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Corrosion behaviour of amorphous Al–Cr and Al–Cr–(N) coatings deposited by dc magnetron sputtering on mild steel substrate
Thin Solid Films 466 (2004) 1 – 9 www.elsevier.com/locate/tsf
Corrosion behaviour of amorphous Al–Cr and Al–Cr–(N) coatings deposited by dc magnetron sputtering on mild steel substrate J. Creus a,*, A. Billard c, F. Sanchette b a
Laboratoire d’Etude des Mate´riaux en Milieux Agressifs, Universite´ de la Rochelle, F17042 La Rochelle Cedex 1, France b Laboratoire des Proce´de´s de Traitement de Surface, CEA DTEN/SMP, F38054 Grenoble Cedex 9, France c Laboratoire de Science et Ge´nie des Surfaces, Ecole des Mines de Nancy, Parc de Saurupt, F54042 Nancy Cedex, France Received 7 March 2003; received in revised form 21 November 2003; accepted 28 January 2004 Available online
Abstract Physical vapour deposition (PVD) Al alloys films are potential alternatives to conventional electrodeposited coatings for sacrificial protection of steel substrates. Even though the structure-mechanical relationships have already been studied for Al – Cr or Al – Cr – (N) coatings, there is no published data on the corrosion behaviour of these alloys deposited on mild steel substrates. Al – Cr or Al – Cr – (N) coatings were deposited by d.c. magnetron sputtering of Al – Cr large size targets with different Ar – N2 mixtures in an industrial PVD system. In this paper, the influence of both chromium and nitrogen contents on the substrate/coating galvanic coupling behaviour in 3% NaCl solution are studied. Only single-phased amorphous films have been synthesised and the microhardness can reach approximately 9 GPa. The results of potentiostatic and impedance spectroscopy electrochemical tests are discussed and linked to composition and morphology of coatings. It is shown that 20 at.% chromium in Al – Cr coating is the limit above which galvanic corrosion of steel substrate occurs. Nitrogen incorporation seems to influence the coating passive layer and favours its pitting resistance. The salt bath results (more than 800 h without corrosion for Al – Cr 18 at.% Cr) are in agreement with electrochemical behaviour. D 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium; Amorphous materials; Coatings; Corrosion
1. Introduction Aluminium alloy coatings are potentially interesting candidates for the cathodic protection of construction steel due to their good corrosion resistance in aggressive media. However, the mechanical properties of aluminium, as well as their pitting corrosion sensitivity in chloride media have often limited their applications. Mechanical properties (microhardness, mechanical and tribological data) of Al coatings can be reinforced by the addition of transition metals such as chromium or titanium, and experimental data were previously published [1– 3]. The main results revealed that the best compromise between mechanical properties is obtained for amorphous Al –Cr coatings with approximately 20 at.% of chromium. * Corresponding author. Tel.: +33-5-46-45-72-94; fax: +33-5-46-4572-72. E-mail address: [email protected] (J. Creus). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.315
In a narrow composition range, it is possible to deposit single-phased amorphous coatings by sputter deposition. In a general way, the corrosion resistance of amorphous alloys is extremely high in various aqueous solutions. This resistance is attributed to their chemical homogeneity and lack of structural defects such as dislocations or grain boundaries [4 –6]. The homogeneous single-phase nature leads to the formation of a uniform passive film, which is able to insulate the alloy from an aggressive environment [7]. Hashimoto [8] proposed that, when active amorphous metal alloys such as iron, cobalt or aluminium contain elements with a high passivation capability (e.g. chromium or titanium), the higher the reactivity of the alloy, the easier it is to passivate. After a fast initial dissolution of the active components, a spontaneous passivity occurs. Katoh [9] remarks that Al – Cr alloy coatings deposited on steel sheets have superior corrosion resistance in 3% NaCl solution when the Cr content is approximately 10 wt.%. The corrosion products
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consist mainly of dense bayerite Al(OH)3 and do not contain Cr compounds. In this paper, the influence of both chromium and nitrogen content on the corrosion behaviour of Al coatings in 3% NaCl solution are studied. In the first part, the intrinsic corrosion behaviour of Al – Cr alloys deposited onto glass substrates is compared to a ‘pure’ Al coating. Then, the protective efficiency of Al – Cr coatings on steel is examined during extended immersion tests and salt spray exposure.
