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Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co
Thin Solid Films 460 (2004) 143–149
Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co ˜ A.G. Munoz*, J.B. Bessone ´ Quımica, ´ ´ Blanca, Argentina INIEC-Dto de Ingenierıa Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahıa Received 3 June 2003; received in revised form 10 January 2004; accepted 6 February 2004 Available Online 12 April 2004
Abstract Surface conditions similar to those found in aluminium alloys of practical use were assessed by cathodic deposition of transitions metals (Ni and Co) from different electrolytes. Fundamental aspects concerning with the growth of anodic oxide films at potentials lower than 10 V in neutral acetate buffer solution on these modified surfaces were analysed by common electrochemical techniques complemented with scanning electron microscopy and transmission electron microscopy. In both potentiodynamic and galvanostatic modes, the growth of aluminium oxide competes with the dissolution of deposited metal particles. The formation of a thin aluminium barrier oxide film beneath them shifts the dissolution potential over to 1.5 V towards more positive values. Some particles get progressively embedded in the matrix of the growing alumina and act as cation sources, increasing the film conductivity and diminishing the established electric field in the oxide. This effect is more pronounced with Co deposits due to its high active dissolution rate, before passivation occurs. Then, the generation of a two-layer film is explained in terms of the precipitation of metal hydroxide at the solution side on the oxide barrier film. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium oxide; Growth mechanism; Nickel; Transmission electron microscopy
1. Introduction The study of the anodic oxide growth on aluminium alloys is getting increasing interest due to the necessity for a better knowledge of the microscopic processes occurring at the surface either during corrosion or different surface treatments. Alloying elements, usually introduced to improve the corrosion resistance and mechanical strength, exert a great influence on the composition and structure of the oxide, thus conducting to different properties of the film w1,2x. In order to get a more corrosion resistant aluminium surface, the oxide film was modified by introduction of inhibitors in the electrolyte w3x or by addition of alloying elements such as Mo, Zr, Ta and Nb w4–6x. Thus, a close examination of the anodic processes is of vital importance for understanding the mechanisms under which the presence of alloying elements improves the corrosion behaviour. *Corresponding author. Tel.: q54-2914-5951-82; fax: q54-29145951-82. ˜ E-mail address: [email protected] (A.G. Munoz).
The cathodic deposition is a suitable method to obtain surface conditions similar to those found in alloys w7,8x. This also opens other possibilities for obtaining information about the highly localised processes as film formation and dissolution. In the present study, Ni and Co were selected as transition metals, the first being present in several special aluminium alloys. However, the presence of these elements as cations in solution was found to reduce the corrosion rate of steel w9x and aluminium w10,11x. In that case, Auger depth profiles have given evidence for the incorporation of Ni2q and Co2q into the forming oxide film. Similar conclusions were addressed in the presence of Ce3q w12,13x. Then, the reduction of the corrosion rate was mainly explained in terms of the suppression of the oxygen reduction reaction by the presence of oxideyhydroxide films. Accordingly, thicker films, with a more open structure, was the cause for a poor protection offered by Co2q. The aim of this work is to get a closer insight into the processes occurring during the anodisation of aluminium surfaces previously modified by cathodic dep-
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.017
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osition of Ni and Co. Thus, an approach to surface conditions during the corrosion of aluminium in the presence of cationic film-forming inhibitors or the anodisation of aluminium alloys will be intended. 2. Experimental details Sheets and rods of aluminium 99.99% embedded in a Teflon holder conforming vertical and horizontal electrodes with an exposed area of 0.5 cm2 and 0.070 cm2, respectively, were used as working electrodes. They were mirror like polished with 1000 emery paper and then with 0.3 mm alumina. Afterwards, they were submerged in a 0.5 M NaOH solution for 10 min, rinsed with pure water and neutralised in acetic acid for 30 s. A reproducible surface covered with a very thin oxide film was expected to be obtained with this method. Experiments were performed with a conventional cell of three compartments in a purified N2 atmosphere at room temperature. A Pt sheet and a saturated sulfate electrode (sse) (E 0s0.64 V vs. nhe) were used as counter and reference electrodes, respectively. Solutions were prepared from analytical grade chemicals. All potentials were referred to the sse. Otherwise stated, all experiments were performed with a vertical electrode. The deposition of alloying elements was made from neutral acetate buffer solutions with a cation concentration of 0.02 M. The subsequent anodic growth was performed in acetate buffer solution without cations. As AlyNi and AlyCo we refer in the text to aluminium surfaces modified by cathodic deposition of Ni and Co, respectively. Sweep voltammetries were performed using a linear voltage sweep generator EG&G PAR model 175 and a potentiostat–galvanostat EG&G PAR model 173. A dual stage ISI DS 130 scanning electron microscope was used to examine the electrode surface. Transmission electron microscopy (TEM) was performed with a Jeol JSM 100CXII. In this case, aluminium sheets were embedded into an epoxy resin, wherefrom sections of 80 nm thickness were obtained by a LKB 8800 Ultratome III ultramicrotome.
Fig. 1. (a) Cyclic voltammograms showing the potentiodynamic deposition in acetate buffer solutions containing 0.02 M of metal cation. vs0.02 V sy1. Es,asy0.8 V. (-----) cyclic voltammetry performed in a solution without cations; (b) anodic scans performed on the Al surface modified by deposition of Ni and Co in acetate buffer solution. vs0.1 V sy1. Es,csy1.0 V, Es,as4 V.
