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Corrosion Science 50 (2008) 15–22 www.elsevier.com/locate/corsci

Synergistic corrosion behavior of coated Ti60 alloys with NaCl deposit in moist air at elevated temperature Yuming Xiong *, Shenglong Zhu, Fuhui Wang * State Key Laboratory for Corrosion and Protection, The Chinese Academy of Sciences, Wencui Road 62, Shenyang 110016, China Received 2 December 2005; accepted 27 June 2007 Available online 18 July 2007

Abstract In the present paper, the corrosion behavior of Ti60 alloys with an aluminide, TiAlCr, and enamel coatings in moist air containing NaCl vapor at 700–800 °C were studied. The results showed that the TiAlCr and aluminide coatings failed to protect the substrate from corrosion due to the cyclic formation of volatile products during corrosion at 800 °C. However, an uneven continuous protective Al2O3 scale could form on the aluminide coating during corrosion at 700 °C. And the enamel coating could protect Ti60 from corrosion due to its high thermochemical stability and matched thermal expansion coefficient with substrates of Ti-base alloys during corrosion. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Titanium; A. Metal coatings; B. X-ray diffraction; C. High temperature corrosion; C. Oxide coatings

1. Introduction It is known that continuous protective Al2O3 scale forms on aluminide (mainly composed of the TiAl3 phase) and nanocrystalline Ti–Al–Cr coatings during oxidation in dry air or oxygen at high temperatures [1–9]. So far, the beneficial effect of Cr on the formation of Al2O3 on Tibased alloys oxidized in dry air is widely accepted. However, several reporters considered that water vapor was detrimental to the oxidation resistance of TiAl-based alloys [10–12]. H2O tends to dissociate at the defects of TiO2 into a free H atom and OH groups when titanium alloys are oxidized in moist air at high temperatures. The anisotropic diffusion of hydrogen in TiO2 was considered as the main reason for the rapid formation of TiO2, which decreased the oxidation resistance of TiAl-based alloys. In addition, the formation of volatile species, such as CrO3 (g), CrO2OH (g) or CrO2(OH)2 (g) results in chromium depletion in alloys and oxide scales to degrade the oxidation *

Corresponding authors. Tel: +86 24 23904856; fax: +86 24 23893624 (Y. Xiong). E-mail addresses: [email protected] (Y. Xiong), [email protected] (F. Wang). 0010-938X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.06.007

resistance [13–17]. Generally, for the coastal applications of titanium alloys, chloride salts might deposit on the surface and accelerate the degradation of titanium alloys. Hence, the corrosion behavior of materials became complex at the service condition containing both water vapor and Cl anion. Wang et al. indicated that the reaction between chromium and Cl could reduce the corrosion resistance of alloys due to cyclic formation of volatile CrCl3 during corrosion of Cl at high temperatures [18]. A recent study indicated that enamels are good candidates for high temperature coating of Ti-based alloys, due to their high thermochemical stability and good compatibility with the substrate. Moreover, they are economically more attractive than coatings produced by means of PVD process [19]. In the present paper, the synergistic corrosion behavior of Ti60 alloys without and with enamel, sputtered nanocrystalline TiAlCr, and aluminide coatings in humid air containing NaCl vapor at 700–800 °C was evaluated. 2. Experimental procedure Ti60 is an a + a2 titanium alloy containing 5.6Al, 4.8Sn, 2Zr, 1Mo, 1Nd and 0.32Si (wt%). Alloy ingots were cut