2. Experimental procedure All Al – Cr and Al – Cr – (N) coatings have been dc sputter-deposited at low pressure (0.5 Pa) and low temperature ( < 200 jC) from large size Al –Cr 18 and Al – Cr 25 (corresponding to 18 at.% and 25 at.% of Cr in Al, respectively) sintered targets (500 140 mm). The main deposition conditions used in this study are summarized in Table 1. The substrates were glass slides and AISI 4135 construction steel plates. The structural investigation of Al alloy coatings was performed by X-ray diffraction (kKaCu = 0.154 nm). The surface morphology before and after immersion tests was examined using optical microscopy (OM), atomic force microscopy (AFM) in contact mode (Autoprobe CP Research) and scanning electron microscopy (SEM). Prior to immersion and electrochemical tests, the samples were successively degreased in ethanol, rinsed in Milli-Qk ultra pure water and dried. The effective surface area of the test samples was approximately 5 cm2. The electrochemical measurements were carried out in an aerated and stirred chlorine solution (3% NaCl solution) using a conventional three-electrode potentiostat. The potential was referenced against a saturated calomel electrode (SCE) and the counter electrode was a large platinum grid. The potentiostatic electrochemical measurements were performed with an EG&G 273A potentiostat driven by M352 software. The open circuit potential of coated steel
Table 1 Main deposition conditions Substrate Target-to-substrate spacing Pre-sputtering time Residual pressure Working pressure DC power Ar flow rate N2 flow rate Deposition rate Bias Deposition time Coating thickness
Glass slide, mild steel (AISI 4135) 100 mm 15 min 10 4 Pa 0.5 Pa 4 kW 100 sccm 0 – 14 sccm 4 Am/h 50 V 45 – 50 min approximately 3 Am
vs. time was recorded during long immersion tests (approx. 50 h). The cyclic polarisation curve i(E) was recorded with a scan rate of 1 mV/s after 1 h of immersion in saline solution. The reverse sweeping was started when the current density value reached 100 AA/ cm2. Cyclic polarisation method was used to estimate the corrosion and pitting resistance of Al alloy coatings in saline solution. The corrosion potential Ecorr and corrosion current density icorr were calculated by using Tafel extrapolation. The electrochemical impedance spectroscopy (EIS) analysis was performed with a 1260 Solartron frequency response analyser (FRA) coupled to a 1287 Solartron potentiostat and driven by Fracom 2.1 software. EIS spectra were acquired at the open circuit potential in a frequency range from 80 kHz to 10 mHz, with an ac excitation amplitude of 10 mV. Results are interpreted in terms of Nyquist and Bode plots after: 0.5, 1.5, 4, 6, 8, 24, 32 and 48 h of immersion in saline solution.
3. Results and discussion 3.1. Morphology and structure of as-sputtered Al based alloys Fig. 1 shows the surface topography of aluminium coatings deposited onto glass (a) and steel (b) substrates. The topography is smoother when coatings are deposited on steel substrates. The coating deposited on the glass substrate grows under conditions at low-energy ion bombardment, due to a low influence of electrical bias on the insulating substrate. According to previous studies (see [2] for example), Al – Cr coatings deposited from large size targets with a chromium content of approximately 18 (Al – Cr 18) and 25 at.% (Al – Cr 25) present a compact morphology and a single phased amorphous microstructure (Fig. 2). It has also been shown previously that coating Al/Cr compositions ratio decreases as the inlet volume fraction of nitrogen increases [1]. This result is consistent with EDS analysis, combined with SEM observations, which reveal that introduction of nitrogen in Al –Cr coating reduces the Al content. Moreover, nitrogen incorporation into the coating stabilises the amorphous phase and favours a dense morphology [1]. Nevertheless, few surface defects like growth defects or small cavities are observed on coatings. 3.2. Intrinsic corrosion behaviour of Al– Cr coatings Polarisation curves of Al and Al– Cr alloys deposited onto glass substrates, measured after 1 h of immersion in a 3% NaCl aqueous solution, are shown in Fig. 3. The electrochemical characteristics determined from these curves are reported in Table 2.
J. Creus et al. / Thin Solid Films 466 (2004) 1–9
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Fig. 3. Cyclic polarisation curves of Al and Al – Cr coatings deposited onto glass substrate after 1 h of immersion in saline solution.