3. Results 3.1. Voltammetric identification of anodic processes Fig. 1b shows the anodisation of vertical sheets whereon Ni or Co was previously deposited by means of potentiodynamic scans from acetate buffer solutions (Fig. 1a). The presence of a thin oxide layer on aluminium makes the charge transfer difficult and deposition begins at high over-potentials. In spite of similar potentials for NiyNi2q(E 0sy0.89 V) and Coy Co2q(E 0sy0.917 V), the deposition of Co seems to be favoured. The hydrogen evolution reaction on the
formed Co deposits predominates at E-y1.9 V, generating a considerable local alkalisation. Thus, the hysteresis observed at the reverse scan may be explained in terms of the cathodic dissolution of the surrounding Al oxide, favouring further deposition. The voltammetries performed in Ni2q containing solutions, however, present various peaks that can be attributed to different mechanisms of nucleation and growth. The deposition of Ni from neutral solutions is a complex process involving the passivation of the growing deposits. An anodic peak is always observed at the beginning of anodisation (overshoot), which can be related with a
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Fig. 2. SEM micrographs of Al surface after potentiodynamic deposition of Co. vs0.02 V sy1 . Es,a sy0.8 V, Es,c sy2.0 V. (a) as deposited; (b) after anodising up to 4 V.
delayed oxide formation. This phenomenon was extensively discussed in the literature w14x and is out of the scope of this paper. A second anodic peak, identified as a shoulder before the overshoot, is observed in the anodisation of AlyCo, it being attributed to the dissolution of Co deposits. At more anodic potentials, a current plateau of approx. 0.3 mA cmy2 extends up to 0.9 V, indicating the growth of a homogeneous oxide film according to the high-field law. At E)0.9 V, the appearance of a characteristic anodic peak may be ascribed to the dissolution of deposits separated from the metallic substrate by the growing anodic film. The corresponding evolved charges Qa were 3.25 mC cmy2 and 23.7 mC cmy2 for AlyNi and AlyCo, respectively, which are much lower than those for deposition (approx. 85 mC cmy2 in both cases). Therefore, this low efficiency may be related with the passivation of the surface of particles andyor the covering of them by the growing film of alumina. At potentials E)2 V, a continuous increase of the current with the presence of superimposed anodic spikes is close related with the transport of Ni2q or Co2q through the growing aluminium oxide film. Thus, the higher currents observed in the case of AlyCo might be a consequence of the higher flux of Co2q brought about by a higher dissolution rate. The changes of surface morphology during the oxide growth were followed by scanning electron microscopy (SEM). Fig. 2a shows the surface of Al after the potentiodynamic deposition of Co (Fig. 1a). Rounded grains evolving to star-like forms of approx. 5 mm can be observed on an initial porous aluminium surface. Certain oxide dissolution, as a consequence of the strong local alkalisation, can be noted around some of them. After the oxide growth up to 4 V, some particles were not dissolved yet (Fig. 2b). A uniform distribution of small white spots can be also distinguished at this stage. That is probably related with the precipitation of hydroxide particles during the dissolution process. The dissolution kinetics of deposited particles at E) 0.9 V is related to their surface state, which in turn depends strongly on the conditions developed during
deposition. Fig. 3 shows the current transients, obtained by applying different potential steps and the corresponding anodic scans after 37 s deposition. In the case of Ni (Fig. 3a), the passivation of growing particles is denoted by a rapid current decay until they reach a constant value (Esy1.85 V). This effect was already analysed in a previous work and attributed to the generation of a surface Ni(OH)2 film arresting further deposition w15x. At more cathodic potentials, deposition takes place by reduction of hydroxide particles precipitated at the surface, which is revealed by transient patterns with the appearance of various peaks (dotted line in Fig. 3a). This is as a consequence of the rapid local alkalisation given by the hydrogen evolution reaction on the substrate. Passivation of Ni particles conducts to a drastic reduction of the anodic dissolution of them at 1.5 V (Fig. 3c). A similar effect, but in a lesser extent, is also observed in the deposition of Co. In this case, the charge passed at y1.9 V is three times that at y1.7 V (Fig. 3b), but the formation of a surface hydroxide layer during deposition would diminish the dissolution rate in the anodic scan (Fig. 3d). Also, the enhanced alkaline dissolution of Al oxide at the most cathodic potentials may bring about the formation of an insulating film beneath the particles. 3.2. Galvanostatic growth The influence of deposits on the oxide growth was further analysed by applying current steps (Fig. 4). Pure aluminium presents a linear potential increase with time, in accord with a high-field growth mechanism w14x. However, a slope value of 0.4 V mCy1 cm2, lower than that reported in the literature of 0.56 V mCy1 cm2, is obtained. This result may be ascribed to a higher real surface area and a loss of current efficiency. Near 10 V, the onset of the oxygen evolution reaction impedes further potential increase. In the case of modified surfaces, three potential regions, coinciding with the different anodic processes observed during the potentiodynamic growth, can be distinguished. Firstly,
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Fig. 3. (a) and (b) i vs. t transients recorded on applying potential steps on Al in acetate buffer solutions containing 0.02 M of metal cations: (a) Ni2q; (b) Co2q. (c) and (d) anodic scans performed in acetate buffer solution after 37 s deposition as shown in (a) and (b), respectively.
the potential increases linearly with an identical slope as aluminium does. Then, two successive shoulders appear at qa)5 mC cmy2 for the AlyNi. They can be related with dissolution of deposits in the metalyelectrolyte and metalyoxide film, respectively. However, the uninhibited dissolution of Co particles maintains the potential at 0.9 V until it reachs practically the deposition charge. Then, the dissolution of Co through the oxide film is characterised by a slow continuous increase of the slope.
