further confirm the coating microstructure observed by SEM, elemental mapping for the Al coating with Y2O3 addition was performed. The mapping indicates that Y element only exists in the outside layer, which is enriched
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with Al and O, while the underneath layer shows the presence of Fe, Cr, Ni, and Al, but very little O and Y. Fig. 3 shows the comparison of mass changes for the coatings with and without Y2O3 addition. The kinetic curves of the uncoated alloys were also plotted for comparison. It can be seen that all coatings with Y2O3 addition showed improvement in hot corrosion resistance after 50 h of exposure. Fig. 4 shows the elemental mapping for the Al coating with Y2O3 addition on Inconel 600 after hot corrosion for 50 h. A three-layer structure can be seen in the cross-section scale. Similar to the Al–Y2O3-coated stainless steel, the outer layer was enriched with Al, O, and Y, indicating that Al2O3 with dispersed Y2O3 particles dominated this layer. The middle layer, however, was enriched with Ni and Al, indicating that NiAl phase formed in this layer. A Cr-rich zone formed on the inner layer, implying that the outward diffusion of Ni resulted in the counterdiffusion of Cr.
Fig. 4. Elemental mapping for Al–Y2O3 coating on Inconel 600 after hot corrosion at 900 8C for 50 h.
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4. Discussion
5. Conclusion
It was known that yttria (Y2O3) is thermodynamically stable. The effect of dispersed Y2O3 phase on the oxidation behaviour of metals has been studied by Stringer et al. [8]. It was proposed that the dispersed particles act as heterogeneous nucleation sites for oxide grains, thereby reducing the internuclear distance, allowing more rapid formation of a continuous Cr oxide film with a finer grain size. Therefore, the scale adhesion can be improved. Li and Ma [9] proposed that the beneficial effects of Y2O3 on reducing the oxidation rates of the composite coatings could arise in two ways. Firstly, the protective property of a scale is enhanced by the effect of Y2O3 on the rapid establishment of scale—the improvement of integrity and adhesion of the scale. Secondly, the dispersed Y2O3 phase stabilizes the microstructure of NiAl coating, thus reducing the degradation rate of the coatings. He and Stott [10] studied the beneficial effects of Al2O3–Y2O3 film on the selective oxidation of Cr in Ni–10Cr alloy. They found that the film accelerated the short-circuit diffusion and reduced the critical content of Cr in the alloy that is required for the formation of a chromia scale. Our observations in the present work generally support these mechanisms. On the other hand, a finding reported by Quadakkers et al. [11] was that Y2O3 incorporated into Al2O3 films, retarding the diffusion of Mn and Cr within the film. They proposed that anion diffusion prevails over cation diffusion in the oxide scale of ODS alloys. This mechanism has been supported by the study of Ikeda et al. [12], who also confirmed that the adhesion of Al2O3 coating could be promoted by the dispersed Y2O3 phase. In the present study, the beneficial effect of Y2O3 addition can be attributed to the following mechanisms: Y2O3 promoted the nucleation and formation of Al2O3 in the Al–Y2O3 coating layer. For the NiCr coating, the dispersed Y2O3 particles promoted selective oxidation to form dense Cr2O3 film with fine grain structure. The oxide layer with fine grain size can then easily release the thermal stress, therefore preventing crack propagation in the Al2O3 layer [4]. Besides, the Y2O3 particles could play a role of retarding the outward diffusion of Fe, Ni, and Cr [13]. Thus, the effect of synergistic dissolution could be minimised. In addition, the inward diffusion of sulphur could be retarded by the trapping effect contributed by Y2O3 incorporated in the outer layer of the coating. This is supported by the elemental mapping obtained on the cross-section of corroded specimens (Fig. 4).