Fig. 1. AFM surface topography of Al coatings on (a) glass and (b) AISI steel substrates.
Fig. 2. Scanning electron micrograph of fracture cross-section of Al – Cr 18 at.% coating deposited on glass with the associated X-ray diffraction pattern.
The open circuit potential of the ‘pure’ Al coating rapidly increases and becomes stable at approximately 730 m/ ECS due to the formation of a protective corrosion product film on the aluminium surface. The polarisation curve of aluminium reveals a drastic increase in current density for a small anodic overpotential. This arises from the fact that the pitting potential of aluminium is very close to the corrosion potential in saline solution [10,11]. Comparison of initial open circuit potentials (E0) of Al – Cr alloy coatings to Al, shows that the incorporation of chromium leads to a shift of the open circuit potential towards positive values, which is in agreement with the literature [6,9]. During 1 h of immersion, the potential of Al – Cr18 coating is quite stable at approximately 850 mV/ECS, whereas the potential of the ‘pure’ Al coating rapidly increases from 1170 to 730 mV/ECS. Thus, the corrosion potential of Al – Cr18 coating, which was calculated after 1 h of immersion by using Tafel extrapolation is more negative than Al. The polarisation curve shape of an Al –Cr alloy coating containing 18 at.% Cr is close to that of aluminium, however the pitting potential is shifted to 690 mV/SCE and a passivation level appears. In the sweep reverse curve, we notice a pitting repassivation, which is absent in the aluminium coating. The incorporation of chromium at 25 at.% induces a spontaneous ennoblement of potential which reaches 565 mV/SCE after 1 h of immersion in saline solution. On the polarisation curve, we notice that the current density slightly increases for anodic overpotential and then reaches a level at higher current density than Al – Cr 18 at.%. At approximately 400 mV/SCE an increase of current density associated to pitting formation occurs. During the immersion, a galvanic corrosion occurs between chromium, the more noble element, and Al which is preferentially attacked. A film, rich in aluminium corrosion products, is formed on the binary amorphous alloys. Katoh [9] emphasises that no Cr compounds were detected in such
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Table 2 Electrochemical characteristics of pure Al and Al – Cr alloy coatings deposited onto glass substrates after 1 h of immersion in a 3% NaCl solution Al Initial open circuit potential E0 (mV/SCE) Ecorr (mV/SCE) icorr (AA cm 2)
Al – Cr 18 1170 715 0.07
Al-rich films. One might expect that the increase of superficial Cr content in the coating could accelerate aluminium dissolution and may rapidly produce ennoblement of the coating potential. Further surface analyses are in progress to determine whether the observed potential ennoblement and modification of corrosion behaviour when Cr content increases, are due either to chromium enrichment of the coating surface or to formation of Al-rich passive film during preferential aluminium dissolution. Nyquist impedance spectra developed after extended immersion in a 3% NaCl solution are presented in Fig. 4 for pure Al and Al– Cr alloy coatings, respectively. The impedance spectra of the Al coating (Fig. 4a) usually exhibit a capacitive semicircle at intermediate frequencies, which is associated with aluminium corrosion, and exhibit an inductive semicircle at low frequencies, which is associated to corrosion product relaxation and removal [12]. During the first few hours of immersion, the open circuit potential of the Al coating evolves from 1170 to 735 mV/SCE and an increase in capacitive semicircle diameter is observed. This corresponds to the formation of a barrier corrosion product film on the aluminium surface. Then the open circuit potential oscillates around a fixed potential value of 730 mV/SCE, and a decrease of the capacitive semicircle diameter (Rt) occurs reaching a value of 14.5 kV cm2 after 48 h of immersion. This decrease is associated with pitting initiation and growth on the aluminium surface confirmed by surface optical observations after 48 h of immersion. Usually the impedance diagrams of Al –Cr coatings (Fig. 4 b and c) present a capacitive semicircle. According to the impedance diagram shape, it is assumed that several reactions may occur, without separation on the diagrams. Thus the overall evolution of the semicircle during the immersion is considered here. After 0.5 h of immersion, the semicircle diameter (Rt) is estimated at 440 kV cm2 and 120 kV cm2, respectively, for Al – Cr 18 and Al – Cr 25, and only at 18 kV cm2 for Al coating. Thus, the incorporation of chromium, and essentially the amorphous character of the coating [4,6] enhances the corrosion resistance. However, the transfer resistance of Al – Cr 25 is approximately four times lower than Al – Cr 18. This seems to be due to a surface preferential dissolution of aluminium accelerated by the higher Cr content provoking a passivation of the coating, which is rapidly covered by protective corrosion products. Therefore, the open circuit potential rapidly ennobles from 740 to 504 mV/SCE for 2 h of immersion. Then, when the surface is completely covered by the protective film, the potential remains virtu-
Al – Cr 25
840 855 0.09
740 565 0.07
ally constant at around 460 mV/SCE, and the transfer resistance slowly increases and then stabilises at 330 kV cm2 after 48 h of immersion.