3.3. Influence of the surface roughness Due to the different local hydrodynamic conditions established by the detachment of hydrogen bubbles during the initial alkaline treatment of the surface, different surface morphologies arise for horizontal or vertical positions of the electrode. Fig. 5 shows the anodic scans performed after depositing Ni in vertical and horizontal positioned aluminium electrodes. The horizontal position generates a rougher surface with a
Fig. 4. Time-dependence of potential, depicted as a function of passed anodic charge, for the anodisation of Al at is0.707 mA cmy2 with and without the presence of deposits. The deposits were generated potentiodynamically as shown in Fig. 1.
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Fig. 5. Anodic scans performed on aluminium electrodes with different positions after depositing Ni from an acetate buffer solution containing 0.02 M Ni2q. vs0.1 V sy1. Es,csy1.0 V, Es,as4 V. Detail: cyclic voltammograms showing the potentiodynamic deposition. vs 0.02 V sy1. Es,asy0.8 V.
real area twice that for the vertical one. In fact, charges of 94 mC cmy2 and 161 mC cmy2, referred to the geometric area, were obtained for the same cathodic scan in vertical and horizontal electrodes, respectively, (see detail at the upper-right side). The initial current plateaux in the subsequent anodisation point out also this fact. Thus, roughness factors of 1.1 and 1.9 can be estimated from a current plateau of 0.25 mA cmy2 reported for an electro-polished surface w14x. Considering the true area, the anodic current of the dissolution processes at E)0.9 V is very much higher for the vertical electrode. This means that the particles deposited onto the rougher surface (horizontal position) would get more rapidly covered with the growing film of alumina.
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a linear relationship is observed, as expected for a highfield growth. Then, the similar slopes of Cy1 vs. E plots suggest that the presence of deposits do not exert any effect on the dielectric behaviour of the oxide. This means that cations generated by dissolution of particles just beneath the growing film of alumina are rapidly transported by the electric field to the filmyelectrolyte interface. In the case of pure aluminium, the slope of the Cy1 vs. E plot for a horizontal electrode is 2.3 times lower than that for the vertical position, as expected for a higher real surface area. The presence of deposits gives rise to an increment of the slope only for a horizontal electrode. Here, a less homogeneous surface, given by a higher roughness, would allow the metal cations to be accumulated into the oxide structure. Thus, a thicker film with a lower dielectric constant may arise. In order to get more information about the morphology of the oxide film, grown in the presence of Ni or Co deposits, ultramicrotomed sections were examined by TEM after anodisation up to 4 V (Fig. 7a,b). A film consisting of two layers is observed after anodising the AlyNi. The outer film has a thickness of approx. 25 nm. This value is almost three times thicker than that expected of 7.2 nm at 4 V for the aluminium oxide film, according to the expression: dsk(EyEOX)
(2)
where k is the formation factor (s1.2 nm Vy1 w14x) and EOX the equilibrium potential of the oxide electrode (sy1.99 V sse). Then, this film could be identified as an upper precipitated nickel hydroxide film formed from the flux of Ni2q transported to the filmyelectrolyte interface. The inner layer, however, would thus correspond to the aluminium oxide contaminated with Ni2q. After anodisation of the AlyCo, however, a much thicker
3.4. Influence of particle dissolution on the oxide film growth The dielectric behaviour of the oxide, grown potentiodynamically by cyclic scans between y1 V and different anodic reverse scan potentials, was analysed by means of impedance spectroscopy at Es0 V. In all cases, a quasi-ideal dielectric behaviour was shown and the oxide capacity was calculated according to: COXs´ ´0 yds(NZN 2p f)y1
(1)
where ´ and d are the dielectric constant and the oxide thickness, respectively. Fig. 6 shows the potential dependence of the inverse of capacity obtained for a vertical electrode with Ni and Co deposits. In all cases,
Fig. 6. Dependence of the inverse of capacity (obtained at Es0 V and fs500 Hz, DEs10 mV) on the anodic limit of cyclic anodic scans performed on Al, AlyNi and AlyCo. Deposition as shown in Fig. 1.
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Fig. 7. TEM examination of ultramicrotomed sections of AlyNi and AlyCo after anodising up to 4 V in acetate buffer solution.
