Surface Films Formed on Amorphous Co-Cr Alloys in 1 N HCl

Transcription

1 Technical Paper Boshoku Gijutsu, 28, (1979) Surface Films Formed on Amorphous Co-Cr Alloys in 1 N HCl Koji Hashimoto* Katsuhiko Asami, Masaaki Naka* and Tsuyoshi Masumoto* *The Research Institute for Iron, Steel and Other Metals, Tohoku University X-ray photoelectron spectroscopy has been used to investigate the effect of the composition of surface films on their protective properties. Amorphous cobalt-chromium alloys passivate by the formation of a certain amount of hydrated chromium oxy-hydroxide in their films. The open circuit corrosion resistance also. depends upon the concentration of hydrated chromium oxy-hydroxide in corrosion-product film, despite the fact that the corrosion potential of the alloys is in the active region. The surface film formed on Co-Cr-20B alloys contains a large amount of borate together with hydrated chromium oxy-hydroxide, whereas the passive film formed on corrosion-resistant amorphous Co-10Cr-20P alloy by spontaneous passivation consists exclusively of hydrated chromium oxy-hydroxide. The presence of a large amount of borate in the surface film increases the corrosion rate and is detrimental for passivation. 1. Introduction Amorphous cobalt-base alloys, because of attractive soft magnetic materials,1-3) are required to possess sufficiently high corrosion resistance The present authors4) have attempted to improve their corrosion resistance by alloying with various elements. The most effective alloying element in increasing the corrosion resistance of amorphous cobalt-base alloys has been found to be chromium. The effect of chromium depends upon the metalloid elements whose addition is necessary for preparing amorphous alloys by rapid quenching of molten alloys. The work reported herein was undertaken to determine correlations between composition and protective quality of surface film formed on amorphous cobalt-chromium alloys in both the active and passive regions in 1 N HCl. X-ray photoelectron spectroscopy was used to determine film compositions on the alloys. Particular attention was given to the effect of metalloid element. 2. Experimental Procedures Molten mixtures of commercial metals and metalloids under argon atmosphere were waterquenched to prepare alloy ingots. Rotating wheel method was applied to the preparation of amorphous Co-0-5OCr-20B and Co-10Cr-20P alloys. This technique consists of impinging a jet of remelted alloy under argon atmosphere on a rapidly rotating wheel. Potentiodynamic polarization was carried out * Katahira 2-1-1, Sendai 980, Japan with a potential sweep rate of 2.37x103V.s-1 in 1 N HCl which was open to the air. Films were formed by immersion or potentiostatic polarization for 3600s in 1 N HCl. X-ray photoelectron spectra from the specimen were measured using an AEI-ES200 electron spectrometer with Mg Ka excitation. In order to remove the error due to adsorbed water on the specimen surface and to avoid the change in specimen temperature, spectral measurements were made at a constant temperature of 323K. It took about 4N8x104 s to measure all photoelectron spectra such as Co 2p3/2, Cr 2p3/2, 01s, B 1s, P 2p, Cl 2p and. C 1s for 4 specimens which can be mounted at once in the spectrometer. The quantitative determination of the thickness and composition of surface film and the composition of underlying alloy from integrated intensities of photoelectrons was made by the method reported previously5'6). Photoionization cross sections relative to the P 1s cross section used for the quantitative determination are summarized in Table 1. The Co 2p3/2 photoelectron cross section was determined as described in the Appendix. 3. Results. Fig. 1 shows potentiodynamic polarization curves of amorphous Co-Cr-20B alloys measured in 1N HCl. The addition of chromium less than 20 at% does not affect polarization curves and not leads to anodic passivation. When the alloys contain 20 at% or more chromium the anodic passivation takes place and the anodic currentt density de-

2 Table 1. Photoelctron cross sections relative to 01 s cross section. Superscripts m and ox denote metallic and oxidized states, respectively. Fig. 1. Potentiodynamic polarization curves of amorphous Co-Cr-20B alloys in 1N HCl. creases with an increase in the chromium content. Spontaneous passivation occurs at 30 at% or more chromium. These changes with the chromium content are similar to the changes in open circuit corrosion rates of these alloys in 1 N HCl at 303K4). The alloys containing less than 20 at% chromium exhibited high corrosion rates similarly to amorphous Co-20B alloy. The corrosion rate decreases with further increase in chromium content. The addition of 50 at% chromium is required for the immunity to corrosion in 1N HCl. One of the most important characteristics is the fact that the alloys which passivate anodically do not show an abrupt rise in current density due to pitting, even by anodic polarization up to transpassive region in 1 N HCl. The alloys which dissolve actively by anodic polarization suffer not pitting but general corrosion in 1N HCl. The modification of alloy by the change of metalloid element from boron to phosphorus effectively increases the corrosion resistance of amorphous cobalt-chromium alloys. For instance, the corrosion rate of amorphous Co-10Cr-20P