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into 16  8  2.5 mm specimens and ground down to 600#-SiC paper. The enamel frit (SiO2 58.2, Al2O3 6.3, ZrO2 5.3, ZnO 9.0, CaO 4.1, and others 17.0, wt%) fabrication and the detailed coating preparation method were described elsewhere [19,20]. The frit was milled for ultrafine to decrease its vitrifying temperature. An ultrafine enamel frit layer formed on round angle alloys after sand-blasting by an air spray technology at room temperature. Then the coating was obtained by vitrifying for 45 min in air at 900 °C. For comparison, an Ti–35.45Al–20.05Cr (at.%) coating was prepared by magnetron-sputtering technique [2], and an aluminide coating was prepared at 850 °C for 3.5 h by means of a traditional pack method [21]. The thickness of the three coatings is about 30 lm. Corrosion tests were carried out in a thermal balance. NaCl was coated on the preheated specimen surface by brushing a saturated solution of NaCl and then drying. The thickness of NaCl scale on specimens keeps about 2.4 mg/cm2 in terms of the difference in weight of specimens before and after coating. The corrosion atmosphere (O2 + water vapor) was obtained by passing pure oxygen through distilled water by means of a glass bubbler. The distilled water was heated to boiling by recycling the water that came from a water bath. The concentration of water vapor was controlled by adjusting the temperature of the water bath. In this case, the concentration of water vapor is 12.2 vol.% (corresponding temperature of water bath is about 50 °C). A counterflow gas (N2) was used to prevent water vapor from condensing on the measuring system of the thermal balance. The protecting gas (N2) and carrier gas (O2) flows are 400 and 60 ml/min, respectively. After the furnace temperature was stabilized at a constant, the specimen was quickly suspended in the furnace tube. The corrosion kinetics of specimens were obtained by an automatic recorder connected with the thermal balance. The experimental setup and process were interpreted in detail elsewhere [22]. The surface morphologies and phases of samples before and after corrosion were characterized by means of scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The specimens were also mounted in epoxy resins after electroless plating of nickel for protection from

mechanical grounding and polishing, cross-sectioned, and then polished to reveal their microstructures.

3. Results 3.1. The microstructures of coatings The enamel frit particles before vitrifying have been milled to about 300 nm (see Fig. 1). The enamel frit layer on Ti60 alloys was vitrified at 900 °C, which is decreased about 150 °C in comparison to the conventional one (20 lm) with the same composition [23]. The cross-sectional microstructures of ultrafine and conventional as-vitrified enamel coatings are shown in Fig. 2. The ultrafine enamel coating was compact and adhesive to the substrate. However, besides a few air bubbles remain in the conventional enamel coating, there were still some discontinuous pores at the interface of coating/substrate after vitrifying [23]. The cross-sectional microstructures of as-depositedTiAlCr and aluminide coatings on Ti60 alloys are shown in Fig. 3. A uniform, compact and adhesive TiAlCr nanocrystalline coating was obtained by means of magnetronsputtering. The coating shows typically columnar grain characteristics. EDS analysis shows that the composition of TiAlCr coating is Ti–41.3Al–18.2Cr (at.%) which is slightly different from the initial target composition. It may be due to the difference in sputterability of each component phase in target material. According to EDS

Fig. 1. The distributing of enamel frit particles after milling 300 h.

Fig. 2. The cross-sectional microstructures of as-vitrified conventional (a) [17] and ultrafine (b) enamel coatings on Ti60 alloys.

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Fig. 3. The cross-sectional microstructures of TiAlCr (a) and aluminide (b) coatings on Ti60 alloys before corrosion.

the aluminide coating implies the brittleness of TiAl3 phase. 3.2. The synergistic corrosion in solid NaCl with water vapor

Fig. 4. SEM micrograph of initial TiAlCr target materials.

analysis, the composition of white and gray phases in TiAlCr target (A and B marked in Fig. 4) is Ti–42Al–27Cr, and Ti–57Al–7Cr (at.%), respectively. X-ray diffraction results reveal that the aluminide coating is mainly composed of TiAl3 phase. In addition, trace amounts of Al2O3 could be detected in aluminide coating, whilst the oxides of titanium are undetectable although there still exist some unknown peaks in the pattern (Fig. 5). In terms of SEM micrograph (Fig. 3b), the formation of some cracks in

Fig. 6 shows the corrosion kinetics of coated and uncoated Ti60 in moist air containing NaCl vapor at 700–800 °C. It is shown that only the enamel coating among the three coatings could reduce the corrosion mass gains of Ti60 alloys at the two temperatures. The influence of temperature on the mass gain of enamel coated alloy can hardly be detected. However, the breakaway corrosion of TiAlCr coated Ti60 indicates that its mass gain is more than that of bare Ti60 alloy and other coated alloys at the both temperatures. After rapid initial corrosion for about 1 h at 700 °C, the kinetics of aluminide coated Ti60 alloy shows approximately horizontal linear law, although its mass gain is more than that of bare alloy during the 10 h corrosion. Interestingly, the corrosion mass gain can be almost invisible for aluminide coated alloy at an initial stage for about 1 h at 800 °C, whereafter it shows a continuous negative mass gain. Aluminide coating Unknown Al2O3 TiAl3

Corrosion at 700 °C Al2O3 NaCl NaTiO2

2

3

4

5

d (A)

Fig. 5. XRD patterns of aluminide coating before and after corrosion at 700 °C.

