Composition and structure of coloured oxide films on stainless steel formed by triangular current scan and cathodic hardening treatment

Corrosion Science 49 (2007) 2303 2314 www.elsevier.com/locate/corsci Composition and structure of coloured oxide films on stainless steel formed by triangular current scan and cathodic hardening treatment E. Kikuti a, N. Bocchi a, J.L. Pastol b, M.G. Ferreira c,d, M.F. Montemor d, M. da Cunha Belo b,d, A.M. Simões d, * a Department of Chemistry, Universidade Federal de São Carlos, Cx Postal 676, 13 560-970 São Carlos, SP, Brazil b Centre National de La Recherche Scientifique, CECM, 15 Rue Georges Urbain, F94407 Vitry-sur-Seine Cedex, France c Department of Ceramics and Glass, University of Aveiro, Aveiro, Portugal d Instituto Superior Técnico, Chemical Engineering Department, Av. Rovisco Pais, 1049-001 Lisboa Codex, Portugal Received 10 July 2006; accepted 8 September 2006 Available online 25 January 2007 Abstract The influence of the electrochemical hardening treatment on the composition and structure of coloured porous films formed by triangular current scan on AISI 304 stainless steel has been examined by impedance spectroscopy and also by SEM, AES, XPS and AFM. It was observed that the films, originally consisting of a porous chromium iron oxide, became more compact and enriched in chromium after the hardening treatment. The development of a high frequency semi-circle in the impedance spectra is explained by the formation of an outer oxide layer. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Stainless steel; A. Acid solutions; C. Hardening; C. Passive films * Corresponding author. Tel.: +35 121 841 7963; fax: +35 121 840 4589. E-mail address: alda.simoes@ist.utl.pt (A.M. Simões). 0010-938X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.09.002

2304 E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 1. Introduction The growth of interference-coloured oxide films on stainless steel is a challenging area, with great interest for architectural and decorative applications. Coloured oxide films with controlled thickness can be grown on stainless steel by either chemical [1,2] or electrochemical oxidation [3 7]. The chemical process consists of immersion in a hot, concentrated solution of chromic and sulphuric acids and leads to the formation of thick films that seem to result from the dissolution of steel and concomitant reduction of chromic acid [1]. Chromic sulphuric acid solutions, however, are hazardous when used at high temperatures, and therefore the use of electrochemical techniques is a potentially interesting alternative. Ogura et al. [5,7] have grown films with interference colours by applying either alternating potential pulses or a triangular current scan to a stainless steel substrate in chromic and sulphuric acid solution at ambient temperature. In the triangular current scan method the anodic currents are attributed to the anodic dissolution of steel, whereas the cathodic process is apparently due to the reduction of Cr(VI) ions to Cr 3+ : M! M zþ þ ze ðm ¼ Fe; Ni; CrÞ ð1þ Cr 2 O 2 7 þ 14H þ þ 6e! 2Cr 3þ þ 7H 2 O ð2þ These reactions occur spontaneously at room temperature, although at a slow rate. The role of cyclic polarization is to accelerate the process. The interference film may result from hydrolysis of both metal ions with precipitation of a mixed oxide [7]: pm zþ þ qcr 3þ þ rh 2 O! M p Cr q O r þ 2rH þ ðzp þ 3q ¼ 2rÞ ð3þ Electrolytic films seem to be thinner than chemically formed films, although they seem to have improved resistance to mechanical damaging. Chemically formed films are highly porous, with a pore fraction of 20 30%, and therefore very prone to scratching [6]. One way to improve the mechanical properties of chemical films is by a cathodic polarization treatment [1], which also leads to further thickening of the film and to changes in the interference colour. The basic composition of these films is oxides and hydroxides, of mainly chromium, and their chemical composition is similar to that of thin passive films formed on stainless steel. However, both the treatments of coloration and of hardening lead to differences of surface composition and structure that may affect corrosion resistance. Wang et al. [8] studied the resistance to pitting of stainless steel coloured by different methods and concluded that the pitting potential increased as a result of the hardening treatment. In this work the hardening treatment was used on films grown by the triangular scan method. The influence of the hardening treatment on the structure and properties of the coloured films was studied by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).