alloy in 1N HCl at 303K was about 0.01mm per year which was about one fortyth of that of amorphous Co-30Cr-20B alloy. X-ray photoelectron spectra from these specimens were measured to determine the composition of surface films. The Co 2p3/2 spectrum showed a main peak corresponding to metallic cobalt in the underlying alloy together with a high binding energy shoulder arising from divalent cobalt in the surface film. In contrast to the fact, the main peak of Cr 2p3/2 spectrum arised from trivalent chromium in the surface film, and metallic chromium in the underlying alloy gave a weak shoulder at the low binding energy region. Both B1s and P 2p spectra exhibited two separate peaks: The high binding energy peak corresponded to the oxidized state in the surface film and the low binding energy peak arised from metalloids in the underlying alloy. The O 1s spectrum consisted of the spectra arising from OM oxygen (metal-o-metal bond with a low binding energy peak, O2-) and OH oxygen (metal- OH and metal-oh2 bonds with a high binding energy peak)7). Oxygen in borate was also include4 in OH oxygen. The Cl 2p spectrum consisted of a single peak corresponding to chloride ion. The C 1s spectrum also appeared arising from contaminant hydrocarbon layer formed on specimen surface in the specrtometer. After integrated intensities of photoelectron spectra were separately obtained for individual species, the quantitative determination of the thickness and composition of the surface film and the composition of the underlying alloy was performed. The analytical results for the amorphous Co- 25Cr-20B alloy are shown in Figs. 2-5 as a function of polarization potential. The film thickness is of the order of 3 nm in the active region and decreases in the passive region (Fig. 2). When transpassive reaction of chromium occurs at higher potentials, the film tends to thicken. This change in the film thickness with polarization potential is similar to that observed on 30Cr ferritic stainless steels in 1 N HCl9). The chromium content in the surface film is remarkably high even in the active region in comparison with that of bulk alloy and further increases in the passive region (Fig. 3). According-

3 Vol. 28, No Fig. 2. Thickness of sufrace films formed on amorphous Co-25Cr-20B alloys immersed or potentiostatically polarized for 3600 s in 1N HCl. Fig. 3. Cationic fractions in the surface film formed on amorphous Co-25Cr-20B alloy in 1 N HCI. Specimens are dentical with those described in Fig. 2. Fig. 4. Numbers of anions linked to a cation in the surface film formed on amorphous Co-25Cr-20B alloys in 1 N HCI. Specimens are identical with those described in Fig. 2. ly, the passive film consists exclusively of chromium as a cation. Fig. 4 shows the concentration of anions as a function of polarization potential. The surface film contains oxy-hydroxide, borate and chloride. Some of bound water is also incorporated in the film. Concentrations of borate and chloride are expressed in units per total number of rations, The ordinate of the top figure corresponds to the sum of electronic charge of oxygen species which are linked to a cation. After subtraction of total negative changes of O2-, chloride and borate ions from total positive charges of cobalt and chromium in the film, the excess positive charge is alloted to OH- ion. The excess OH oxygen after subtraction of oxygen in borate and OH- ions from total OH oxygen analyzed is assigned to OH2. The number of 2O2-+OH- in the passive film is nearly, constant and is higher than that in the film formed in the active region. The rise in this number by passivation may relate to the increase in the chromium content of the film. The change Fig. 5. Atomic fractions of element in the alloy surface immeridiately under the surface film formed on amorphous Co-25Cr-20B alloys in 1N HCl. Specimens are identical with those described in Fig. 2. in the concentration of borate in the film with polarization potential is not clear. The concentration of chloride ion in the film formed in the active region is fairly high; one in ten cations is linked to chloride ion. However, when anodic passivation takes place, the chloride content of the film lowers. Fig. 5 shows atomic fractions of

4 274 Boshoku Gijutsu elements in the alloy surface immediately under the surface film. The change in the concentrations of elements in the underlying alloy is not: clear but cobalt appears to be slightly enriched. As an example, thee average composition of passive film formed on amorphous Co-25Cr-2O alloy by polarization for 3600 s at 0.6 V (SCE) in 1N HCl can be expressed as: (CII0.04 CrOiii96) (B02)0.48 C10, (OH) H2O. Consequently, the passive film consists of hydrated chromium oxy-hydroxide and borate. On the other hand, the corrosion rate degreased with the increase in the chromium content of amorphous Co-Cr-20B alloys in 1 N HCI. In this connection, the analyses were made for composition of surface films formed on amorphous Co-Cr-20B alloys containing, chromium of 20, 25 and 30 at % which were simply immersed for 360s in 1 N HCI. The results are shown in Fig.s 6 and 7. The chromium content of the surface film on the amorphous Co-20Cr-20B alloy exceeds that of bulk alloy by a factor, of two (Fig. 6). With increasing chromium content of the alloy, chromium is further enriched and comprises nearly 100 