Fig. 4. Nyquist impedance spectra evolution during 48 h immersion tests in 3% NaCl solution for (a) Al, (b) Al – Cr 18 at.% and (c) Al – Cr 25 at.% coatings deposited onto glass substrate. ( 0.5 h, D 8 h, w 24 h and 5 48 h of immersion in saline solution).
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For the Al –Cr 18, the preferential dissolution of aluminium and the formation of the corrosion product film also occur and explain the ennoblement of potential from 840 to 506 mV/SCE in 48 h of immersion. The kinetic of the ennoblement mechanism is reduced, and obviously linked to the Cr content. After 48 h of immersion, the transfer resistance of both Al –Cr coatings are quite similar at approximately 300 kV cm2. The surface optical observations do not reveal modification of coating morphology during the immersion. Ultimately, the incorporation of Cr allows, in a narrow range of Cr content to elaborate an amorphous alloy coating which presents better corrosion resistance than a pure Al coating in saline solution thanks to the shift of the pitting potential toward positive values. An ennoblement of coating potential occurs when Cr content increases. We notice that Cr content above 20 at.% leads to rapid passivation of the coating with a higher Al dissolution at the beginning of the immersion when Cr content increases. 3.3. Corrosion behaviour of Al and Al – Cr alloy coatings onto mild steel Fig. 5 presents the polarisation curves of Al – Cr alloy coatings deposited onto steel substrates after 1 h of immersion in saline solution compared to Al and Cr coatings. The characteristics determined from these curves are reported in Table 3. The open circuit potential evolution of coated steel in saline solution during a 50 h immersion test is measured. Impedance measurements were performed for different immersion durations and results in terms of Nyquist diagrams are presented in Fig. 6. The potential of chromium coated steel is slowly shifted during immersion towards negative potential, indicating that the steel substrate is corroding through the open porosity.
Fig. 5. Single sweeping of polarisation curves of Al, Cr and Al – Cr alloy coatings deposited onto AISI 4135 steel substrate after 1 h of immersion in saline solution.
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The low corrosion current density and EIS measurements (Fig. 6a) show that Cr coating is very dense, with a low porosity. This metallic coating offers good barrier protection against the solution and local rust formation seems to partially seal the open porosity. After extended immersion tests, only few rust points are detected on the Cr coating surface. Aluminium coated steel presents a corrosion behaviour in saline solution slightly different from the coating deposited on the glass substrate. The difference is associated with the coating morphology modification following the substrate nature. The potential evolution shows a slow ennoblement during the first 20 h of immersion, associated with the formation of a protective corrosion product film on the aluminium surface. The Al coated steel impedance diagrams (Fig. 6b) in saline solution are in general composed of two capacitive semicircles (intermediate and low frequency), associated, respectively, to Al dissolution and the formation of a corrosion product film on aluminium. During the first few hours of immersion, the transfer resistance of the two semicircles increases, which is in agreement with the formation of a protective film. After 24 h of immersion, the potential oscillates around a fixed value of 730 mV/SCE. In the meanwhile, the transfer resistance of both semicircles decreases, which indicates an instability of the protective film. After 48 h of immersion, few deep and blackish pitting are observed on aluminium surface (Fig. 7a). The incorporation of chromium in the Al deposit ennobles the corrosion potential of the coated steel. The potential is intermediary between the potentials of the Al and Cr coatings. It is shifted toward a positive value when Cr content increases. The corrosion current densities of Al – Cr alloy coated steel are very low and close to that of the pure Al coating. The polarisation curve of Al – Cr 18 coated steel is very similar to the one of Al coated steel, with a sufficiently negative corrosion potential value to ensure cathodic protection of the steel substrate. A modification of oxidation kinetics seems to occur around 600 mV/SCE. The possible mechanism at this potential corresponds either to pitting