Protective Properties of High Temperature Oxide Films on Ni-based Superalloys in 3.5% NaCl Solution

J. Mater. Sci. Technol., Vol.23 No.4, 2007 541 Protective Properties of High Temperature Oxide Films on Ni-based Superalloys in 3.5% NaCl Solution Huiping BAI and Fuhui WANG State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received April 30, 2007, in revised form May 28, 2007] The electrochemical behaviors of high temperature oxide film formed on the sputtered microcrystalline coating of M38 alloy (mc-m38) were investigated by potentiodynamic and electrochemical impedance spectroscopy (EIS) techniques in 3.5% NaCl solution. Mott-Schottky analysis was used to study the semi-conductive properties of the surface oxide. The results of the capacitance measurements showed that the oxide films on both the coating and the cast alloy were p-type semiconducting characteristics. Both the carrier density (N a ) and the flat band potential (E fb ) were obviously frequency-dependent, and the optimal frequency range was from 1000 to 1500 Hz. The oxidized coating exhibited higher protectivity than the oxidized cast alloy due to the lower carrier density compared with that of the oxidized cast alloy. The EIS data of the long-term immersing tests suggested that the oxide film served as an inner-barrier layer against chloride ions. The penetration of the aggressive ions into the surface oxide resulted in the decreased polarization resistance as a function of the immersion time. KEY WORDS: High temperature oxide film; Protective properties; EIS; Mott-Schottky analysis 1. Introduction Ni-based superalloys are widely used as turbine blade materials for advanced engines which may suffer from corrosion problems induced by chlorides when they are deployed in service or in rest in marine environments. In addition to the oxidation performance, the electrochemical behavior of alloys can also affect the integrity and service life of engines. Engine performance at high temperatures at which protective oxide films are formed on the substrates, can be hindered by chloride attack when the aggressive ions penetrate the oxide films. So it is important to study the electrochemical behavior of high temperature oxide films in chloride-containing solution. In recent times, the corrosion performances of high temperature materials (metals, alloys, coatings) have been controlled by modifying their microstructures. Wide-ranging studies, for instance, have been undertaken on the influence of micro and sub microcrystalline grain size on the high temperature corrosion resistance of many alloys. Lou and Wang et al. [1 5] had performed extensive research on the oxidation behavior of sputter-deposited Ni-based superalloy coatings on the same materials. Their results indicated that the microcrystalline coatings altered oxidation behavior and possessed much better oxidation resistance than the uncoated alloys. Among various electrochemical techniques, electrochemical impedance spectroscopy (EIS) technique is particularly valuable to investigate corrosion mechanisms [6]. EIS has found its usefulness in evaluation of corrosion behavior of coatings synthesized through PVD (physical vapor deposition) methods [7 9], as well as the electrical properties of multilayers [10]. EIS has been successfully used for characterization of oxide films, such as anodic oxides Prof., Ph.D., to whom correspondence should be addressed, E-mail: fhwang@imr.ac.cn. on Al [11,12], Zr [13 15], as well as high temperature oxide layers on zircalloy and iron-based superalloy [16]. With moderate success, EIS has also been used to monitor and predict degradation of paint coatings [17,18], based on the fact that evolution of EIS spectra with exposure time reflects corrosion progression of the coated systems. Mott-Schottky theories have been widely used to explain the semiconductor properties of the passive films formed on metals or alloys [19 22], but few reports exist on their application to semiconductor properties of the high temperature oxide films in the aqueous solution. The effect of frequency-selection on carrier density (N a ) and flat band potential (E fb ) is a critical aspect in Mott- Schottky analysis; however, the evaluation of this effect has not been reported. The aim of this work is to study the performance of oxide films on the cast and mc-m38 in a chloridecontaining environment, and elucidate the corrosion mechanism thereof by EIS technique. The effect of frequency-selection on N a and E fb, were assessed by Mott-Schottky analysis. 