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a 14

b 25 Mass Change (mg/cm2)

Mass Change (mg/cm2)

12 10 Ti60+TiAlCr

8 6 Ti60+Aluminide

4 2

Ti60

0

2

4

6

8

Ti60+TiAlCr 15 10 5

Ti60+Aluminide

Ti60+Enamel

0

Ti60+enamel

0

Ti60 20

10

Corrosion Time (hrs)

0

2

4

6

8

10

Corrosion Time (hrs)

Fig. 6. The kinetics of Ti60 alloys with or without coatings corroded in solid NaCl with water vapor at 700 °C (a) to 800 °C (b) in air.

The cross-sectional microstructures of Ti60 alloys after corrosion for 10 h in humid air containing NaCl vapor at 700 °C are shown in Fig. 7. It is seen that thick and porous oxide scales form on the bare Ti60 alloy after corrosion. The scales can be divided into four layers. The EDS results show that the external oxide A is mainly composed of TiO2, followed by oxygen-rich titanium oxides layer (white region B), C layer is the mixture of TiN, SnO2, TiO2, ZrO2 and Al2O3, followed by a porous Ti depletion layer (D region) resulting from its outward diffusion. An uneven continuous scale forms on the aluminide coating after corrosion for 10 h at 700 °C. XRD results reveal that the scale is mainly composed of Al2O3 (as shown in Fig. 5). The corrosion product (NaTiO2) of titanium and a small amount of Cl could be also detected. The TiAlCr coating almost flakes from the substrate due to heavy corrosion. The

EDS analysis shows that the residual corrosion products on the coating are mostly composed of TiO2 and Cr2O3. However, the enamel coating remains uniform and intact after corrosion. No interfacial corrosion products and diffusion can be detected. Fig. 8 shows surface morphologies of bare Ti60 and enamel coated alloys after corrosion at 800 °C. Typical rutile TiO2 forms on bare alloy after corrosion at 800 °C. But the enamel coating still remains even after corrosion for 10 h. Fig. 9 shows cross-sectional microstructures of examined NaCl coated Ti60 alloys after corrosion for 10 h in moist air at 800 °C. For the bare alloys, the surface corrosive scale at 800 °C is thicker than that at 700 °C. Similarly, it consists of four layer oxides (I–IV) whose composition is, respectively, similar to the corresponding scale at 700 °C

Fig. 7. The cross-sectional microstructures of Ti60 without (a) and with aluminide (b), TiAlCr (c) and enamel (d) coatings after corrosion for 10 h at 700 °C.

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Fig. 8. The surface morphologies of Ti60 alloys without (a) and with (b) enamel coating after corrosion for 10 h in moist air with NaCl at 800 °C.

Fig. 9. The cross-sectional microstructures of Ti60 without (a) and with aluminide (b), TiAlCr (c) and enamel (d) coatings after corrosion for 10 h at 800 °C.

according to EDS analysis. The TiAlCr coating is attacked and cracks during corrosion. The EDS results show that the corrosion products are primarily composed of TiO2 and Cr2O3. Interestingly, no protective Al2O3 scale forms on the aluminide coating after corrosion for 10 h at 800 °C, in comparison with 700 °C. The thickness of coating decreases markedly during corrosion in comparison with the original coating and corroded one at 700 °C. In addition, heavy corrosion occurs inwards along the inherent cracks of the aluminide coating, and then wide corrosive channels with porous filling form during corrosion. Subsequently, the aluminide coating tends to flaking from the substrate due to formation of porous corrosion products with high thermal stress at the interface of coating/ substrate by the rapid diffusion inwards of corrosives. However, the interface of enamel/substrate still remains intact although the coating exterior surface becomes

uneven due to the possible dissolution of some enamel components in corrosive environment. 4. Discussion Due to the similar oxygen appetency of both Ti and Al, over 50% (approximately 60–70 at.%) Al critical concentration is needed to obtain a continuous protective Al2O3 scale on binary Ti–Al intermetallics at high temperature [24]. However, it is accepted that Cr addition can promote the formation of protective alumina scale on TiAl intermetallics resulting from the reduction of critical aluminum concentration [3,4,7,24–27]. Thus, nowadays, TiAlCr becomes one of the most promising protective coatings for Ti-based alloys and intermetallics at high temperatures. In addition, the aluminide coating, which is mainly composed of TiAl3 phase, can also be utilized in some environments due to its