EPD technique was used to produce alloy coatings on stainless steels and Ni-based alloys. A dispersed Y2O3 particle phase was added into the coating layer. AFM observation showed that the grain size of the coating was around 50 nm. Optical microscopy showed that the thickness of the coating was about 50 Am. Hot corrosion tests at 900 8C with mixed salts of Na2SO4+10% NaCl indicated that the coatings containing dispersed Y2O3 have significantly improved hot corrosion resistance. EDX analysis indicated that the scales formed on the Y2O3-dispersed coatings have much lower contents of the substrate elements Fe and Cr than the coatings without Y2O3 addition. It is therefore proposed that Y2O3 particles retarded the outward diffusion of Fe and Cr, and improved the adhesion of Al2O3 layer on the coating surface.
Acknowledgements J. Liang would like to thank the Postdoctoral Fellowship from the New Zealand Government (FRST). The authors appreciate the help from the Department of Chemical and Materials Engineering, and the Research Centre for Surface and Materials Science.
References [1] K.L. Luthra, E.L. Hall, Oxidation of Metals 26 (5/6) (1986) 385. [2] L. Zhou, R. Ye, S. Zhang, L. Gao, Corrosion Science 32 (3) (1991) 337 – 346. [3] T. Li, X. Ma, in International Symposium on High Temperature Corrosion and Protection. 1990. Shenyang, China. [4] J.P. Kim, H.G. Jung, K.Y. Kim, Surface and Coatings Technology 112 (1999) 91 – 97. [5] X. Lu, R. Zhu, Y. He, Oxidation of Metals 43 (1995) 353. [6] Y. He, Z. Huang, H. Qi, D. Wang, Z. Li, W. Gao, Materials Letters 45 (2) (2000) 79 – 85. [7] H. Qi, H. Pang, Y. He, W. Gao, in International Symposium on High Temperature Corrosion and Protection 2000. 2000. Hokkaido, Japan. [8] J. Stringer, B.A. Wilcox, R.I. Jaffee, Oxidation of Metals 5 (1972) 11. [9] T. Li, X. Ma, Journal of Rare Earths 10 (1992) 121. [10] Y. He, F.H. Stott, Corrosion Science 38 (1996) 1853. [11] W.J. Quadakkers, H. Halzbrechen, K.G. Brief, H. Beske, Oxidation of Metals 32 (1989) 67. [12] Y. Ikeda, K. Nii, M. Yata, ISIJ International 33 (1993) 298. [13] R.A. Rapp, Corrosion Science 44 (2002) 209.
Hot corrosion resistance of electrospark-deposited Al and Ni Cr coatings containing dispersed Y2O3 particles J. Lianga, W. Gaoa,*, Z. Lia, Y. Heb a
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Aukland, New Zealand b The University of Science and Technology Beijing, China Received 14 June 2004; accepted 20 June 2004 Available online 20 July 2004
Abstract A novel electrospark deposition technique has been developed to produce high-temperature coatings. Al and Ni–20Cr were used as coating materials. Dispersed Y2O3 particles were added into the coating layer. Hot corrosion tests at 900 8C in a vapour of mixed salts of Na2SO4+10% NaCl indicated that both NiCr and Al coatings improved the corrosion resistance of the substrates. Scanning electron microscopy (SEM) observation, combined with energy-dispersive X-ray (EDX) analysis, indicated that a thick layer of Al2O3 still existed after hot corrosion exposure. Elemental mapping analysis showed that the dispersed Y2O3 particles form a layer inside the Al2O3 scale, which retards the outward diffusion of the substrate elements and promotes the adhesion of Al2O3 layer onto the coating surface. D 2004 Elsevier B.V. All rights reserved. Keywords: Electrospark deposition; Molten salt vapour corrosion; Oxide dispersive alloy coating; Dispersive Y2O3
1. Introduction It is known that high-temperature corrosion failure of an alloy often results from failure of its protective oxide scale. Yttria (Y2O3) phase, when being finely dispersed in an alloy, can promote the formation of protective oxide scale and increase its adhesion effectively [1,2]. Oxide dispersionstrengthened (ODS) alloys have been developed success-