layer of 180 nm is observed. The higher constrast presented by this film suggests that it probably corresponds to a Co(OH)2 film. In spite of the similar solubility constants of nickel and cobalt hydroxides, pKss15.8 and 15.6, respectively, a thicker film is the result of a more rapid dissolution of Co. 4. Discussion Different types of deposits seem to be generated on Al in the potentiodynamic scans due to the potentialdependence of the deposition mechanism. In fact, the different cathodic peaks observed in the deposition of Ni are related to passivation processes caused by precipitation of a nickel hydroxide film on the growing clusters w15–17x. The morphology and surface chemistry of deposits will determine the processes occurring during the anodisation of the modified surface. These clusters begin to dissolve in the anodic scan simultaneously with the growth of aluminium oxide. At the beginning of the anodic scan, the dissolution of some particles takes place at the deposityelectrolyte interface (Esy1.0 V). This process also coincides with the overshoot generated by a delayed growth of the aluminium oxide. Although the anodic dissolution of particles of alloying elements is clearly identified on AlyCo (Fig. 1b), it is probably masked by the overshoot in the case of AlyNi. With the
progress of anodisation, the particles lose their contact with the metal substrate due to the growth of an aluminium barrier type oxide film just in between. At approx. 0.9 V, the valence band edge of the aluminium oxide reaches the redox level of the couple Ni2q yNi (Co2q yCo) and the dissolution of particles starts again through tunnelling of holes. Then, the dissolution kinetics of the deposits is reflected by the different heights of peaks after depositing the alloying elements by different potentials steps (Fig. 3c,d). As a consequence of the continuous growth of the aluminium oxide, part of the deposits is possibly retained at the aluminiumyoxide interface. An enrichment of the depositing element should be also taken into account. This fact was extensively discussed in a series of papers dealing with the anodisation of different aluminium alloys w1,2,18x, wherein the enrichment of the alloying element was related to a higher Gibbs free energy of oxide formation with respect to that of Al (DGOXs y326.9 Kcal moly1). This would be here the case, considering that DGOX is y51.7 Kcal moly1 and y49.0 Kcal moly1 for NiO and CoO, respectively w19x. At low over-potentials, the current transients tend to a constant current on applying potentiostatic steps in Ni2q and Co2q containing buffer solutions (Fig. 3). This type of transients resembles those of a threedimensional growth of deposits controlled by lattice incorporation of ad-atoms w20,21x. According to this, the last part of transients is related with the overlapping of growing crystallites generating an almost compact film. This may also explain the formation of a homogeneous hydroxide film in a nano-scale on the major part of the surface (Fig. 7). The formation of a surface alloy is another possibility that cannot be disregarded. However, the growth of several out-coming deposits is also favoured in several points during the potentiodynamic scan (Fig. 2). At E)4 V, the anodic dissolution of deposited clusters covered by the oxide film is maintained by migration of cations from the substrateyoxide to the oxideyelectrolyte interface. Then, the reduction of the electric field necessary for the growth of the aluminium oxide film is reflected in lower slopes of the E vs. qa plots in the galvanostatic experiments (Fig. 4) Other researchers reported similar results on anodising Al–Mg alloys w22x. Further, the higher dissolution rate of Co generates a higher flux of Co2q and the potential increases more slowly. Towards potentials near 10 V, the particles of alloying elements gradually lose their contact with the surface and the behaviour of pure aluminium is again observed. However, the frequency response of the oxide film is practically not affected by the dissolution of alloying elements just placed in the substrateyfilm interface. It is believed that Ni2q or Co2q, once incorporated into the alumina film, migrates outwards at a greater rate than Al3q. Probably, the dissolution of deposits on
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the substrateyoxide interface proceeds in a similar way as found on Al–Cu alloys by other researchers w1x. They suggested that the local formation of copper oxide would provide a low resistance pathway for further concentration of current. This would be also a reliable explanation for the anodic current spikes observed during the anodic scans. However, a more detailed study of the composition of the film as well as the structure of deposits is necessary to understand the transport mechanism. 5. Conclusions The anodic dissolution of Ni and Co, previously deposited on Al occurs simultaneously with the formation of a barrier aluminium oxide film. This latter progressively covers some of the clusters of alloying element and the dissolution continues by tunnelling of holes at potentials more positive than 0.9 V. Part of the deposits is retained at the aluminiumyfilm interface and its dissolution occurs by injection of Ni2q or Co2q into the alumina film. Then, cations are rapidly transported by the electric field to the filmyelectrolyte interface. This process is evidenced by a reduction of the electric field for oxide growth as well as by current spikes in the anodic scans. The continuous ejection of cations to the electrolyte, by dissolution of particles either in direct contact with the electrolyte or just beneath the growing alumina film, gives rise to the precipitation of a low resistive hydroxide layer at the uppermost part of the oxide film. Acknowledgments ´ The Consejo Nacional de Investigaciones Cientıficas ´ y Tecnicas (CONICET) is gratefully acknowledged for the financial support.