formation on the Al –Cr coating, or steel dissolution through open porosity. For Al– Cr 25, the corrosion potential is very close to that of bare steel. For anodic overpotential, the current density corresponds to the contribution of the steel substrate dissolution through open porosity and also to the coating oxidation reaction (the pitting corrosion occurs around 450 mV/SCE). During long immersion tests, we observe that the potential of Al– Cr 25 evolves from 710 to 560 mV/SCE in 2 h of immersion, whereas 24 h of immersion are necessary for Al– Cr 18 potential to evolve from 880 to 560 mV/ SCE. The impedance diagrams (Fig. 6c,d) after 0.5 h of immersion show a single capacitive semicircle which corresponds to the electrochemical contribution of Al – Cr coatings. We notice a transfer resistance of 1300 kV cm2
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Table 3 Electrochemical characteristics of Al, Cr and Al – Cr alloy coatings deposited onto AISI 4135 steel substrates after 1 h of immersion in a 3% NaCl solution Steel Ecorr (mV/SCE) icorr (AA.cm 2)
590 145
Al/steel 1072 0.3
and 530 kV cm2, respectively, for Al– Cr 18 and Al – Cr 25, which confirms an accelerated dissolution of the latter coating due to micro-galvanic cells between Cr and Al and then, a spontaneous passivation. After 30 h of immersion for Al –Cr 18, and only 10 h of immersion for Al –Cr 25, the impedance diagrams present two capacitive semicircles. The first semicircle at intermediate frequencies is associated with steel dissolution through open porosity. The high value of transfer resistance indicates that the coatings are very dense with negligible macroscopic porosity. After immersion, a maximum number of two rust points were observed on the different samples surface tested in saline solution. It appears then that the amorphous Al – Cr coating is dense enough to be an effective barrier between the steel substrate and the saline solution. The second semicircle is attributed to the formation of corrosion products composed of steel and aluminium compounds.
Cr/steel 385 25
Al – Cr 18/steel 1030 0.5
Al – Cr 25/steel 610 0.5
Optical observations after extended immersion tests show that Al – Cr coatings are characterised by good adhesion and a dense morphology. In general, the coating morphology is not affected by the immersion test, however, we observed some local blackish marks (Fig. 7b), which correspond to pitting formation. The pitting shape is changed compared to Al coating, in fact it seems that the Cr addition prevents the depth propagation of pitting, which only is localised at the surface. We have previously emphasized that a Cr content of 25 at.% induces a fast ennoblement of coating potential, which significantly reduces galvanic protection of the steel substrate. The corrosion resistance of coated steel is then reduced, even if the coating presents an excellent intrinsic corrosion resistance. The Al – Cr 18 coating deposited onto steel presents the best corrosion resistance because it combines long-term cathodic protection of the steel ensured (by non-localised sacrificial dissolution of the coating) with an
Fig. 6. Nyquist impedance spectra evolution during a 48 h immersion test in 3% NaCl solution for (a) Cr, (b) Al, (c) Al – Cr 18% and (d) Al – Cr 25% coatings deposited onto AISI 4135 steel. ( 0.5 h, D 8 h, w 24 h and 5 48 h of immersion in saline solution).
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Fig. 7. Optical observations of pitting formation on (a) Al and (b) Al – Cr 18% coatings deposited on steel after 48 h of immersion in saline solution.
effective low-porosity protective barrier against the saline solution. 3.4. Influence of nitrogen incorporation on corrosion behaviour of single phased amorphous Al – Cr18 at.% coated steel in saline solution The exact nitrogen content has not been measured. Al – Cr – N7 and Al – Cr – N14 correspond, respectively, to 7 sccm and 14 sccm flow rates of nitrogen in the gas mixture during coating deposition. Fig. 8 presents the polarisation curves of Al –Cr18– N with two nitrogen contents (compared to the standard Al – Cr 18 coating) deposited onto glass substrates. The electrochemical characteristics determined from these curves are reported in Table 4. The incorporation of nitrogen in Al – Cr 18 coating deposited onto glass substrates still shifts the potential
Fig. 8. Cyclic polarisation curves of Al – Cr18 – N coatings deposited onto glass substrate compared to Al – Cr 18 coating after 1 h of immersion in saline solution.