fully in the last 20 years. Studies have also been conducted to develop ODS alloy coatings by employing various coating techniques. For instance, coatings with rare earth oxide dispersion were produced by electroplating followed by aluminising [3]. NiCrAl coatings containing various dispersed oxides (MgO, La2O3, and Y2O3) were produced by low-pressure plasma spraying (LPPS) [1]. Y–Al oxides coatings were made by EB-PVD [4]. An overlay alloy
Table 1 Nominal composition of the materials used (wt.%) Materials
Fe
Cr
Ni
Si
Mn
AISI 304 AISI 310 AISI 430 Inconel 600 NiCr (NICHROME 80) Al
Bal Bal Bal 8.0 V5.00 –
18.0–20.0 24.0–26.0 16.0–18.0 15.5 18–21 –
8–10 19–22 – Bal Bal –
1.0 1.5 1.0 – V1.00 –
2.0 2.0 1.0 –
* Corresponding author. Tel.: +64 9 3737599; fax: +64 9 3737463. E-mail address: [email protected] (W. Gao). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.06.018
–
Al
– 0.5–1.8 99.9
J. Liang et al. / Materials Letters 58 (2004) 3280–3284
3281
coating with dispersed Y2O3 phase was achieved through processes combined with electrophoretic deposition and pack cementation [5]. We have developed two novel electrospark deposition (EPD) techniques to produce Ni–Cr-based ODS coatings [6,7]. In both cases, the mechanically alloyed MGH754 (Ni–20Cr–Y2O3) was used as the depositing electrode, and microcrystalline structure was obtained in the coatings. The high-temperature oxidation tests showed that the formation of Cr2O3 scale has been promoted, and the scale spallation resistance was greatly improved. In the present paper, an in situ formation of alloy coatings with Y2O3 dispersion produced by EPD is studied. The hot corrosion resistance of the coating is evaluated by comparing with coatings without Y2O3 dispersion.
2. Experimental Fig. 1. AFM micrograph showing that the grain size of Al coating is ~50 nm.
Commercial AISI 304, 310, and 430 stainless steel and Inconel 600 alloy were selected as the substrate materials. The commercial pure Al plate and Ni20Cr alloy were used
Fig. 2. SEM image of a cross-section of Al–Y2O3 coating on AISI 304 stainless steel after etching with Marble’s reagent (10 g of CuSO4+50 ml of HCl+50 ml of water). (a) Coating morphology; (b) high-magnification morphology of area A in the cross-section; and (c) high-magnification morphology of area B.
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J. Liang et al. / Materials Letters 58 (2004) 3280–3284
as coating materials. The nominal compositions of all materials used are listed in Table 1. To produce coatings containing dispersed Y2O3 particles, fine Y2O3 (99.9%) powder with particle size ~100 nm was spread onto on the surface of the specimen to be coated. Powder spreading was carried out for four times during the five-run coverage deposition on each side of the specimen. Surface morphologies of the coatings were observed using a high-resolution scanning electron microscopy (SEM) (Philips XL30 FEG) and an atomic force microscope (Digital Instruments Nano Scope IIIa). The phases of the coatings were identified using X-ray diffraction (XRD; Phillips PW1050/25 with Co Ka radiation). The thickness of the coatings was measured using an optical microscope (Olympus BX 60M). Isothermal hot corrosion tests at 900 8C, in the presence of Na2SO4/NaCl deposits, were performed on all coatings. The specimens were preheated at about 90 8C on a hot plate. Na2SO4+10% NaCl were applied to the specimens by brushing an aqueous solution of the salts onto the hot samples. A layer of salt, 2.5–3.0 mg/cm2, was deposited on the specimen surface. The specimens were held in alumina crucibles individually during the tests. The exposure time
was selected as 5, 10, 20, 30, and 50 h, after which the specimens were withdrawn and weighed together with the crucible. Mass gains of the specimens were measured using an electronic balance with an accuracy of 0.01 mg. The morphology and composition of the sample surfaces and cross-sections after hot corrosion were examined using SEM and energy-dispersive X-ray (EDX) elemental mapping. The corrosion products were also characterised by using XRD.