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References w1x M.A. Paez, ´ T.M. Foong, C.T. Ni, G.E. Thompson, K. Shimizu, H. Habazaki, P. Skeldon, G.C. Wood, Corros. Sci. 38 (1996) 59. w2x G.M. Brown, K. Shimizu, K. Kobayashi, P. Skeldon, G.E. Thompson, G.C. Wood, Corros. Sci. 40 (1998) 1575. w3x S. Menezes, R. Haak, G. Hagen, M. Kendig, J. Electrochem. Soc. 136 (1989) 1884. w4x E. MacCafferty, Corros. Sci. 45 (2003) 301. w5x J.O’M. Bockris, Y. Kang, J. Solid State Electrochem. 1 (1997) 17. w6x R.B. Inturi, Z. Szklarska-Smialowska, Corros. Sci. 34 (1993) 705. w7x S.B. Saidman, A.G. Munoz, ˜ J.B. Bessone, J. Appl. Electrochem. 29 (1999) 245. w8x S.B. Saidman, S.G. Garcıa, ´ J.B. Bessone, J. Appl. Electrochem. 25 (1995) 252. w9x H. Leidheiser Jr, I. Suzuki, J. Electrochem. Soc. 128 (1981) 242. w10x D.R. Arnott, B.R.W. Hinton, N.E. Ryan, Mater. Perform. 26 (1987) 42. w11x M.G.A. Khedr, A.M.S. Lashien, Corros. Sci. 33 (1992) 137. w12x J.D. Gorman, A.E. Hughes, D. Jamieson, P.J.K. Paterson, Corros. Sci. 45 (2003) 1103. w13x D.R. Arnott, N.E. Ryan, B.R.W. Hinton, B.A. Sexton, A.E. Hughes, Appl. Surf. Sci. 22y23 (1985) 236. w14x M.M. Lohrengel, Mater. Sci. Eng. R 11 (1993) 243. w15x A.G. Munoz, ˜ D.R. Salinas, J.B. Bessone, Thin Solid Films 429 (2003) 119. w16x A.G. Ives, J.W. Edington, G.P. Rothwell, Electrochim. Acta 15 (1970) 1797. w17x F. Lantelme, A. Seghiouer, J. Appl. Electrochem. 28 (1998) 907. w18x H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Corros. Sci. 39 (1997) 731. ´ w19x M. Pourbaix, Atlas d’Equilibres Electrochimiques, Gautier Villars, Paris, 1963, p. 323. w20x M. Fleischmann, H.R. Thirsk, Electrochim. Acta 1 (1959) 146. w21x M. Palomar-Pardave, ´ I. Gonzalez, A.B. Soto, E.M. Arce, J. Electroanal. Chem. 443 (1998) 125. w22x Y. Liu, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, Corros. Sci. 44 (2002) 1133.
Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co ˜ A.G. Munoz*, J.B. Bessone ´ Quımica, ´ ´ Blanca, Argentina INIEC-Dto de Ingenierıa Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahıa Received 3 June 2003; received in revised form 10 January 2004; accepted 6 February 2004 Available Online 12 April 2004
Abstract Surface conditions similar to those found in aluminium alloys of practical use were assessed by cathodic deposition of transitions metals (Ni and Co) from different electrolytes. Fundamental aspects concerning with the growth of anodic oxide films at potentials lower than 10 V in neutral acetate buffer solution on these modified surfaces were analysed by common electrochemical techniques complemented with scanning electron microscopy and transmission electron microscopy. In both potentiodynamic and galvanostatic modes, the growth of aluminium oxide competes with the dissolution of deposited metal particles. The formation of a thin aluminium barrier oxide film beneath them shifts the dissolution potential over to 1.5 V towards more positive values. Some particles get progressively embedded in the matrix of the growing alumina and act as cation sources, increasing the film conductivity and diminishing the established electric field in the oxide. This effect is more pronounced with Co deposits due to its high active dissolution rate, before passivation occurs. Then, the generation of a two-layer film is explained in terms of the precipitation of metal hydroxide at the solution side on the oxide barrier film. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium oxide; Growth mechanism; Nickel; Transmission electron microscopy
1. Introduction The study of the anodic oxide growth on aluminium alloys is getting increasing interest due to the necessity for a better knowledge of the microscopic processes occurring at the surface either during corrosion or different surface treatments. Alloying elements, usually introduced to improve the corrosion resistance and mechanical strength, exert a great influence on the composition and structure of the oxide, thus conducting to different properties of the film w1,2x. In order to get a more corrosion resistant aluminium surface, the oxide film was modified by introduction of inhibitors in the electrolyte w3x or by addition of alloying elements such as Mo, Zr, Ta and Nb w4–6x. Thus, a close examination of the anodic processes is of vital importance for understanding the mechanisms under which the presence of alloying elements improves the corrosion behaviour. *Corresponding author. Tel.: q54-2914-5951-82; fax: q54-29145951-82. ˜ E-mail address: [email protected] (A.G. Munoz).
The cathodic deposition is a suitable method to obtain surface conditions similar to those found in alloys w7,8x. This also opens other possibilities for obtaining information about the highly localised processes as film formation and dissolution. In the present study, Ni and Co were selected as transition metals, the first being present in several special aluminium alloys. However, the presence of these elements as cations in solution was found to reduce the corrosion rate of steel w9x and aluminium w10,11x. In that case, Auger depth profiles have given evidence for the incorporation of Ni2q and Co2q into the forming oxide film. Similar conclusions were addressed in the presence of Ce3q w12,13x. Then, the reduction of the corrosion rate was mainly explained in terms of the suppression of the oxygen reduction reaction by the presence of oxideyhydroxide films. Accordingly, thicker films, with a more open structure, was the cause for a poor protection offered by Co2q. The aim of this work is to get a closer insight into the processes occurring during the anodisation of aluminium surfaces previously modified by cathodic dep-
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.017
144
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osition of Ni and Co. Thus, an approach to surface conditions during the corrosion of aluminium in the presence of cationic film-forming inhibitors or the anodisation of aluminium alloys will be intended. 2. Experimental details Sheets and rods of aluminium 99.99% embedded in a Teflon holder conforming vertical and horizontal electrodes with an exposed area of 0.5 cm2 and 0.070 cm2, respectively, were used as working electrodes. They were mirror like polished with 1000 emery paper and then with 0.3 mm alumina. Afterwards, they were submerged in a 0.5 M NaOH solution for 10 min, rinsed with pure water and neutralised in acetic acid for 30 s. A reproducible surface covered with a very thin oxide film was expected to be obtained with this method. Experiments were performed with a conventional cell of three compartments in a purified N2 atmosphere at room temperature. A Pt sheet and a saturated sulfate electrode (sse) (E 0s0.64 V vs. nhe) were used as counter and reference electrodes, respectively. Solutions were prepared from analytical grade chemicals. All potentials were referred to the sse. Otherwise stated, all experiments were performed with a vertical electrode. The deposition of alloying elements was made from neutral acetate buffer solutions with a cation concentration of 0.02 M. The subsequent anodic growth was performed in acetate buffer solution without cations. As AlyNi and AlyCo we refer in the text to aluminium surfaces modified by cathodic deposition of Ni and Co, respectively. Sweep voltammetries were performed using a linear voltage sweep generator EG&G PAR model 175 and a potentiostat–galvanostat EG&G PAR model 173. A dual stage ISI DS 130 scanning electron microscope was used to examine the electrode surface. Transmission electron microscopy (TEM) was performed with a Jeol JSM 100CXII. In this case, aluminium sheets were embedded into an epoxy resin, wherefrom sections of 80 nm thickness were obtained by a LKB 8800 Ultratome III ultramicrotome.