of cations in the film on the Co-30Cr-20B alloy. The increase in the chromium content of the alloy leads not to an appreciable change in the borate content of the surface film but to increase the content of oxy-hydroxide as shown in Fig. 7. The chloride content of the film is significantly high on the Co-20Cr-20B alloy but decreases with the increase in the chromium content of the alloy. The average compositions of these films are Fig. 6. Change in cationic fractions in the surface film formed on amorphous Co-20, 25, 30Cr-20B alloys by open circuit corrosion for 3600 s in 1 N HCl as a function of the ratio of the amount of chromium to the sum of those of cobalt and chromium in the bulk alloy. (Coo45 Crot5) (B02)0.47 Clo.2s Oo.53 (OH) H2O on Co-20Cr-20B alloy, (Cooi19 Croza1) (BO2j0;49 C1o.12 O:88 (OH) H2O on Co-25Cr-20B alloy, (Co 01 Cr0i99) (B02) (OH) on Co-30Cr-20B alloy. Accordingly, with the rise in the chromium content of the alloy the chromium content of the film increases but the film always contains a large amount of borate. On the other hand, the corrosion resistance of amorphous Co-10Cr-20P alloy in 1 N ICI was enormously higher than that of amorphous Co- 30Cr-20B alloy. The average composition of the Fig. 7. Numbers of anions linked to a cation in the surface film formed on amorphous Co-Cr-20B alloys in 1N HCI Specimens are identical with those described in Fig. 6. surface film formed on the, Co-10Cr-20P alloy which was simply immersed for 3600 s in 1 N HCl can be expressed as: (Copi05 Cr0.85) (P043) v2 (H)2.86-2x (x-0.90) H2O. Because the 0 is spectrum could not clearly be separated to 02-, OH and bound water, the

5 Vol. 28, No content of O2- is expressed by x. The chromium content of the film is very high but the phosphate content is very low. Therefore, the surface film formed on the amorphous Co-10Cr-20P alloy consists exclusively of hydrated chromium oxy-hydroxide. 4. Discussion The main constituent of passive films formed on various iron- and nickel-base alloys containing chromium is hydrated chromium oxy-hydroxide, CrOx(OH)3-2x nh2o, where x and n change with alloy composition and condition of film formation6,8-11). In this study, the surface film formed on amorphous Co-10Cr-20P alloy by spontaneous passivation in 1N HCl consists exclusively of hydrated chromium oxy-hydroxide. When the polarization potential for amorphous Co-25Cr-20B alloy is switched from the active region to the passive region, the concentration of hydrated chromium oxy-hydroxide of the film increases, whereas the borate content remains almost unchanged. In addition, the corrosion potential of Co-Cr-20B alloys rises with the increase in the concentration of hydrated chromium oxy-hydroxide in the film. The composition of the film formed on the Co- 30Cr-20B alloy by spontaneous passivation is identical with that of the passive film formed on the Co-25Cr-20B alloy by anodic polarization. Consequently, amorphous cobalt-chromium alloys passivate by the formation of a passive hydrated chromium oxy-hydroxide film similarly to ironand nickel-base alloys containing chromium. In other words, when a certain amount of hydrated chromium oxy-hydroxide is contained in the surface film, passivation takes place. On the other hand, the cationic fraction of chromium in the surface film formed on the cobalt-chromium alloys in the active region is extraordinarily high and the open circuit corrosion rate lowers with increasing chromium content of the film. As reported previously12), amorphous Fe-3Cr-13P-7C-1-3M alloys in which M is various metallic elements have revealed that the corrosion resistance in 1 N HCl depends upon the concentration of trivalent chromium in the film, despite the fact that their corrosion potentials are in the active region. As far as amorphous alloys are concerned, immersion or polarization in the active region usually results in covering alloy surfaces by corrosion-product film which more or less acts as a diffusion barrier.. When alloying with various elements less noble than base metal lowers the corrosion rate of alloys at open circuit potentials in the active region, their corrosion resistance often depends upon the passivating capabilities not of base metal but of alloying elements, themselves13): The passivating species of alloying elements included in the corrosion-product film seem to improve the protective properties of the film. This interpretation also can be applied to the present results: The corrosion resistance increases with increasing chromium content of corrosion-product film formed on amorphous Co- Cr-20B alloys. The fact that cobalt is more noble than iron seems responsible for the extraordinarily high chromium content of corrosion-product film on cobaltchromium alloys in comparison with iron-chromium alloys. The chromium content of corrosionproduct film on Co-Cr-20B alloys far exceeds that of the surface film formed on 30Cr ferritic stainless steels in the active region in 1N HCl and is comparable to or higher than that of the passive film on those steels9). Nevertheless, spontaneous passivation does not occur on the amorphous Co-20, 25Cr-20B alloys in 1 N HCI. The surface films on these amorphous alloys contain large amounts of borate together with oxy-hydroxide. In contrast to the fact, a passive hydrated chromium oxyhydroxide film formed on amorphous Co-10Cr- 20P