3. Results Observation of the coating gain size using atomic force microscopy (AFM) is shown in Fig. 1, indicating that the grain size is around 50 nm. The interface between Al coating and substrate was not clear when observed directly from a polished cross-section. Metallographic etching with Marble’s reagent (10 g of CuSO4+50 ml of HCl+50 ml of H2O) was therefore employed to reveal the boundary between the coating and the base metal. The SEM observation of etched Al coating provided more detailed information about the coating structure, as shown in Fig. 2. The coating consists two sublayers. The outside layer
Fig. 3. Comparison of hot corrosion kinetics for the coatings with and without Y2O3 addition on different alloys after exposure at 900 8C for 50 h: (a) NiCr coatings on AISI 310; (b) Al coatings on AISI 304; (c) Al coatings on AISI 310; and (d) Al coatings on Inconel 600.
J. Liang et al. / Materials Letters 58 (2004) 3280–3284
consists of mainly Al2O3, which maintains a smooth surface due to its good resistance to the corrosive Marble’s reagent. Underneath the Al2O3 is a layer of mixed FeAl, Fe3Al, NiAl, and Ni3Al alloys, which was confirmed by XRD analysis. These intermetallic alloys are vulnerable to the Marble’s reagent, therefore exhibiting a typical corroded morphology. The gaps and holes between the two layers are believed to be due to the spallation of the brittle phases when the surrounding phases are etched away. Interestingly, Al coating with Y2O3 addition showed that, among the Al2O3 phase, there is an evenly dispersed particle phase, which was identified as Y2O3 by EDX and shown in Fig. 2(b). On the contrary, no Y2O3 particle was found in the rest of the coating zone (Fig. 2(c)). To further confirm the coating microstructure observed by SEM, elemental mapping for the Al coating with Y2O3 addition was performed. The mapping indicates that Y element only exists in the outside layer, which is enriched
3283
with Al and O, while the underneath layer shows the presence of Fe, Cr, Ni, and Al, but very little O and Y. Fig. 3 shows the comparison of mass changes for the coatings with and without Y2O3 addition. The kinetic curves of the uncoated alloys were also plotted for comparison. It can be seen that all coatings with Y2O3 addition showed improvement in hot corrosion resistance after 50 h of exposure. Fig. 4 shows the elemental mapping for the Al coating with Y2O3 addition on Inconel 600 after hot corrosion for 50 h. A three-layer structure can be seen in the cross-section scale. Similar to the Al–Y2O3-coated stainless steel, the outer layer was enriched with Al, O, and Y, indicating that Al2O3 with dispersed Y2O3 particles dominated this layer. The middle layer, however, was enriched with Ni and Al, indicating that NiAl phase formed in this layer. A Cr-rich zone formed on the inner layer, implying that the outward diffusion of Ni resulted in the counterdiffusion of Cr.
Fig. 4. Elemental mapping for Al–Y2O3 coating on Inconel 600 after hot corrosion at 900 8C for 50 h.