Fig. 1. (a) Cyclic voltammograms showing the potentiodynamic deposition in acetate buffer solutions containing 0.02 M of metal cation. vs0.02 V sy1. Es,asy0.8 V. (-----) cyclic voltammetry performed in a solution without cations; (b) anodic scans performed on the Al surface modified by deposition of Ni and Co in acetate buffer solution. vs0.1 V sy1. Es,csy1.0 V, Es,as4 V.
3. Results 3.1. Voltammetric identification of anodic processes Fig. 1b shows the anodisation of vertical sheets whereon Ni or Co was previously deposited by means of potentiodynamic scans from acetate buffer solutions (Fig. 1a). The presence of a thin oxide layer on aluminium makes the charge transfer difficult and deposition begins at high over-potentials. In spite of similar potentials for NiyNi2q(E 0sy0.89 V) and Coy Co2q(E 0sy0.917 V), the deposition of Co seems to be favoured. The hydrogen evolution reaction on the
formed Co deposits predominates at E-y1.9 V, generating a considerable local alkalisation. Thus, the hysteresis observed at the reverse scan may be explained in terms of the cathodic dissolution of the surrounding Al oxide, favouring further deposition. The voltammetries performed in Ni2q containing solutions, however, present various peaks that can be attributed to different mechanisms of nucleation and growth. The deposition of Ni from neutral solutions is a complex process involving the passivation of the growing deposits. An anodic peak is always observed at the beginning of anodisation (overshoot), which can be related with a
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Fig. 2. SEM micrographs of Al surface after potentiodynamic deposition of Co. vs0.02 V sy1 . Es,a sy0.8 V, Es,c sy2.0 V. (a) as deposited; (b) after anodising up to 4 V.
delayed oxide formation. This phenomenon was extensively discussed in the literature w14x and is out of the scope of this paper. A second anodic peak, identified as a shoulder before the overshoot, is observed in the anodisation of AlyCo, it being attributed to the dissolution of Co deposits. At more anodic potentials, a current plateau of approx. 0.3 mA cmy2 extends up to 0.9 V, indicating the growth of a homogeneous oxide film according to the high-field law. At E)0.9 V, the appearance of a characteristic anodic peak may be ascribed to the dissolution of deposits separated from the metallic substrate by the growing anodic film. The corresponding evolved charges Qa were 3.25 mC cmy2 and 23.7 mC cmy2 for AlyNi and AlyCo, respectively, which are much lower than those for deposition (approx. 85 mC cmy2 in both cases). Therefore, this low efficiency may be related with the passivation of the surface of particles andyor the covering of them by the growing film of alumina. At potentials E)2 V, a continuous increase of the current with the presence of superimposed anodic spikes is close related with the transport of Ni2q or Co2q through the growing aluminium oxide film. Thus, the higher currents observed in the case of AlyCo might be a consequence of the higher flux of Co2q brought about by a higher dissolution rate. The changes of surface morphology during the oxide growth were followed by scanning electron microscopy (SEM). Fig. 2a shows the surface of Al after the potentiodynamic deposition of Co (Fig. 1a). Rounded grains evolving to star-like forms of approx. 5 mm can be observed on an initial porous aluminium surface. Certain oxide dissolution, as a consequence of the strong local alkalisation, can be noted around some of them. After the oxide growth up to 4 V, some particles were not dissolved yet (Fig. 2b). A uniform distribution of small white spots can be also distinguished at this stage. That is probably related with the precipitation of hydroxide particles during the dissolution process. The dissolution kinetics of deposited particles at E) 0.9 V is related to their surface state, which in turn depends strongly on the conditions developed during
deposition. Fig. 3 shows the current transients, obtained by applying different potential steps and the corresponding anodic scans after 37 s deposition. In the case of Ni (Fig. 3a), the passivation of growing particles is denoted by a rapid current decay until they reach a constant value (Esy1.85 V). This effect was already analysed in a previous work and attributed to the generation of a surface Ni(OH)2 film arresting further deposition w15x. At more cathodic potentials, deposition takes place by reduction of hydroxide particles precipitated at the surface, which is revealed by transient patterns with the appearance of various peaks (dotted line in Fig. 3a). This is as a consequence of the rapid local alkalisation given by the hydrogen evolution reaction on the substrate. Passivation of Ni particles conducts to a drastic reduction of the anodic dissolution of them at 1.5 V (Fig. 3c). A similar effect, but in a lesser extent, is also observed in the deposition of Co. In this case, the charge passed at y1.9 V is three times that at y1.7 V (Fig. 3b), but the formation of a surface hydroxide layer during deposition would diminish the dissolution rate in the anodic scan (Fig. 3d). Also, the enhanced alkaline dissolution of Al oxide at the most cathodic potentials may bring about the formation of an insulating film beneath the particles. 3.2. Galvanostatic growth The influence of deposits on the oxide growth was further analysed by applying current steps (Fig. 4). Pure aluminium presents a linear potential increase with time, in accord with a high-field growth mechanism w14x. However, a slope value of 0.4 V mCy1 cm2, lower than that reported in the literature of 0.56 V mCy1 cm2, is obtained. This result may be ascribed to a higher real surface area and a loss of current efficiency. Near 10 V, the onset of the oxygen evolution reaction impedes further potential increase. In the case of modified surfaces, three potential regions, coinciding with the different anodic processes observed during the potentiodynamic growth, can be distinguished. Firstly,
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Fig. 3. (a) and (b) i vs. t transients recorded on applying potential steps on Al in acetate buffer solutions containing 0.02 M of metal cations: (a) Ni2q; (b) Co2q. (c) and (d) anodic scans performed in acetate buffer solution after 37 s deposition as shown in (a) and (b), respectively.