E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 2305 2. Experimental 2.1. Material A 1.0 mm thick plate of bright annealed austenitic AISI-304 stainless steel was used, with the following composition: 17.7% Cr; 7.4% Ni; 1.2% Mn; 0.4% Si; 0.01% S; 0.07% C and Fe balanced. Specimens of 10 mm 30 mm in size were cut from the steel sheet, degreased for 10 min with acetone under ultrasonic stirring and then rinsed in distilled water. Prior to colouring, samples were electrochemically reduced (i = 1.0 ma cm 2 ) for 20 min in an aqueous 1.0 mol L 1 HNO 3 solution, in order to provide a uniform state of the surface to all the samples. Electrochemical colouring was made in a 5.0 mol L 1 H 2 SO 4 (Carlo Erba p.a.) + 2.5 mol L 1 CrO 3 (Carlo Erba, 99%) solution, at room temperature, in a 150-mL cell, using two spiral Pt wires as auxiliary electrodes. The current density values were alternated between 0.81 ma cm 2 and +2.0 ma cm 2, at a scan rate of 9 ma s 1, following the method reported by Ogura et al. [7]. Electrolysis was carried out at 25 C for 40 min. The thickness of the layer achieved in this was 240 ± 10 nm [9]. The cathodic hardening treatment was made at a constant current density of 4mAcm 2 in 0.026 mol L 1 H 3 PO 4 (Mallinchrot p.a.) + 2.5 mol L 1 CrO 3 (Carlo Erba, 99%) aqueous solution for 15 min at room temperature. A Hg/Hg 2 SO 4 electrode in 5.0 mol L 1 H 2 SO 4 was used as reference. 2.2. Electrochemical measurements The electrochemical study was conducted in a borate buffer solution of ph 9.2 [0.05 mol L 1 H 3 BO 3 (Merck p.a.) + 0.075 mol L 1 Na 2 B 4 O 7 Æ 10 H 2 O (Riedel-de Haen analytical reagent)], at room temperature. EIS measurements were obtained at the open circuit potential, using a 10 mv sine wave perturbation, by means of a Solartron 1255 lock-in amplifier and a 1260 Electrochemical Interface. The results were normalized to the geometrical area of the sample. 2.3. Surface analysis Auger and XPS measurements were made using a 310F Microlab equipped with a field emission electron gun and a concentric hemispheric analyzer. Auger spectra were taken using a 10 kev, 50 na primary electron beam. Ion etching was performed at a pressure of 10 7 mbar using high-purity argon, generating a current of 1 la mm 2 on a crater with a diameter of 1 mm. The electron beam was aligned with the centre of the sputtered region and had a spatial resolution of 100 nm. Although the roughness of the surface increased with the sputtered depth, the high spatial resolution of the electron beam limits the deleterious effects of long sputtering. The concentrations of Fe, Cr, Ni and O were normalized to 100%. The thickness of the film can be estimated by the point at which the oxygen signal reduces to half of its original value. X-ray photoelectron spectroscopy (XPS) was made using a non-monochromated Mg (Ka) photon

2306 E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 source. Spectra were taken in constant analyser energy mode (CAE = 30 ev). For these conditions the energy resolution is approximately 0.9 ev. Deconvolution and quantification of the peaks was made using a Shirley baseline and Eclipse software (Fisons Surface Science, UK). 3. AFM AFM mapping of the dry samples was made in the tapping mode, using a Nanoscope system (Digital Instruments, USA). Measurements were made on a 50 lm 50 lm area. 3.1. SEM observations A LEO Gernini 1530, operating at a tension of 3 kv, was used for surface inspection of the samples. 4. Results 4.1. D.c. electrochemical behaviour The anodic polarization plots of the stainless steel in the three situations studied is shown in Fig. 1. The untreated steel has a passive plateau that extends to the potential of oxidation of water, at approximately +0.9 V. This plateau consists in fact of one region of lower passive current and another, above +0.5 V, at which chromium oxides become soluble, increasing the passive current. The growth of the coloured film led to a drop in the corrosion potential by nearly 0.2 V and also to an increase of both the corrosion rate and the passive dissolution current. This increase of the passive current agrees with the growth of a porous film on the surface, which results in increased area and in anodic depolarization. The hardening treatment did not significantly affect the currents measured (curve 3), -2-3 log i / A cm -2-4 -5-6 -7 3 2-8 1-9 -1.0-0.5 0.0 0.5 1.0 1.5 U / V Fig. 1. Potentiodynamic curves (1 mv s 1 ) obtained in borate buffer solution at ph 9.2, for AISI-304 stainless steel non-treated samples (1) and coloured samples, before (2) and after (3) the cathodic hardening treatment.

