Passivity and passivity breakdown in nickel aluminide

Corro.~ion ,')'cteme. Vol. 31. pp. 47 I-~178, It)0()

0010-938X/90 $3.00+0.00 Pergamon Press pie

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Passivity and Passivity Breakdown

In Nickel Alumlnlde

U. Bertocci, J. L. link, D. E. Hall, P. V. Madsen, R. E. Ricker National Institute of Standards and Technology Gaithersburg, MD 20899 U. S. A.

ABSTRACT The electrochemical behavior of ductile nickel aluminide has been examined in alkaline, acidic and neutral solutions. Conditions for passive behavior, as well as for passivity breakdown, have been defined. The experimental results are compared with those obtained with the pure constituent metals, Ni and AJ. Pitting has been observed not only in chloride-containing solutions, but in sulfuric acid in a certain potential range. This pitting is believed to be caused by a potential-dependent transition from the passive to the active state. The corrosion resistance of nickel aluminide appears to be similar to that of pure nickel. KEYWORDS Electrochemical measurements; nickel aluminide; passivity; pitting. INTRODUCTION Nickel aluminide (Ni3AI) is an intermetallic compound that forms when ordering occurs in a nickelaluminum alloy. The aluminum atoms are ordered on the comers of a FCC unit call while the nickel atoms occupy the centers of the faces, so that each AI has only Ni atoms for nearest neighbors, forming a L12 supedatUce. Single phase nickel aluminide can be made from alloys of 25 atomic percent aluminum, but polycrystalline samples have, in contrast with single crystals, very low strength and ductiity. However, it has been found that the polycrystalline material can be made ductile by microalloying with boron (200 ppm by weight) and by keeping the aluminum content slightly substolchlometric (Taub et al, 1989). Ductile nickel aluminide (NI3AI+200 ppm B) has many properties that make it attractive for high temperature applications, such as a high yield strength that does not decrease significantly with temperature up to 1000 C and good oxidation resistance. Although originally developed for high temperature applications, many of its properties also make it attractive for low temperature applications. These are a high yield strength, excellent ductility (up to 50%), a high work hardening coefficient and a relatively low density as compared to other nickel alloys. These properties, coupled with a good corrosion resistance, would yield a high strength alloy with excellent wear, erosioncorrosion and cavitation-corrosion characteristics. Ductile nickel aluminide and its alloys with Cr, Mo or other elements are being considered for numerous low temperature applications such as aircraft fasteners, dental drills, forging dies, bearings, tubing and other components for corrosive environments (oil and gas wells, chemical process industry, marine environments, injection molding), expansion joint bellows, metal matrix composites, aerospace engines, gas and steam turbines, casting melds for aluminum and glass, and even heating elements. However, to date, the published information on the corrosion of Ni3AI alloys is limited (Buchanan, 1988), (Kim, 1989), (Ricker, 1989). Therefore, an investigation into the aqueous corrosion behavior of a ductile Ni~6,1was undertaken, to help the development of this material for new applications. From a more fundamental viewpoint, it was also thought interesting to investigate the behavior of a compound of two metals which form passive films in different environments, and to study their interactions concerning passivity. The chemical potentials of the elements in this ordered alloy will be different from those of the pure elemental phases or the disordered solid solution but, for an initial analysis, we may consider the 471

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The purpose of this paper is to present the electrochemical evidence so far collected, with the understanding that other techniques for the analysis of the passive films will be necessary in order to form a comprehensive picture of the passive behavior of this material. EXPERIMENTAL METHODS The material used for this study was provided by the Oak Ridge National Laboratory, Oak Ridge, TN. Its composition was 77.0 At % Ni, 22.2 At % AI, 0.42 At % Hf and 0.087 At % B. Part of the material, which was received in the cold worked condition, was vacuum annealed at 1100 C for I hour, and specimens in both conditions were used for the electrochemical tests. Since no significant differences in electrochemical behavior were detected in the experiments reported here, indication of the metallurgical conditions of the samples employed has been omitted. Solutions were made with reagent grade chemicals and doubly distilled water. Electrochemical measurements were conducted with conventional laboratory instruments, operated in general under computer control and driven by inhouse developed software. Electrochemical noise techniques have been described previously (Bertocci, 1980, 1981). EXPERIMENTAL RESULTS Fig. 2 shows the open circuit potentials of nickel aluminide as well as on the pure constituent metals, measured in aerated solutions of various compositions. These values, as it is well known, have a significant degree of variation; however, they can be taken to indicate that Ni3AI is closer in behavior to Ni than to AI in all solutions. 2. Alkaline solutions. Nickel is the metal of choice in many alkaline environments, such as alkaline batteries, because it forms a highly protective oxide film. However, a well known method for preparing an effective hydrogenation catalyst, Raney nickel, involves the anodic attack of a Ni-AI alloy containing 50 atomic percent of AI, with dissolution of the At in alkali. It was therefore interesting to determine how Ni3AI, which has an intermediate composition, behaves in alkaline solutions. The one used here was 0.5 M NaOH.