the potential increases linearly with an identical slope as aluminium does. Then, two successive shoulders appear at qa)5 mC cmy2 for the AlyNi. They can be related with dissolution of deposits in the metalyelectrolyte and metalyoxide film, respectively. However, the uninhibited dissolution of Co particles maintains the potential at 0.9 V until it reachs practically the deposition charge. Then, the dissolution of Co through the oxide film is characterised by a slow continuous increase of the slope.
3.3. Influence of the surface roughness Due to the different local hydrodynamic conditions established by the detachment of hydrogen bubbles during the initial alkaline treatment of the surface, different surface morphologies arise for horizontal or vertical positions of the electrode. Fig. 5 shows the anodic scans performed after depositing Ni in vertical and horizontal positioned aluminium electrodes. The horizontal position generates a rougher surface with a
Fig. 4. Time-dependence of potential, depicted as a function of passed anodic charge, for the anodisation of Al at is0.707 mA cmy2 with and without the presence of deposits. The deposits were generated potentiodynamically as shown in Fig. 1.
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Fig. 5. Anodic scans performed on aluminium electrodes with different positions after depositing Ni from an acetate buffer solution containing 0.02 M Ni2q. vs0.1 V sy1. Es,csy1.0 V, Es,as4 V. Detail: cyclic voltammograms showing the potentiodynamic deposition. vs 0.02 V sy1. Es,asy0.8 V.
real area twice that for the vertical one. In fact, charges of 94 mC cmy2 and 161 mC cmy2, referred to the geometric area, were obtained for the same cathodic scan in vertical and horizontal electrodes, respectively, (see detail at the upper-right side). The initial current plateaux in the subsequent anodisation point out also this fact. Thus, roughness factors of 1.1 and 1.9 can be estimated from a current plateau of 0.25 mA cmy2 reported for an electro-polished surface w14x. Considering the true area, the anodic current of the dissolution processes at E)0.9 V is very much higher for the vertical electrode. This means that the particles deposited onto the rougher surface (horizontal position) would get more rapidly covered with the growing film of alumina.
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a linear relationship is observed, as expected for a highfield growth. Then, the similar slopes of Cy1 vs. E plots suggest that the presence of deposits do not exert any effect on the dielectric behaviour of the oxide. This means that cations generated by dissolution of particles just beneath the growing film of alumina are rapidly transported by the electric field to the filmyelectrolyte interface. In the case of pure aluminium, the slope of the Cy1 vs. E plot for a horizontal electrode is 2.3 times lower than that for the vertical position, as expected for a higher real surface area. The presence of deposits gives rise to an increment of the slope only for a horizontal electrode. Here, a less homogeneous surface, given by a higher roughness, would allow the metal cations to be accumulated into the oxide structure. Thus, a thicker film with a lower dielectric constant may arise. In order to get more information about the morphology of the oxide film, grown in the presence of Ni or Co deposits, ultramicrotomed sections were examined by TEM after anodisation up to 4 V (Fig. 7a,b). A film consisting of two layers is observed after anodising the AlyNi. The outer film has a thickness of approx. 25 nm. This value is almost three times thicker than that expected of 7.2 nm at 4 V for the aluminium oxide film, according to the expression: dsk(EyEOX)
(2)
where k is the formation factor (s1.2 nm Vy1 w14x) and EOX the equilibrium potential of the oxide electrode (sy1.99 V sse). Then, this film could be identified as an upper precipitated nickel hydroxide film formed from the flux of Ni2q transported to the filmyelectrolyte interface. The inner layer, however, would thus correspond to the aluminium oxide contaminated with Ni2q. After anodisation of the AlyCo, however, a much thicker
3.4. Influence of particle dissolution on the oxide film growth The dielectric behaviour of the oxide, grown potentiodynamically by cyclic scans between y1 V and different anodic reverse scan potentials, was analysed by means of impedance spectroscopy at Es0 V. In all cases, a quasi-ideal dielectric behaviour was shown and the oxide capacity was calculated according to: COXs´ ´0 yds(NZN 2p f)y1
(1)
where ´ and d are the dielectric constant and the oxide thickness, respectively. Fig. 6 shows the potential dependence of the inverse of capacity obtained for a vertical electrode with Ni and Co deposits. In all cases,
Fig. 6. Dependence of the inverse of capacity (obtained at Es0 V and fs500 Hz, DEs10 mV) on the anodic limit of cyclic anodic scans performed on Al, AlyNi and AlyCo. Deposition as shown in Fig. 1.