2. Experimental Magnetron sputter deposition was used to prepare M38 microcrystalline coating on M38 cast alloy. Samples for oxidation tests in static air were held in alumina crucibles in a furnace at 900 C for 100 h. The electrolyte solution used in this work was 3.5% NaCl. The solution was prepared with analytical grade reagents and distilled water. A conventional three-electrode cell was employed for the impedance measurements. A saturated calomel electrode (SCE) was used as a reference and a platinum net as a counter electrode. Potentiodynamic curves were obtained with a scan rate of 0.330 mv/s. Mott- Schottky plots were obtained from Parstat 2273. The measurements were performed within the passive zone of the polarization curve with a frequency range from

542 J. Mater. Sci. Technol., Vol.23 No.4, 2007 Fig.1 Surface morphology of M38 cast alloy (a), and its sputtered coating (b) after 100 h oxidation at 900 C Fig.2 Cross-section view of M38 cast alloy (a), and its sputtered coating (b) after 100 h oxidation at 900 C 6 khz to 10 Hz. The EIS measurements were carried out by using a computer controlling measuring equipment, which consisted of a potentiostat (EG&G 273) and a frequency response analyzer (EG&G 5210). The impedance spectra were recorded in the frequency range between 100 khz and 10 MHz from open circuit potential to 1000 mv with a scan of 200 mv. EIS measurements were interpreted by fitting to an electrical equivalent circuit based on a physically plausible model of the oxide films. From the film resistance and capacitance values, the resistive and capacitive properties of the oxide films were evaluated. Under the long-time immersing tests, the impedance spectra were recorded at the open circuit potential and equivalent circuit modeling was performed using ZView 2 software. During the long time immersion, the electrolyte solution was changed periodically; the threeelectrode cell was placed air-tightly and the working electrode was sure of enveloping entirety before each test. All tests were carried out at room temperature. 3. Results and Discussion Fig.3 Potentiodynamic curves of the materials oxidized at 900 C for 100 h. The data were measured after an immersion in 3.5% NaCl for 4 h, and the scan rate was set as 0.33 mv/s 3.1 Microstructure An oxide film rapidly forms on the surface during high temperature oxidation of metals and alloys and the rate of film growth depends on the transport of cations, anions, or their vacancies across the film. The surface and cross-section morphologies of cast alloy and sputtered coating after 100 h oxidation at 900 C are shown in Figs.1 and 2. The cast alloy had a rougher surface and a deep oxide scale with internal oxidation, but the oxide scale on the sputtered coating was uniform and thin without internal oxidation and nitridation. What s more, the oxide scale on the coating was composed of two different layers: the external oxide layer was rich in Cr and Ti, according to the results of XRD, where the oxides were Cr 2 O 3 and TiO 2 ; the internal oxide layer was composed of Al 2 O 3. The different oxidation behavior was due to the effect of the microcrystallization on the selective oxidation of Al 2 O 3 [5,23]. 3.2 Polarization plots High temperature oxide films are often nonstoichiometric and disordered, with a high level of different defects. These defects have a strong influence on the electrical resistance of the films. Figure 3 demonstrated the overall polarization behavior of the hightemperature oxide films in the chloride-containing solution. The surface oxide on the M38 coating exhibited better protective ability than that on cast alloy.

J. Mater. Sci. Technol., Vol.23 No.4, 2007 543 about 1.2 V, according to the small current density, which was insufficient to account the reaction involving oxygen. Moreover we observed stable pits in the surface oxide (Fig.4) after the completion of the polarization measurement. Fig.4 Morphology of the pit formed on the oxidized sputtered coating in 3.5% NaCl solution Fig.5 Bode plots of the M38 microcrystalline coating after the oxidation: (a) frequency dependence of the modulus, (b) frequency dependence of the phase angle. The EIS was measured at a series of potential ranging from the open circuit potential to 1.0 V The corrosion potential for the microcrystalline coating shifted positively by 400 mv, and the passive current density was greatly reduced by about three orders. For the microcrystalline coating, three distinct regions of various polarization behaviors could be seen from the potentiodynamic