E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 2307 but it again shifted the spontaneous potential to more negative values. This suggests that the active area did not increase further and that cathodic polarization resulted from hindered diffusion of oxygen across a porous film. During the cathodic galvanostatic treatment the potential evolution revealed a small step at 0.4 V (Fig. 2), possibly due to the reduction of Fe 2+ ions occluded in the pores of the oxide, and after that it became stable at 1.1 V, at which simultaneous reduction of chromate ions and of water is likely to occur. 4.2. Surface morphology The micrograph of the surface reveals a grain structure with grains of roughly 10 20 lm (Fig. 3a). This morphology reflects the underlying grain structure of the metal substrate and is in good agreement with the morphological properties reported before [10]. At high magnifications a fine porous structure was revealed by numerous dark spots scattered along the oxide layer (Fig. 3b). The topography of the surface inspected by AFM reveals a rough surface both before and after the hardening treatment, although the crystallites at the surface were slightly U / V (vs. Hg / Hg 2 SO 4 ) 0.4 0.0-0.4-0.8-1.2 0 200 400 600 800 1000 t / s Fig. 2. Evolution of the potential during the galvanostatic cathodic treatment, at 4 ma cm 2. Fig. 3. Micrographs of the coloured oxide film, showing the grain (a) and the intragranular pattern (b).

2308 E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 coarser after the treatment (Fig. 4). This can be due either to the deposition of a layer above the previous one or to narrowing of the pores due to some blocking effect caused by the precipitation of chromate at pore walls or at the pore mouth. 4.3. Impedance measurements The impedance response of the film before hardening was capacitive for most of the spectrum. It was also identical to that of the native passive film and according to the literature [11]. The hardening treatment led to the appearance of a new time constant at the high frequencies (Fig. 5). This means that the surface has two distinct types of behaviour, responding at high and low frequencies. Fig. 4. Topography of the surface obtained by AFM, for the coloured (a) and the hardened (b) oxide film. Z / ohm.cm 2 10 6 10 5 10 4 10 3 10 2 10 1 c b a 10 0 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) phase angle /deg -90-70 -50-30 -10 c b a 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) Fig. 5. Impedance spectra of the stainless steel with a native oxide film (a) and with the coloured oxide, before (b) and after hardening (c); curves obtained in borate buffer solution.

E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 2309 Fitting of the capacitive part of the spectra was made using a constant phase element (CPE), whose impedance is given by Z CPE ¼ j Y 0 x n where x = angular frequency, Y 0 is the frequency-independent parameter, n is related to the phase angle and gives information on the type of element (n = 1 for a capacitor and n = 0 for a resistor) and j = p 1. For the native and the unhardened films, the equivalent circuit (EC) as well as the results of the fitting is presented in Fig. 6 and Table 1. There was an increase of the capacitance (measured by Y 0 ) due to the presence of the film. Since there was no change in the equivalent circuit of the spectrum and the capacitance is proportional to the area, this suggests that the area contributing to the capacitance has increased. Since the surface was already covered by an oxide film, then the growth of the area can be explained by an increased surface area in the film, i.e., by the thickening of a porous film, as seen in the polarization plots. The fact that there is only one time constant also suggests identical properties for the walls and the bottom of the pores. There was also a decrease of the exponent n, which reveals a distribution of the values of EC along the surface, i.e., a more heterogeneous surface, as could be expected after the growth of a porous layer. R ct accounts for the charge transfer at the film surface. Its estimate is affected by a relatively high error since its response comes mostly below the low frequency limit of the spectrum. Nevertheless, the fact that it decreases with the triangular polarization is in agreement with the development of an extended area. For the hardened film the spectrum is described by a different EC, with two time constants (Fig. 7). A similar EC has been proposed by Jüttner et al. [12] for metals covered with porous layers. In this EC it is assumed that the film has two layers and also that the outer layer partially blocked the pores. Thus, Q out and R out correspond to the outer part of the film, whereas R por is the resistance of the pores in the outer layer; Q in reveals the capacitance of the pore walls and R in is the charge transfer Z / ohm.cm 2 10 5 10 4 10 3 10 2 FitResult 10 1 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) R s Q film phase angle /deg -75-50 -25 0 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) Fig. 6. Fitting and EC of spectrum obtained with the unhardened film. R ct