towards positive values and decreases the corrosion current density. We notice that the anodic part of the polarisation curve is modified when nitrogen is incorporated into Al – Cr coating. This incorporation increases the intrinsic corrosion resistance of the Al –Cr coatings because pitting corrosion disappears with an increase of nitrogen content. According to previous studies [1– 3] the nitrogen addition seems to favour amorphisation of the Al –Cr alloy. Therefore, one might reasonably suppose that the corrosion product film formed on the amorphous coating is uniform, adherent and homogeneous. It remains to be seen, however, whether the corrosion product film composition is now a mixture of Al and Cr compounds. Fig. 9a,b presents the polarisation curves of Al – Cr –N with two nitrogen contents (compared to the standard Al – Cr 18 coating) deposited onto the AISI 4315 steel substrate. The polarisation curve shape of Al –Cr – N alloy coated steel is similar to the standard Al – Cr coating, the main difference being observed at high anodic overpotentials where no pitting corrosion of the Al – Cr– N alloy films occurs. The optical observations do not reveal pit formation on the coating surface. Indeed, a modification of the oxidation slope, which corresponds to a new dissolution mechanism, also occurs around 585 mV/SCE for both Al – Cr– N coatings. We observe, on the reverse polarisation curve, that the corrosion potential is similar for both samples. In fact the additional reaction should be associated to the aggressive accelerated corrosion test, which suppresses the protection ensured by the sacrificial coating. Therefore, on the reverse polarisation, the electrochemical behaviour should correspond to steel corrosion through the open porosity of the corroded coating. Potential evolution and EIS measurements after extended immersion times show a fast potential ennoblement and, after 35 h of immersion, the impedance diagrams for both nitrogen contents represent the steel electrochemical characteristics, for which the high and constant value of
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Table 4 Electrochemical characteristics of Al – Cr and Al – C – N alloy coatings deposited onto glass substrates after 1 h of immersion in a 3% NaCl solution Al – Cr 18 Ecorr (mV/SCE) icorr (AA cm 2) Ep (pitting potential mV/SCE)
857 0.09 667
transfer resistance lead us to conclude that porosity rate is negligible. In fact, the surface observations after 48 h of immersion show only a few rust points. The coating remains adherent and no localised degradation were detected.
4. Conclusions The corrosion behaviour of Al – Cr and Al –Cr – (N) alloy coatings deposited by d.c. magnetron sputtering on glass and mild steel substrates was studied and compared to the corrosion behaviour of pure Al and Cr coatings.
Al – Cr18 N7 635 0.02 540
Al – Cr18 N14 490 0.015 none
In a narrow range of Cr content, the incorporation of chromium in an aluminium coating allows an amorphous coating to be deposited. These coatings present excellent intrinsic corrosion resistance in saline solution, with low corrosion rate and a pitting potential shifted towards more positive values. The corrosion potential of Al – Cr alloys is also shifted when Cr content increases with potential ennoblement occurring during long immersion tests. The ennoblement mechanism seems to be associated to preferential aluminium dissolution, which leads either to formation of a protective corrosion product film, or to an enriched chromium surface coating. Thorough research using local electrochemical techniques would permit to clarify the mechanism involved in the corrosion of amorphous Al– Cr coatings. The corrosion behaviour of coated steel is strongly dependant on the Cr content in the Al based coating. All Al – Cr alloy coatings are compact and dense, and thus provide improved barrier protection (for a given thickness) against the aggressive media. For a Cr content of 18 at.%, the coating also provides cathodic protection of steel, ensured by evenly distributed sacrificial dissolution of the Al – Cr coating. The cathodic protection efficiency decreases with further increasing Cr content, due to spontaneous passivation of the coating. Thus an Al – Cr coating with 18 at.% of Cr presents the best corrosion resistance in saline solution, combining an effective barrier and uniform sacrificial properties which continue to protect steel substrates in contact with the solution once open porosity develops. Nitrogen incorporation in Al –Cr coatings enhances the pitting resistance in saline solution, and for high nitrogen content, pitting corrosion is suppressed. However, this incorporation induces a potential shift towards positive values. Thus, sacrificial protection of steel substrate is reduced. The barrier protection effect is, however conserved and perhaps enhanced since nitrogen incorporation is expected to stabilise the amorphous phase and improve coating densification. A thorough research using local electrochemical techniques would permit to determine the principal dissolution mechanism of coated steel when its potential is shifted towards the oxidation domain of steel.