curves. The coating showed extremely low current density with the order of 10 9 A/cm 2 ; we referred the state to as the entire passivity (region I). In region II, the current density increased substantially and linearly up to 10 6 A/cm 2 with increasing the potential in the range of 0.4 1.2 V; however, the current remained very small, and the surface therefore still processed good protectivity, based on our common knowledge of passive current. In region III, the stronger linear potential dependence of the current emerges compared with that of region II. Nonetheless, we ruled out the possibility of oxygen evolution occurring at potential 3.3 EIS plots In current investigation, the oxide impedance varies many orders of magnitude over the frequency range, which can be clearly observed in Bode plots; while the vital information in the Nyquist plot was obscured. Three relaxation-time constants were apparent in the phase angle-frequency (Fig.5). The low frequency feature was corresponding to transport phenomena in the oxide film. The modulus-frequency relationship revealed the almost identical behavior in high and medium frequency ranging from 10 to 10 5 Hz for the measurements held at the different potentials. This result indicated that the surface oxide played a role as a perfect barrier against penetration of chloride ions, which is also evidenced by the extremely low current density of the polarization curve. However, in the low frequency ranging from 10 2 to 10 Hz, the modulus of the impedance decreased with increasing the potential, which demonstrated that microscopic structures of outer Helmholtz layer or diffusion layer with the interface were dramatically modified in the presence of chloride ions. 3.4 Mott-Schottky plots The Mott-Schottky equation [24,25] predicted the linear relationships between 1/CSC 2 and the electrode potential, E, based on the assumption that the capacitance of the Helmholtz layer can be neglected and that the interfacial capacitance is approximately equal to the space charge capacitance (C SC ). N d and N a can be determined from the slope, and E fb from the extrapolation to 1/CSC 2 =0. 1 2 = (E E fb kt ) εε 0 en d e C 2 SC (n-type semiconductors) (1) 1 2 = (E E fb kt ) εε 0 en a e C 2 SC (p-type semiconductors) (2) where ε 0 is the vacuum permittivity constant (8.85 10 14 F cm 1 ), ε is the dielectric constant of the passive film (taken as 13.3 [26] ), e is the electron charge, N d and N a are the donor and acceptor densities respectively, E fb is the flat band potential, k is Boltzman s constant and T is the absolute temperature. Figure 6(a) depicted that although the carrier density (N a ) was frequency-dependent, it stayed at a stable value in the region of (3.1 3.8) 10 16 when the frequency varied from 800 to 4000 Hz. Figure 6(b) showed that the flat band potential (E fb ) were strongly frequency-dependent and lineally increased with increasing the frequency, and that the narrow range of frequency (1000 1500 Hz) corresponded to relatively stable E fb in the range of 4.0 4.3 V. The result suggested that the optimal frequency range was from 1000 Hz to 1500 Hz. The Mott-Schottky plots (Fig.7, f =1000 Hz) for

544 J. Mater. Sci. Technol., Vol.23 No.4, 2007 Table 1 Calculated parameters from the Mott-Schottky curves Specimen Semiconductor film type Carrier density, N a/cm 3 Oxidized coating p 3.3 10 16 Oxidized cast p 2.0 10 18 Fig.6 Changes in carrier density N a (a) and the flat band potential E fb (b) with frequency in 3.5% NaCl solution. The frequency range was from 10 Hz to 6000 Hz Fig.7 Mott-Schottky curves of the oxidized coating and the oxidized cast alloy in 3.5% NaCl solution. The frequency range was set at 1000 Hz the oxidized mc-m38 and the oxidized cast alloy in 3.5% NaCl solution depicted that the oxide films were p-type semiconductor. The calculated parameters are listed in Table 1. The oxidized coating had lower carrier density than the oxidized cast alloy, which led to the higher stability and higher corrosion resistance. 3.5 EIS data of long time immersion tests The typical EIS spectra of the high temperature oxide film during 28 d immersion test in 3.5% NaCl solution are shown in Fig.8. Figure 8(a) is the Nyquist plot of Z Re vs Z Im, Fig.8(b) the frequency dependence of the modulus, and Fig.8(c) the frequency dependence of the phase angle. In the plots, the three different frequency regions, which correspond to three capacitance arcs in the phase angle-frequency plot, reflected the distinct interfacial behaviors of the coated sheets. The surface oxide exhibits a capacitive behavior because of the dielectric nature of the oxide. Because of the dispersion effect, a rather empirical constant phase element -CPE substitutes the capacitance. So the high frequency capacitance characterized the porous external oxide layer, the medium frequency capacitance described the compact internal Fig.8 Nyquist and Bode plots of the oxidized microcrystalline coating in 3.5% NaCl solution within 28 days immersion: (a) the Nyquist plot of Z Re vs Z Im, (b) frequency dependence of the modulus, (c) frequency dependence of the phase angle