alloy by spontaneous passivation contains a very low amount of phosphate. Also the amorphous Co-30Cr-20B alloy passivate spontaneously in 1N HCI. However, the passive film on this alloy contains a large amount of borate and the open circuit corrosion rate of this alloy is forty times as high as that of the Co-10Cr-20P alloy. It is evident that the corrosion rate of alloys containing boron is raised by the fact that the concentration of oxy-hydroxide in the film is reduced by borate. Therefore, even if the chromium enrichment in the surface film is extraordinarily high, the concentration of hydrated chromium oxy-hydroxide is not high enough to passivate spontaneously unless borate is removed from the film. Hoar14) has suggested that poly-oxy-anions can take part in formation of cross linked monolithic amorphous filmm which enhance corrosion resistance. However, as reported previously8), the formation of silicate, borate and/or phosphate in the film reduces the corrosion resistance of amorphous alloys in acidic solutions. Okamoto15' has stated that the most important parameter controlling the corrosion resistance of Type 304 stainless steel is the amorphous nature of the pas-

6 276 Boshoku Gijutsu sive film in which bound water is included. Revesz and Kruger16) also have emphasized the importance of the amorphous nature of passive film. Okamoto15) has suggested the following dual function of the bound water: The bound water reacts to give various species or is replaced by the other ions coming from surroundings, resulting in degradation of the passive film. On the other hand, the bound water acts as the effective species to capture the dissolving metal ions and forms the new film resisting against a further attack by surroundings. His conclusion has been drawn on the basis of the analytical result of the passive film with the aid of tritiated water17), and hence the bound water includes OH- ion together with H2O. Accordingly, the bound water corresponds to constituents of hydrated chromium oxy-hydroxide. The fact that the formation of hydrated oxy-hydroxide leads to passivation of various amorphous and crystalline alloys suggests that the bound water, OH- and H2O, in the film mainly acts as beneficial species, even in hydrochloric acids. The question has been raised why passivation takes place at a certain concentration of hydrated chromium oxy-hydroxide in the surface film, despite the fact that the concentration in the film changes continuously with the chromium content of the alloy. The experimental results imply that a small shift of polarization potential toward noble direction or a small increase in the chromium content of alloys leads to small increase in the chromium content of the surface film and to passivation of amorphous cobalt-chromium alloys. A sudden change in the film thickness and a sudden decrease of several orders of magnitude in the current density by passivation suggest the significant change in the protective properties of the film. One conceivable explanation is that the small increase in the concentration of hydrated chromium oxy-hydroxide in the corrosion-product film gives rise to a significant change in the structure of the film and consequently diminishes the activity of diffusion channels in the film. In spite of the fact that both hydrated oxyhydroxide and poly-oxy-anions such as borate can take part in formation of amorphous films, hydrated oxy-hydroxide can act as passivating species while poly-oxy-anions are detrimental. Even if a large amount of borate is included in the film, passivation takes place by the formation of hydrated oxy-hydroxide. Accordingly, borate may influence mainly not to the structure of the film but to the function of the film. For instance, if a majority of borate was linked not to proton but to metallic rations in the film, borate could not act to capture dissolving metal ions as has been thought possible for hydrated oxy-hydroxide: Borate ions are likely to be obstacles to form a monolithic film. Appendix Determination of Co 2p3/2 photoelectron cross section relative to O 1s cross section X-ray photoelectron spectra from standard substances Coo and binary Fe-Co alloys (20.41, 39.85, 60.32, at% Co) were measured. The Coo specimen was prepared by oxidation of a cobalt sheet for s in pure oxygen at 1 atm. and 1373 K. Prior to spectral measurement, the Coo specimen was heated at 653 K to remove adsorbed water in the spectrometer. The Fe-Co alloys were polished mechanically by siliconcarbide paper (1500) in cyclohexane. As a first approximation, the cross section for the oxidized state cco2p3/2/01s was estimated from integrated intensities of spectra of the CoO specimen using equation (4) in Reference 7. The cross section for the metallic state QC o 2p312'601s was estimated on the basis of the method described in Reference 5 from the value of o2p3/2/qo1s and integrated intensities of spectra from binary Fe-Co alloys. The value of c.2p3/2/601s obtained was very close to that of o2p3/2/01s. If one used not the peak hights but the integrated intensities of Fig. 8. Analytical result of XPS for binary Fe-Co alloys polished mechanically by siliconcarbide paper (1500) in cyclohexane. Cationic and atomic fractions of cobalt in the surface film and the underlying alloy as a function of the atomic fraction of cobalt in the bulk alloy.