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Fig. 7. TEM examination of ultramicrotomed sections of AlyNi and AlyCo after anodising up to 4 V in acetate buffer solution.
layer of 180 nm is observed. The higher constrast presented by this film suggests that it probably corresponds to a Co(OH)2 film. In spite of the similar solubility constants of nickel and cobalt hydroxides, pKss15.8 and 15.6, respectively, a thicker film is the result of a more rapid dissolution of Co. 4. Discussion Different types of deposits seem to be generated on Al in the potentiodynamic scans due to the potentialdependence of the deposition mechanism. In fact, the different cathodic peaks observed in the deposition of Ni are related to passivation processes caused by precipitation of a nickel hydroxide film on the growing clusters w15–17x. The morphology and surface chemistry of deposits will determine the processes occurring during the anodisation of the modified surface. These clusters begin to dissolve in the anodic scan simultaneously with the growth of aluminium oxide. At the beginning of the anodic scan, the dissolution of some particles takes place at the deposityelectrolyte interface (Esy1.0 V). This process also coincides with the overshoot generated by a delayed growth of the aluminium oxide. Although the anodic dissolution of particles of alloying elements is clearly identified on AlyCo (Fig. 1b), it is probably masked by the overshoot in the case of AlyNi. With the
progress of anodisation, the particles lose their contact with the metal substrate due to the growth of an aluminium barrier type oxide film just in between. At approx. 0.9 V, the valence band edge of the aluminium oxide reaches the redox level of the couple Ni2q yNi (Co2q yCo) and the dissolution of particles starts again through tunnelling of holes. Then, the dissolution kinetics of the deposits is reflected by the different heights of peaks after depositing the alloying elements by different potentials steps (Fig. 3c,d). As a consequence of the continuous growth of the aluminium oxide, part of the deposits is possibly retained at the aluminiumyoxide interface. An enrichment of the depositing element should be also taken into account. This fact was extensively discussed in a series of papers dealing with the anodisation of different aluminium alloys w1,2,18x, wherein the enrichment of the alloying element was related to a higher Gibbs free energy of oxide formation with respect to that of Al (DGOXs y326.9 Kcal moly1). This would be here the case, considering that DGOX is y51.7 Kcal moly1 and y49.0 Kcal moly1 for NiO and CoO, respectively w19x. At low over-potentials, the current transients tend to a constant current on applying potentiostatic steps in Ni2q and Co2q containing buffer solutions (Fig. 3). This type of transients resembles those of a threedimensional growth of deposits controlled by lattice incorporation of ad-atoms w20,21x. According to this, the last part of transients is related with the overlapping of growing crystallites generating an almost compact film. This may also explain the formation of a homogeneous hydroxide film in a nano-scale on the major part of the surface (Fig. 7). The formation of a surface alloy is another possibility that cannot be disregarded. However, the growth of several out-coming deposits is also favoured in several points during the potentiodynamic scan (Fig. 2). At E)4 V, the anodic dissolution of deposited clusters covered by the oxide film is maintained by migration of cations from the substrateyoxide to the oxideyelectrolyte interface. Then, the reduction of the electric field necessary for the growth of the aluminium oxide film is reflected in lower slopes of the E vs. qa plots in the galvanostatic experiments (Fig. 4) Other researchers reported similar results on anodising Al–Mg alloys w22x. Further, the higher dissolution rate of Co generates a higher flux of Co2q and the potential increases more slowly. Towards potentials near 10 V, the particles of alloying elements gradually lose their contact with the surface and the behaviour of pure aluminium is again observed. However, the frequency response of the oxide film is practically not affected by the dissolution of alloying elements just placed in the substrateyfilm interface. It is believed that Ni2q or Co2q, once incorporated into the alumina film, migrates outwards at a greater rate than Al3q. Probably, the dissolution of deposits on
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the substrateyoxide interface proceeds in a similar way as found on Al–Cu alloys by other researchers w1x. They suggested that the local formation of copper oxide would provide a low resistance pathway for further concentration of current. This would be also a reliable explanation for the anodic current spikes observed during the anodic scans. However, a more detailed study of the composition of the film as well as the structure of deposits is necessary to understand the transport mechanism. 5. Conclusions The anodic dissolution of Ni and Co, previously deposited on Al occurs simultaneously with the formation of a barrier aluminium oxide film. This latter progressively covers some of the clusters of alloying element and the dissolution continues by tunnelling of holes at potentials more positive than 0.9 V. Part of the deposits is retained at the aluminiumyfilm interface and its dissolution occurs by injection of Ni2q or Co2q into the alumina film. Then, cations are rapidly transported by the electric field to the filmyelectrolyte interface. This process is evidenced by a reduction of the electric field for oxide growth as well as by current spikes in the anodic scans. The continuous ejection of cations to the electrolyte, by dissolution of particles either in direct contact with the electrolyte or just beneath the growing alumina film, gives rise to the precipitation of a low resistive hydroxide layer at the uppermost part of the oxide film. Acknowledgments ´ The Consejo Nacional de Investigaciones Cientıficas ´ y Tecnicas (CONICET) is gratefully acknowledged for the financial support.
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