J. Mater. Sci. Technol., Vol.23 No.4, 2007 545 Table 2 Equivalent circuit parameters of EIS results Immersing Q-CP E1 R1 α-cp E1 Q-CP E2 R2 α-cp E2 Q-CP E3 R3 α-cp E3 time /F cm 2 /Ω cm 2 /F cm 2 /Ω cm 2 /F cm 2 /Ω m 2 0 h 2.35 10 7 2.89 10 3 0.78 9.35 10 7 6.91 10 4 0.75 3.12 10 6 0.91 3 h 2.29 10 7 2.56 10 3 0.78 1.03 10 6 6.87 10 4 0.73 3.36 10 6 0.89 10 h 1.77 10 7 2.23 10 3 0.80 1.25 10 6 7.29 10 4 0.71 3.57 10 6 0.86 21 h 2.26 10 7 2.68 10 3 0.78 1.16 10 6 7.87 10 4 0.72 3.85 10 6 0.86 31 h 1.65 10 7 2.48 10 3 0.80 1.24 10 6 8.51 10 4 0.70 4.06 10 6 0.86 49 h 2.00 10 7 2.53 10 3 0.79 1.20 10 6 7.48 10 4 0.71 4.15 10 6 0.85 4 d 2.01 10 7 2.57 10 3 0.79 1.19 10 6 7.31 10 4 0.71 4.26 10 6 2.06 10 7 0.82 8 d 2.17 10 7 2.40 10 3 0.79 1.21 10 6 6.71 10 4 0.71 4.37 10 6 5.73 10 6 0.81 12 d 2.24 10 7 2.48 10 3 0.78 1.22 10 6 6.81 10 4 0.71 4.47 10 6 5.44 10 6 0.81 15 d 1.99 10 7 2.48 10 3 0.79 1.32 10 6 7.34 10 4 0.70 4.29 10 6 5.30 10 6 0.81 22 d 1.95 10 7 2.05 10 3 0.80 1.51 10 6 5.87 10 4 0.70 5.25 10 6 1.25 10 6 0.80 28 d 1.71 10 7 1.86 10 3 0.81 1.85 10 6 4.48 10 5 0.67 9.86 10 6 1.15 10 5 0.80 Notes: Q is the constant phase element, and α is the dispersion exponent. Fig.9 Diagram of an equivalent circuit model employed in analysis of electrochemical impedance data Fig.10 EIS plots and the fitted plots with the equivalent circuit model: (a) the Nyquist plot of Z Re vs Z Im, (b) frequency dependence of the modulus, (c) frequency dependence of the phase angle oxide layer, and the low frequency capacitance corresponded to the electron transport through the oxide film, which represented the protectiveness of the surface film. Interfacial processes in the solution were modeled by the electrical equivalent circuit (Fig.9). The values of the circuit element such as the solution resistance (R s ), the porous oxide capacitance (CP E1), the porous resistance (R1), the double layer capacitance (CP E2), the compact resistance (R2), the compact oxide capacitance (CP E3) and polarization (the chloride ions transport) resistance (R3) were used to characterize the performance of the oxide film in the corrosive environment. Figure 10 shows the fitted curves (Fig.10(a) is the Nyquist plot of Z Re vs Z Im, Fig.10(b) is the frequency dependence of the modulus and Fig.10(c) is the frequency dependence of the phase angle), and the fitted parameters are given in Table 2. During the initial immersion period (up to 49 h) the compact oxide film served as an inner-barrier layer. As a result, diffusion of ions into micro-pores in the oxide film experienced difficulties because of the large resistance to charge transfer. Accordingly the fitted resistance was very large with the value of 10 19 Ω cm 2. Under this condition, the changes of CP E3 with exposure time could be used to determine water/ions uptake of the oxide film. With increasing the immersion time, the oxide film capacitance (CP E3) increased, indicating that chloride ions had penetrated into the oxide film. After then, some interconnected micro-pores were filled with the electrolyte solution, which enables further measurements of the pore s resistance reasonable. The variation of the polarization (chloride ion transport) resistance (R3) demonstrated the protective ability of the oxide film. During immersion time from 4 to 28 d, the polarization resistance (R3) decreased because defects such as pores, voids, channels, or cracks were present inside the oxide film, which accelerated the chloride ions penetration into the oxide film. Although the polarization resistance (R3) decreased noticeably after prolonged immersion (28 d), the value was still considerably high (1.15 10 5 Ω cm 2 ), implying that the oxide film still possessed high protective ability. This effect results from the gradual erosion of the inner barrier layer function of the oxide film such that chloride ions could penetrate into the substrate across the oxide film, which would then function as the diffusion layer.