2310 E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 Table 1 Values used in the fitting of EIS spectra R s /X cm 2 Y 0 (Q)/F cm 2 s n n R ct /X cm 2 Native film 29 2.73 10 5 0.88 7 10 5 Unhardened film 24 1.41 10 4 0.82 7 10 4 Z / ohm.cm 2 10 5 FitResult 10 4 10 3 10 2 10 1 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) R s Q out phase angle /deg -70-60 -50-40 -30-20 -10 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) R out R por Q in R in Fig. 7. Fitting of impedance spectra and EC of the sample with the hardened film; Q out = 1.2 10 4 Fcm 2 ; R por = 236 X cm 2 ; Q in = 6.4 10 4 Fcm 2. resistance inside the pores. R s is the series resistance due to the bulk solution. This circuit becomes in practice simplified because of the passivity of the system that makes the charge transfer resistances R out and R in very high and therefore not measurable. 4.4. Surface analysis The depth profile for the elemental composition of the films shows that the Cr/Fe atomic ratio was strongly modified by the hardening treatment. The film grown by the triangular scan method was composed of approximately similar contents of chromium and iron in the oxidized state (Fig. 8a). After the hardening treatment the film had thickened by 40 50% and the outer layer was highly enriched in chromium and depleted in nickel (Fig. 8b). This outer layer results from the cathodic reduction of dichromate ions and should therefore consist only of chromium oxide. The signal from iron can have its origin in the underlying layers. XPS analysis of the surface of the film before and after the hardening treatment revealed the presence of Ni 0, Ni(II), Fe 0, Fe(II), Fe(III), Cr(III) and oxygen Fig. 9, with no signs of sulphur. Both films revealed a high content of Cr at the surface. Chromium was observed only in one oxidation state, Cr(III). This agrees with results obtained with films formed by square wave pulse polarization [13] and reinforces the idea that the changes caused by the hardening treatment result from further reduction of dichromate ions; the

E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 2311 a atomic % 80 70 60 50 40 30 20 10 0 O Fe Cr Ni 0 200 400 600 800 1000 1200 Sputtering time / s b 70 O Fe 60 Cr Ni 50 40 30 20 10 0 0 500 1000 1500 Sputtering time / s Fig. 8. Auger depth profiles of the coloured oxide films, before (a) and after (b) the electrolytic hardening treatment. Fig. 9. XPS of the oxides formed on stainless steel, before (upper row) and after hardening (bottom). composition is different from films formed chemically, for which both Cr(VI) and Cr(III) were found [14]. Iron was incorporated in the film in two oxidized forms, detected at 708.83 ev and 711.21 ev and attributed to Fe(II) and Fe(III), respectively (Table 2). This last peak probably results from a mixture of Fe 2 O 3 and FeOOH. It became slightly shifted

2312 E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 Table 2 XPS results for coloured oxide films before and after electrolytic hardening Peak Chemical species Binding energy/ev Atomic % Unhardened Hardened Unhardened Hardened Ni2p 3/2 Ni(III) 855.01 855.50 0.8 0.2 Ni 0 852.77 852.83 1.8 0.5 Fe2p 3/2 Fe(III) 711.21 710.80 2.1 0.6 Fe(II) 708.83 709.00 3.0 1.0 Fe 0 706.67 706.7 3.2 1.5 Cr2p 3/2 Cr(III) 576.65 576.66 34.4 37.6 O1s OH 531.61 531.6 27.8 24.9 O 2 530.13 530.1 27.0 33.8 to lower energies after the hardening treatment due to some loss of hydration, as confirmed by a decrease in the OH /O 2 ratio. The distribution of the various oxidation states for each element remained practically unaltered after the hardening treatment, but the total content of Ni and Fe decreased substantially. In spite of the thickening of the film, observed in the Auger profiles, it is interesting to note that the metallic forms of Fe and Ni are detected both before and after the hardening treatment. This differs from the observations on anodic passive films, for which thickening of the film leads to a gradual decrease of the relative intensity of the peak from metallic iron [15] and it is a symptom of film porosity. 5. Discussion At the potential reached during the cathodic treatment, the reactions occurring on the surface are both hydrogen reduction and reduction of chromate to the tri-valent form, either Cr 2 O 3 or Cr(OH) 3. These lead to an increase of ph near the surface and to the deposition of chromium oxide. This deposition, made over a highly porous layer, will occur on the pore walls. In a study by transmission electron microscopy on porous films produced by the chemical method, Wood and co-workers [16] concluded that the porosity of the film did not result from well-defined parallel-sided pores, but rather from random crystallites with size 6 14 nm, the spaces among them being a tortuous network of inter-linking pathways of high effective length and of diameter 1 2 nm. Fujimoto et al. [17] have shown that films formed by square wave polarization consisted of grains with 20 30 nm in diameter and concluded that the grain size changed with the applied potentials and the duration of the pulses. With such narrow pores it is not surprising that diffusion of species in and out of the film is difficult. The results of the potentiodynamic analysis have shown the differences between the two stages of the treatment. In the colouring process the currents increased due to the high porosity of the film, whereas the hardening process led to cathodic polarization, i.e., to difficulties in the transport of species across the film, which suggests narrowing of the pores during the hardening treatment. This is also in good agreement with the changes observed in the AFM and explains why the film becomes harder with the cathodic treatment. Concerning the EIS results, the development of a high-frequency process as a result of the hardening treatment was a marked characteristic of the electrochemical study. The small value of the resistance in this time constant