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low cost and the formation of a uniform and dense Al2O3 scale during oxidation [21,28,29], although there exist some inherent cracks in the coating due to the brittleness of TiAl3 phase. However, the existence of Cl anion and water vapor in air or pure oxygen would be a challenge for the metallic coatings to protect the alloy from high temperature corrosion. In other words, it should be a problem whether a continuous Al2O3 scale could form and be stabilized on the coatings or not at such a harsh corrosive condition. In recent years, some authors reported the beneficial effect of trace amounts of chlorine or chlorides on the oxidation resistance of Ti-base alloys at high temperature. The dense Al2O3 scale could be formed on TiAl in O2 with trace amounts of NaCl vapor, other than mixed scale (TiO2 + Al2O3) in pure O2. This is due to the saturation oxidation and chlorination in the initial period according to Hara and Schutze [30–32]. However, no protective Cr2O3 formed on Cr in O2 containing NaCl vapor at high temperature. Hara et al. considered that it is because of the rapid formation of volatile chlorides of chromium at high temperature [33]. In the present case, the condition is more complex because of the co-existence of corrosives (NaCl vapor, O2, and H2O vapor). The beneficial effect of trace amounts of chlorides vapor on oxidation resistance seems not be concealed by the synergistic acceleration effect of large amounts of the mixture on the corrosion rate. Some thermochemical equilibriums presumed and the corresponding Gibbs energy changes and Equilibrium constants in high temperature moist air containing NaCl and water vapor can be calculated from the thermochemical data [34], except the Eq. (2) (Table 1) due to the absence of data, as shown in Table 1. However, from the XRD analysis,

NaTiO2 peaks are really existed. In addition, since the specimens are coated by NaCl, the synergistic corrosion (oxidation and chlorination) would be significantly influenced by the high water vapor and NaCl vapor contents on the coating. According to the equilibrium constants and Gibbs energy changes, one can deduce the possible corrosion mechanisms which are quite different on between TiAl3 and TiAlCr alloys under the present environment. Besides the oxidation reactions between alloying elements and pure oxygen as shown in Eqs. (1), (6) and (12) in Table 1, the existence of NaCl and water vapor will accelerate corrosion and interrupt the formation of protective scales on alloys. As shown by Eqs. (2) and (7) (Table 1) with large equilibrium constants, the synergistic oxidation of Ti and Al may proceed quickly to form Cl2, The formation of Cl2 will accelerate the chlorination of alloying elements (Ti and Al) to form evaporative TiCl4 and AlCl3, respectively (as given by Eqs. (3) and (8) (Table 1)). Besides evaporation, TiCl4 and AlCl3 could react with O2 to form an oxide scale consisting of rutile and Al2O3 (Eqs. (4) and (9) (Table 1)). Due to the high content of Al in TiAl3 coating, the oxide scale is mainly composed of Al2O3 which can protect the coating from further corrosion. However, the oxides of Ti and Al might be consumed by the synergistic corrosion of NaCl (g) and H2O (g) according to Eqs. (5) and (10) (Table 1) due to their high contents in the coating. Nevertheless, the synergistic oxidation and chlorination reactions of Cr are wider. In initial corrosion stage, the synergistic oxidation of Cr with NaCl and O2 proceeds quickly in terms of its large equilibrium constants as given by Eq. (11) in Table 1, and then the product of Cl2 could react with Cr to form volatile CrCl4 and CrCl3 (Eqs. (13)–(14) (Table 1)). Similarly, the oxide Cr2O3 can be formed when the volatile chlorides react with O2 on the surface and at

Table 1 Equilibrium reactions presumed and corresponding standard Gibbs energy changes and equilibrium constants for TiAl-base alloys in moist air with NaCl vapor at 1073 K No.