546 J. Mater. Sci. Technol., Vol.23 No.4, 2007 4. Conclusion The high temperature oxide film on the mc-m38 possessed excellent protectiveness in chloride solution. Although the carrier density (N a ) and the flat potential (E fb ) were dependent with the frequency, the optimal frequency range was from 1000 to 1500 Hz. The higher corrosion resistance of the oxidized coating was due to the lower carrier density compared with the cast alloy. In the immersing tests, the oxide film worked as an inner-barrier layer, exhibiting a capacitive behavior because of the dielectric nature of the oxide. The electrolyte penetrated into the film and led to a decrease in electrical resistance due to defects such as pores, voids, channels, or cracks in the oxide film. REFERENCES [1 ] H.Lou, S.Zhu and F.Wang: Oxid. Met., 1995, 43, 317. [2 ] H.Lou, F.Wang and L.Bai: Mater. Sci. Eng., 1990, A123, 123. [3 ] F.Wang, H.Lou and W.Wu: Vacuum, 1992, 43, 749. [4 ] H.Lou, Y.Tang, X.Sun and H.Guan: Mater. Sci. Eng., 1996, A207, 121. [5 ] S.Geng, F.Wang and S.Zhu: Oxid. Met., 2002, 57, 231. [6 ] A.Carnot, I.Frateur, S.Zanna and B.Tribollet: Corros. Sci., 2003, 45, 2513. [7 ] C.Liu, Q.Bi and A.Matthews: Corros. Sci., 2001, 43, 1953. [8 ] C.Liu, Q.Bi, A.leyland and A.mattthews: Corros. Sci., 2003, 45, 1243. [9 ] C.Liu, Q.Bi, A.leyland and A.mattthews: Corros. Sci., 2003, 45, 1257. [10] Tapan K.Rout: Corros. Sci., 2007, 49, 794. [11] F.Snogan, C.Blanc, G.Mankowski and N.Pebere: Surf. Coat Technol., 2002, 154, 94. [12] F.J.Martin, G.T.Cheek, W.E.O Grady and P.M.Natishan: Corros. Sci., 2005, 47, 3187. [13] S.C.Chung, J.R.Cheng, S.D.Chiou and H.C.Shin: Corros. Sci., 2000, 42, 1249. [14] V.Barranco, S.Feliu Jr. and S.Feliu: Corros. Sci., 2004, 46, 2203. [15] V.Barranco, S.Feliu Jr. and S.Feliu: Corros. Sci., 2004, 46, 2221. [16] J.Pan, C.Leygraf, R.F.A.Jargelius-Pettersson and J.Linden: Oxid. Met., 1998, 50, 431. [17] C.G.Oliveria and M.G.S.Ferreira: Corros. Sci., 2003, 45, 123. [18] C.G.Oliveria and M.G.S.Ferreira: Corros. Sci., 2003, 45, 139. [19] A.M.P.Simoes, M.G.S.Ferreira, B.Rondot and M.da Cunha Belo: J. Electrochem. Soc., 1990, 137, 82. [20] U.Stimming: Electrochim. Acta, 1986, 314, 15. [21] P.C.Searson, R.M.Latanision and U.Stimming: J. Electrochem. Soc., 1988, 135, 1358. [22] Elzbieta Sikora and Digby D.Macdonald: Electrochim. Acta, 2002, 48, 69. [23] F.H.Wang: Oxid. Met., 1997, 48, 215. [24] N.B.Hakiki, S.Boudin, B.Rondot and M.Da Cunha Belo: Corros. Sci., 1995, 37, 1809. [25] N.E.Hakiki and M.D.C.Belo: J. Electrochem. Soc., 1996, 143, 3088. [26] David R.Lide: Handbook of Chemistry and Physics, 84th Edn, CRC Press, Florida, 2003.