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Lack of any significant dealloying or roughening of the surface in alkaline solutions was also demonstrated by measuring the a.c. impedance of a Ni3AI electrode before and alter soaking in in concentrated KOH for more than 1 hour. The impedance spectra are nearly identical. In particular, there is no significant increase of the electrode capacitance (Fig. 4). 3. Acidic solutions 3.1 Nitric Acid The solution employed for the measurements was 1 M HNO3. Both Ni and Ni3AI con'ode in nitric add: the surface becomes deeply etched, revealing the grain structure. Ni3AI did not show any large preferential attack at the grain boundaries, where the duclility-enhancing additions lend to concentrate. The open circuit potential is fairly stable and reproducible, and it is about 70 mV more negative (-120 mV vs. SCE) than in pure nickel. 16(3(}

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Potentiodynamic scans at sweep rates of 50 and 5 mV/s are shown for both Ni3AI (Fig. 5) and Ni (Fig. 6). Their qualitative behavior is similar, with passive currents between 300 and 1000 mV vs. SCE about the same. The stability of the passive film depends on the potential at which it was formed, and its breakdown is marked by an anodic peak during the negative-going scan. The breakdown peak can be almost completely suppressed by raising the upper limit of the potential above 1 Volt (as in Fig. 5), and if the scan rate is last. When these conditions are not met (as in Fig. 7), the breakdown peak is quite pronounced.

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Passivity and passivity breakdown in nickel aluminide

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higher, the current decays approximately as the square root of time, while at lower potentials, active corrosion is the long term outcome. At intermediate values, such as 200 mV, after a rapid passivation transient, the current steadily increases again.

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Lo 9 FREQUENCY. Hz Fig. 8. Impedancespectraof Ni3AIin HNO3 Impedance spectra taken close to the open circuit potential (-50 mY) and in the passive range (600 mV), do not show any peculiar behavior: the corrosion rate in the active range can be estimated to be of the order of 1 mNcm 2, from a low frequency resistance of about 80 ~.cm2, as shown in Fig. 8. 3.2 Sulfudc Acid The solution employed was 0.5 M H2SO4. The differences in volfammetric behavior in H2SO4 and HNO3 are of quantitative rather than qualitative nature. The open circuit potential is more negative in H2SO4, about -230 mV vs. SCE; impedance spectra (Fig. 9) indicate that at all potentials tested the reaction resistance is greater in H2SO4, reflecting the lower corrosion rates. 4

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The conditions for the formation, stability and breakdown of the passive film that can form on nickel aluminide in sulfuric acid have a close analogy with those in nitric acid. Like pure nickel, as reported by Tousek (Tousek, 1966), two anodic current peaks are detected during potentiodynamic scans in the positive direction, and their relative size appears to be related to scan rate, as shown in Fig10. The passive film, trom 100 mV up, is stable and offers good protection. A very interesting finding was that in the potential region where the passive film is unstable, pitting can be observed. Repeated tests have shown that pitting occurs between -125 and +50 mV vs. SCE, but

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crystallographic features, with faceted interiors, as shown in Fig. 11, indicating that the metal exposed in the pits is in the active state. The breakdown of the passive film has been followed also by recording the spectra of the fluctuations in the current. The results show that the noise level increases remarkably when the potential is lowered from the stable to the unstable passive range, as shown in Fig. 12, indicating passive film breakdown. 4. Neutral solutions 1600

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pitting potential, induction times and the rate of pitting attack. However, it is noteworthy that substantial amounts of AI in the material have no influence on the pitting potential, which is hardly distinguishable from that of pure Ni. A rather interesting and unexpected finding was the observation of pitting in a strongly acidic solution such as 0.5 M H2SO4. Since local acidification cannot be invoked as the cause of stable pit growth, the most reasonable explanation is that an ohmic drop inside the pit drives the potential into the active region, with a mechanism similar to that studied and described by Pickering (Picketing, 1989). The voltage drop, particularly at the beginning, cannot be too large, and this agrees with the fact that the pitting range is close to the potential at which the active-to-passive transition occurs. We have not been able to cause pitting in analogous circumstances on pure nickel, and this raises the question of the role of aluminum in this phenomenon. Perhaps some AI dissolution might cause hydrogen bubble formation, which in turn can increase the voltage drop inside the pits. Another line of reasoning might link the local breakdown to the fact that it occurs approximately at potentials between the two anodic peaks, which have been attributed to the formation of two nickel' oxides, which may have quite different protective properties. Acknowledgments The authors would like to thank Dr. V. K. Sikka and J. R. Weir of the Oak Ridge National Laboratory for supplying the Ni3AI samples used in this work, and for their helpful discussions. One of the authors (P. V. M.) thanks the Danmarks Ingenior Akademi for financial support.

REFERENCES Bertocci, U. (1980), Applications of a Low Noise Potentiostat in Electrochemical Measurements, =U~[.~P,.~,~, 127, 1931-1934. Bertocci, U. (1981). Separation Between Deterministic Response and Random Fluctuations by Means of the Cross-Power Spectrum in the Study of Electrochemical Noise, ~ , 128, 520-523. Buchanan, R. A. and J. G. Kim (1988). Aqueous Corrosion Characteristics of Nickel Aluminides, Final Report to The Nickel Development Institute, Toronto, Ont. Kim, J. G., R.A. Buchanan, V. K. Sikka and J. R. Weir (1989). The aqueous Corrosion Characteristics of Nickel Alurninides, NACE Corrosion Research Symposium, p. 3. New Orleans, LA. Pickering, H. W. (1989). The significance of the local electrode potential within pits, crevices and cracks, Corrosioq ,,~ci., 29, 325-341. Ricker, R. E., D. E. Hall, U. Bertocci and J. L. Fink (1989). Corrosion and Stress corrosion Cracking of Ductile Nickel aluminide, NACE Corrosion Research Symposium, p. 1. New Orleans, LA. Taub, A. I. and R. L. Fleischer, (1989). Intermetallic Compounds for High-Temperature Structural Use, Science, 243, 3616-621. Tousek, J. (1966). Beitrag zur Nickelpassivit&t, Coll. Czech. Chem. Comm.. 31, 3083-3090.