7 Vol. 28, No Com and Coox 2p3/2 spectra, the value of 6co 2p3I21 601s should be equal to that of o Co 2p312/90is. Consequently, the value of oco 2p32/oo is is determined as the mean value of oao 2p321oo is and 23,2/0 15 and is In the course of the determination, we analyzed the composition of the surface film and the underlying alloys as shown in Fig. 8. Cobalt contents in surface films formed on Fe-Co alloys by mechanical polishing in cyclohexane are lower than those of bulk alloys, whereas the composition of underlying alloys are identical with that of bulk alloys. Similar behavior has been found by some of the authors (K. H. & K. A.) on binary Fe-Ni alloys polished mechanically in cyclohexane. (Received September 12, 1978) References 1) R. C. Sherwood, E. M. Gyorgy, H. S. Chen, S. D. Ferris, G. Norman and H. J. Leamy: AIP Conf. Proc., 24, 745 (1975). 2) H. Fujimori, M. Kikuchi, Y. Obi and T. Matsumoto: Sci. Rep. Res. Inst. Tohoku Univ. A26, 36 (1976). 3) R. C. O'Handly, L. I. McCarry and J. J. Becker: Proc. 2nd Int, Conf. on Rapidly Quenched Metals, ed. by N. J. Grant and B. C. Gissen, MIT Press, Cambridge, Mass., (1976), p ) M. Naka, K. Hashimoto and T. Masumoto: Proc. 3rd Int. Conf. on Rapidly Quenched Metals, 1978, The Metal Society, London, (1978), p ) K. Asami, K. Hashimoto and S. Shimodaira: Corros. Sci., 17, 713 (1977). 6) K. Hashimoto; M. Kasaya, K. Asami and T. Masumoto: Corros. Engng. (Boshoku Gijutsu), 26, 445 (1977). 7) K. Asami and K. Hashimoto: Corros. Sc., 17, 559 (1977). 8) K. Hashimoto, M. Naka, K. Asami and T. Masumoto: Corros. Engng. (Boshoku Gijutsu), 27, 279 (1978). 9) K. Hashimoto, K. Asami and K. Teramoto: Corros. Sc., 19, (1979) in press. 10) K. Asami, K. Hashimoto and S. Shimodaira: Corros. Sc., 18, 151 (1978). 11) K. Teramoto. K, Asami and K. Hashimoto: Corros: Engng. (Boshoku Gijutsu), 27, 57 (1978). 12) K. Hashimoto, J. Noguchi, M. Naka, K. Asami and T. Masumoto: "Passivity of Metals' Proc. Int. Symp. on Passivity, 1977, ed by J. Kruger and R. B. Frankenthal. The Electrochem. Sac. Prinston, N. J. (1978), p ) M. Naka, K. Hashimoto and T. Masumoto: J. Non-Cryst. Solid, in press. 14) T. P. Hoar: J. Electrochem. Soc., 117, 17C (1970). 15) G. Okamoto: Corros, Sci., 13, 471 (1973). 16) A. G. Revesz and J. Kruger: Ref. 12, p ) K. Kudo, T. Shibata, G. Okamoto and N. Sato: Corros., Sci., 8, 809 (1968).