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4. Discussion
5. Conclusion
It was known that yttria (Y2O3) is thermodynamically stable. The effect of dispersed Y2O3 phase on the oxidation behaviour of metals has been studied by Stringer et al. [8]. It was proposed that the dispersed particles act as heterogeneous nucleation sites for oxide grains, thereby reducing the internuclear distance, allowing more rapid formation of a continuous Cr oxide film with a finer grain size. Therefore, the scale adhesion can be improved. Li and Ma [9] proposed that the beneficial effects of Y2O3 on reducing the oxidation rates of the composite coatings could arise in two ways. Firstly, the protective property of a scale is enhanced by the effect of Y2O3 on the rapid establishment of scale—the improvement of integrity and adhesion of the scale. Secondly, the dispersed Y2O3 phase stabilizes the microstructure of NiAl coating, thus reducing the degradation rate of the coatings. He and Stott [10] studied the beneficial effects of Al2O3–Y2O3 film on the selective oxidation of Cr in Ni–10Cr alloy. They found that the film accelerated the short-circuit diffusion and reduced the critical content of Cr in the alloy that is required for the formation of a chromia scale. Our observations in the present work generally support these mechanisms. On the other hand, a finding reported by Quadakkers et al. [11] was that Y2O3 incorporated into Al2O3 films, retarding the diffusion of Mn and Cr within the film. They proposed that anion diffusion prevails over cation diffusion in the oxide scale of ODS alloys. This mechanism has been supported by the study of Ikeda et al. [12], who also confirmed that the adhesion of Al2O3 coating could be promoted by the dispersed Y2O3 phase. In the present study, the beneficial effect of Y2O3 addition can be attributed to the following mechanisms: Y2O3 promoted the nucleation and formation of Al2O3 in the Al–Y2O3 coating layer. For the NiCr coating, the dispersed Y2O3 particles promoted selective oxidation to form dense Cr2O3 film with fine grain structure. The oxide layer with fine grain size can then easily release the thermal stress, therefore preventing crack propagation in the Al2O3 layer [4]. Besides, the Y2O3 particles could play a role of retarding the outward diffusion of Fe, Ni, and Cr [13]. Thus, the effect of synergistic dissolution could be minimised. In addition, the inward diffusion of sulphur could be retarded by the trapping effect contributed by Y2O3 incorporated in the outer layer of the coating. This is supported by the elemental mapping obtained on the cross-section of corroded specimens (Fig. 4).
EPD technique was used to produce alloy coatings on stainless steels and Ni-based alloys. A dispersed Y2O3 particle phase was added into the coating layer. AFM observation showed that the grain size of the coating was around 50 nm. Optical microscopy showed that the thickness of the coating was about 50 Am. Hot corrosion tests at 900 8C with mixed salts of Na2SO4+10% NaCl indicated that the coatings containing dispersed Y2O3 have significantly improved hot corrosion resistance. EDX analysis indicated that the scales formed on the Y2O3-dispersed coatings have much lower contents of the substrate elements Fe and Cr than the coatings without Y2O3 addition. It is therefore proposed that Y2O3 particles retarded the outward diffusion of Fe and Cr, and improved the adhesion of Al2O3 layer on the coating surface.
Acknowledgements J. Liang would like to thank the Postdoctoral Fellowship from the New Zealand Government (FRST). The authors appreciate the help from the Department of Chemical and Materials Engineering, and the Research Centre for Surface and Materials Science.
References [1] K.L. Luthra, E.L. Hall, Oxidation of Metals 26 (5/6) (1986) 385. [2] L. Zhou, R. Ye, S. Zhang, L. Gao, Corrosion Science 32 (3) (1991) 337 – 346. [3] T. Li, X. Ma, in International Symposium on High Temperature Corrosion and Protection. 1990. Shenyang, China. [4] J.P. Kim, H.G. Jung, K.Y. Kim, Surface and Coatings Technology 112 (1999) 91 – 97. [5] X. Lu, R. Zhu, Y. He, Oxidation of Metals 43 (1995) 353. [6] Y. He, Z. Huang, H. Qi, D. Wang, Z. Li, W. Gao, Materials Letters 45 (2) (2000) 79 – 85. [7] H. Qi, H. Pang, Y. He, W. Gao, in International Symposium on High Temperature Corrosion and Protection 2000. 2000. Hokkaido, Japan. [8] J. Stringer, B.A. Wilcox, R.I. Jaffee, Oxidation of Metals 5 (1972) 11. [9] T. Li, X. Ma, Journal of Rare Earths 10 (1992) 121. [10] Y. He, F.H. Stott, Corrosion Science 38 (1996) 1853. [11] W.J. Quadakkers, H. Halzbrechen, K.G. Brief, H. Beske, Oxidation of Metals 32 (1989) 67. [12] Y. Ikeda, K. Nii, M. Yata, ISIJ International 33 (1993) 298. [13] R.A. Rapp, Corrosion Science 44 (2002) 209.