Equilibrium equation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Ti + O2 = TiO2 Ti(s) + NaCl(g) + O2(g) = NaTiO2(s) + 1/2Cl2(g) Ti(s) + 2Cl2(g) = TiCl4(g) TiCl4(g) + O2(g) = TiO2(s) + 2Cl2(g) TiO2(s) + 2NaCl(g) + H2O(g) = Na2TiO3 + 2HCl 2Al + 3/2O2 = Al2O3 Al(s) + NaCl(g) + O2(g) = NaAlO2(s) + 1/2Cl2(g) Al(l) + 3/2Cl2(g) = AlCl3(g) 2AlCl3(g) + 3/2O2(g) = Al2O3(s) + 3Cl2(g) Al2O3(s) + 2NaCl(g) + H2O(g) = 2NaAlO2(s) + 2HCl(g) 2NaCl(g) + 11/4O2(g) + 2Cr = 1/2Cr2O3(s) + Na2CrO4(s) + Cl2(g) 2Cr + 3/2O2 = Cr2O3 2Cr(s) + 2Cl2(g) = CrCl4(g) 2Cr(s) + 3/2Cl2(g) = 2CrCl3(g) Cr(s) + 4HCl + O2(g) = CrCl4(g) + 2H2O(g) Cr(s) + 4HCl + O2(g) = CrCl4(g) + 2H2O(g) 2Cr(s) + 6HCl + 3/2O2(g) = 2CrCl3(g) + 3H2O(g) 2CrCl3(g) + 3/2O2(g) = Cr2O3(s) + 3Cl2(g) 2CrCl4(g) + 3/2O2(g) = Cr2O3(s) + 4Cl2(g) Cr2O3(s) + 4NaCl(g) + 2H2O(g) + 5/2O2(g) = 2Na2CrO4(s) + 4HCl(g)

DG0973 K (kJ mol 1) 767.30 / 645.80 121.5 2.02 1370.11 682.73 535.34 299.43 2.35 964.71 883.10 326.20 674.44 312.20 312.20 653.44 208.66 230.70 177.22

Log K973 K 41.22 / 34.69 6.53 0.11 73.61 36.68 28.76 16.09 0.13 51.83 47.44 17.52 36.23 16.77 16.77 35.10 11.21 12.39 9.52

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the grain boundaries of nanocrystalline TiAlCr to give out Cl2 (Eqs. (18) and (19) (Table 1)). In the meanwhile, Cr2O3 would mainly be consumed by the avenue of Eq. (20) (Table 1), and then the products HCl would promote the formation of volatile CrCl3 and CrCl4 (Eqs. (15)–(17) (Table 1)). In summary, the cyclic formation and reactions of Cl2, HCl and volatile chlorides would increase the corrosion of alloys and metallic coatings. Especially, the significant consumption of Cr would weaken its beneficial effect in the TiAlCr coating on the formation of protective Al2O3 and Cr2O3. On the contrary, the coating containing Cr failed quickly with the volatile reactions in H2O and NaCl vapor environment. Also, for aluminide coating, the consumption of Al would also interrupt the formation of protective Al2O3 scales due to the cyclic corrosion. Actually, the consumption of Al2O3 by the reactions with NaCl and H2O vapor is very slow at low temperature. However, the decrease of O2 content weakens formation of oxide (Al2O3) on TiAl3 coating with the increase of corrosion temperature, but the corresponding increase of NaCl (g) and H2O (g) contents accelerated the consumption rate of Al2O3. Hence, even though Al2O3 scale formed on TiAl3 coating at 700 °C, the coating suffered from heavy corrosion without protective scale formation at 800 °C. At 800 °C, the oxidation and chlorination of Al in aluminide coating became faster than at 700 °C. During the initial corrosion of about 1 h, the mass gain due to oxidation of Al, Ti and their chlorides was approximately equivalent to the mass reduction for the volatile chlorination of Al and Ti. Thus, the kinetics showed the approximate zero mass gain of aluminide coated Ti60 alloy. At the subsequent stages of corrosion, the severe corrosion attack along the inherent cracks in aluminide coating [28,29] could be contributed to the formation of discontinuous non-protective products on the coating. Due to the rapid inward diffusion of corrosives through the porous corrosive channels in the coating, the corrosion at the interface of aluminide/ Ti60 would lead to the spallation of coating. Thus, the corrosion kinetics of aluminide coated Ti60 alloy showed a continuously negative mass gain at 800 °C. Furthermore, the ‘‘H2O effect” on the corrosion of TiAlbased alloys, which has been reported [10–12], also should be emphasized in the present condition. According to the theoretical and experimental work on the H2O dissociation and adsorption on TiO2 [35], H2O tended to dissociate at the defects on TiO2/(110) planes into free H atoms and OH groups. Thus, the anisotropic diffusion and adsorption of hydrogen atoms in TiO2 might have resulted in an anisotropic distribution of such defects. The anisotropic distribution of defects could thus trigger the favored growth of TiO2 along a certain direction. Enamel coating consists of some oxides, such as Al2O3, ZrO2, SiO2, which are stable in the service condition containing Cl anion and water vapor at high temperatures. The uniform and compact ultrafine enamel coating with high thermochemical stability could separate the alloys