Acknowledgements Fig. 9. (a and b): Cyclic polarisation curves of Al – Cr18 – N coatings deposited onto AISI 4135 steel substrates compared to Al – Cr 18 coating after 1 h of immersion in saline solution.
B. Peraudeau and C. Savall are gratefully acknowledged for their technical support during the project.
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References [1] F. Sanchette, T.H. Loi, A. Billard, C. Frantz, Surf. Coat. Technol. 74 – 75 (1995) 903. [2] F. Sanchette, A. Billard, C. Frantz, Surf. Coat. Technol. 98 (1998) 1162. [3] F. Sanchette, A. Billard, Surf. Coat. Technol. 142 – 144 (2001) 218. [4] K. Asami, B.P. Zhang, M. Mehmood, H. Habazaki, K. Hashimoto, Scripta Mater. 44 (2001) 1655. [5] M. Mehmood, E. Akiyama, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 41 (1999) 477. [6] E. Akiyama, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 38 (1996) 279.
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[7] A.A. El Moneim, B.P. Zhang, E. Akiyama, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 39 (1997) 305. [8] H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Corros. Sci. 31 (1990) 349. [9] J. Katoh, J. Kawafuku, K. Araga, A. Hihara, Hyomen Gijutsu 51 (2000) 87. [10] T. Foley, Corrosion 42 (1986) 277. [11] J. Creus, Ph.D., Thesis, INSA de Lyon, France, 1997. [12] S.F. Frers, M.M. Stefenel, C. Mayer, T. Chierchie, J. Appl. Electrochem. 20 (1990) 996.
Corrosion behaviour of amorphous Al–Cr and Al–Cr–(N) coatings deposited by dc magnetron sputtering on mild steel substrate J. Creus a,*, A. Billard c, F. Sanchette b a
Laboratoire d’Etude des Mate´riaux en Milieux Agressifs, Universite´ de la Rochelle, F17042 La Rochelle Cedex 1, France b Laboratoire des Proce´de´s de Traitement de Surface, CEA DTEN/SMP, F38054 Grenoble Cedex 9, France c Laboratoire de Science et Ge´nie des Surfaces, Ecole des Mines de Nancy, Parc de Saurupt, F54042 Nancy Cedex, France Received 7 March 2003; received in revised form 21 November 2003; accepted 28 January 2004 Available online
Abstract Physical vapour deposition (PVD) Al alloys films are potential alternatives to conventional electrodeposited coatings for sacrificial protection of steel substrates. Even though the structure-mechanical relationships have already been studied for Al – Cr or Al – Cr – (N) coatings, there is no published data on the corrosion behaviour of these alloys deposited on mild steel substrates. Al – Cr or Al – Cr – (N) coatings were deposited by d.c. magnetron sputtering of Al – Cr large size targets with different Ar – N2 mixtures in an industrial PVD system. In this paper, the influence of both chromium and nitrogen contents on the substrate/coating galvanic coupling behaviour in 3% NaCl solution are studied. Only single-phased amorphous films have been synthesised and the microhardness can reach approximately 9 GPa. The results of potentiostatic and impedance spectroscopy electrochemical tests are discussed and linked to composition and morphology of coatings. It is shown that 20 at.% chromium in Al – Cr coating is the limit above which galvanic corrosion of steel substrate occurs. Nitrogen incorporation seems to influence the coating passive layer and favours its pitting resistance. The salt bath results (more than 800 h without corrosion for Al – Cr 18 at.% Cr) are in agreement with electrochemical behaviour. D 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium; Amorphous materials; Coatings; Corrosion
1. Introduction Aluminium alloy coatings are potentially interesting candidates for the cathodic protection of construction steel due to their good corrosion resistance in aggressive media. However, the mechanical properties of aluminium, as well as their pitting corrosion sensitivity in chloride media have often limited their applications. Mechanical properties (microhardness, mechanical and tribological data) of Al coatings can be reinforced by the addition of transition metals such as chromium or titanium, and experimental data were previously published [1– 3]. The main results revealed that the best compromise between mechanical properties is obtained for amorphous Al –Cr coatings with approximately 20 at.% of chromium. * Corresponding author. Tel.: +33-5-46-45-72-94; fax: +33-5-46-4572-72. E-mail address: [email protected] (J. Creus). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.315