E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 2313 Fig. 10. Model of the oxide film formed on stainless steel. suggests that it may be due to the resistance of an outer layer formed during the hardening treatment, which has to be overcome before ions can react at the pores. According to our interpretation the film can be described by the model in Fig. 10, in which the two oxide layers correspond to the film formed by the triangular scan treatment and by the hardening treatment. According to this model the Cr(VI) ion content inside each pore at the beginning of the cathodic treatment is limited. These ions become quickly deposited but after that further diffusion from the bulk solution is not easy and therefore most of the Cr deposition has to occur around the mouth of the pores, where the surface is more available. As this outer deposition proceeds, the openings of the pores become narrower, further hindering mass transport to the bottom and inner walls. Consequently, only the initial thin layer is formed inside the pores, whereas an outer layer grows at the mouth of the pores, thus resulting in a partial sealing of the pores. This sealing, unlike the classical boiling water process, did not result from uptake of hydration water, since the oxides became in fact less hydrated. 6. Conclusions The oxide film grown on stainless steel by the triangular current method is highly porous and homogeneous in terms of impedance behaviour. The hardening cathodic

2314 E. Kikuti et al. / Corrosion Science 49 (2007) 2303 2314 treatment performed on this film leads to the formation of a layer of chromium oxide over the original porous oxide. The final film is thicker, less hydrated and less porous than the one formed originally by the triangular scan. Chromium is incorporated in the film exclusively in the form Cr(III). Iron and nickel were also present in the film, but apparently only in the inner layer, formed by the triangular current method. The impedance spectra under non-corrosive conditions revealed a high-frequency time constant associated with the hardening layer. The model proposed considers the existence of two layers of oxide in a porous structure, in which the oxide formed in the hardening treatment is located mainly at the pore mouth, thus hindering the diffusion of species from the bulk solution. Acknowledgements The scholarships granted by CNPq and CAPES to E. Kikuti are gratefully acknowledged. References [1] T.E. Evans, R. Blower, Sheet. Met. Indust. 51 (1974) 230. [2] T.E. Evans, Corros. Sci. 17 (1977) 105. [3] C.J. Lin, J.G. Duh, Surf. Coat. Technol. 70 (1994) 79. [4] S. Fujimoto, T. Shibata, K. Wada, T. Tsutae, Corros. Sci. 35 (1993) 147. [5] K. Ogura, K. Sakurai, S. Uehara, J. Electrochem. Soc. 141 (1994) 648. [6] J.H. Wang, J.G. Duh, H.C. Shih, J. Mater. Sci. Lett. 14 (1995) 53. [7] K. Ogura, W. Lou, M. Nakayama, Electrochim. Acta 41 (1996) 2849. [8] J.H. Wang, J.G. Duh, H.C. Shih, Surf. Coat. Technol. 78 (1996) 248. [9] E. Kikuti, R. Conrrado, N. Bocchi, S.R. Biaggio, R.C. Rocha-Filho, J. Braz. Chem. Soc. 15 (2004) 472. [10] R. Conrrado, N. Bocchi, R.C. Rocha-Filho, S.R. Biaggio, Electrochim. Acta 48 (2003) 2417. [11] S. Fujimoto, S. Kawachi, T. Nishio, T. Shibata, J. Electroanal. Chem. 473 (1999) 265. [12] K. Jüttner, W.J. Lorenz, W. Paatsch, Corros. Sci. 29 (1989) 279. [13] Shinji Fujimoto, Kiyoshi Tsujino, Toshio Shibata, Electrochim. Acta 47 (2001) 543. [14] R.O. Ansell, T. Dickinson, A.F. Povey, Corros. Sci. 18 (1978) 245. [15] Petra Keller, Hans-Henning Strehblow, Corros. Sci. 46 (2004) 1939. [16] R.C. Furneaux, G.E. Thompson, G.C. Wood, Corros. Sci. 21 (1981) 23. [17] S. Fujimoto, H. Nakatsu, S. Hata, T. Shibata, Corros. Sci. 38 (1996) 1473.