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from the corrosives to increase the corrosion resistance of Ti60 alloys at 700–800 °C. 5. Conclusions TiAlCr nanocrystalline coating on Ti60 failed quickly in the moist air containing NaCl vapor at high temperatures due to the synergistic cyclical volatile reactions of Cr with Cl2,O2, and water vapor. Moreover, Al2O3 could be obtain and consumed by means of the synergistic oxidation and chlorination with NaCl and H2O vapor at high temperature. The consumption could be increased due to the decrease of O2 content and the increase of NaCl and H2O vapor correspondingly with the temperature. Thus, as a result of competition between consumption and formation of Al2O3, an uneven protective Al2O3 scale was formed on the aluminide coating at 700 °C, but not at 800 °C. Enamel coating could protect Ti60 alloy from synergistic corrosion of mixed O2, NaCl, and H2O vapor at the two temperatures due to its high thermochemical stability and matched thermal expansion coefficient with Ti-base alloys. Acknowledgements This project was supported by the NSFC for Outstanding Young Scientists and BaiRen Plan of Sinica. The authors want to give thanks to associate Professor Yaming Zhang in Institute of Metal Research Chinese Academy of Sciences for his offering some specimens coated aluminide. References [1] D.W. Mckee, L. Luthra, Plasma-sprayed coatings for titanium alloy oxidation protection, Surf. Coat. Technol. 56 (1993) 109–117. [2] Z. Tang, F. Wang, W. Wu, Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound, Oxid. Met. 48 (1997) 511–525. [3] M.P. Brady, J.L. Smialek, J. Smith, D.L. Humphrey, The role of Cr in promoting protective alumina scale formation by c-based Ti–Al–Cr alloys – I. Compatibility with alumina and oxidation behavior in oxygen, Acta Mater. 45 (1997) 2357–2369. [4] M.P. Brady, J.L. Smialek, D.L. Humphrey, J. Smith, The role of Cr in promoting protective alumina scale formation by c-based Ti–Al–Cr alloys – II. Oxidation behavior in air, Acta Mater. 45 (1997) 2371–2382. [5] Z. Tang, F. Wang, W. Wu, The effects of several coatings on cyclic oxidation resistance of TiAl intermetallics, Surf. Coat. Technol. 110 (1998) 57–61. [6] C. Leyens, J.-W. van Liere, M. Peters, W.A. Kaysser, Magnetronsputtered Ti–Cr–Al coatings for oxidation protection of titanium alloys, Surf. Coat. Technol. 108–109 (1998) 30–35. [7] G.S. Fox-Rabinovich, G.C. Weatherly, D.S. Wilkinson, A.I. Kovalev, D.L. Wainstein, The role of chromium in protective alumina scale formation during the oxidation of ternary TiAlCr alloys in air, Intermetallics 12 (2004) 165–180. [8] Y. Xiong, S. Zhu, F. Wang, The oxidation behavior and mechanical performance of Ti60 alloy with enamel coating, Surf. Coat. Technol. 190 (2005) 195–199. [9] Y. Xiong, S. zhu, F. Wang, The oxidation behavior of TiAlNb intermetallics with coatings at 800 °C, Surf. Coat. Technol. 197 (2005) 322–326.

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