In a narrow composition range, it is possible to deposit single-phased amorphous coatings by sputter deposition. In a general way, the corrosion resistance of amorphous alloys is extremely high in various aqueous solutions. This resistance is attributed to their chemical homogeneity and lack of structural defects such as dislocations or grain boundaries [4 –6]. The homogeneous single-phase nature leads to the formation of a uniform passive film, which is able to insulate the alloy from an aggressive environment [7]. Hashimoto [8] proposed that, when active amorphous metal alloys such as iron, cobalt or aluminium contain elements with a high passivation capability (e.g. chromium or titanium), the higher the reactivity of the alloy, the easier it is to passivate. After a fast initial dissolution of the active components, a spontaneous passivity occurs. Katoh [9] remarks that Al – Cr alloy coatings deposited on steel sheets have superior corrosion resistance in 3% NaCl solution when the Cr content is approximately 10 wt.%. The corrosion products
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J. Creus et al. / Thin Solid Films 466 (2004) 1–9
consist mainly of dense bayerite Al(OH)3 and do not contain Cr compounds. In this paper, the influence of both chromium and nitrogen content on the corrosion behaviour of Al coatings in 3% NaCl solution are studied. In the first part, the intrinsic corrosion behaviour of Al – Cr alloys deposited onto glass substrates is compared to a ‘pure’ Al coating. Then, the protective efficiency of Al – Cr coatings on steel is examined during extended immersion tests and salt spray exposure.
2. Experimental procedure All Al – Cr and Al – Cr – (N) coatings have been dc sputter-deposited at low pressure (0.5 Pa) and low temperature ( < 200 jC) from large size Al –Cr 18 and Al – Cr 25 (corresponding to 18 at.% and 25 at.% of Cr in Al, respectively) sintered targets (500 140 mm). The main deposition conditions used in this study are summarized in Table 1. The substrates were glass slides and AISI 4135 construction steel plates. The structural investigation of Al alloy coatings was performed by X-ray diffraction (kKaCu = 0.154 nm). The surface morphology before and after immersion tests was examined using optical microscopy (OM), atomic force microscopy (AFM) in contact mode (Autoprobe CP Research) and scanning electron microscopy (SEM). Prior to immersion and electrochemical tests, the samples were successively degreased in ethanol, rinsed in Milli-Qk ultra pure water and dried. The effective surface area of the test samples was approximately 5 cm2. The electrochemical measurements were carried out in an aerated and stirred chlorine solution (3% NaCl solution) using a conventional three-electrode potentiostat. The potential was referenced against a saturated calomel electrode (SCE) and the counter electrode was a large platinum grid. The potentiostatic electrochemical measurements were performed with an EG&G 273A potentiostat driven by M352 software. The open circuit potential of coated steel
Table 1 Main deposition conditions Substrate Target-to-substrate spacing Pre-sputtering time Residual pressure Working pressure DC power Ar flow rate N2 flow rate Deposition rate Bias Deposition time Coating thickness
Glass slide, mild steel (AISI 4135) 100 mm 15 min 10 4 Pa 0.5 Pa 4 kW 100 sccm 0 – 14 sccm 4 Am/h 50 V 45 – 50 min approximately 3 Am
vs. time was recorded during long immersion tests (approx. 50 h). The cyclic polarisation curve i(E) was recorded with a scan rate of 1 mV/s after 1 h of immersion in saline solution. The reverse sweeping was started when the current density value reached 100 AA/ cm2. Cyclic polarisation method was used to estimate the corrosion and pitting resistance of Al alloy coatings in saline solution. The corrosion potential Ecorr and corrosion current density icorr were calculated by using Tafel extrapolation. The electrochemical impedance spectroscopy (EIS) analysis was performed with a 1260 Solartron frequency response analyser (FRA) coupled to a 1287 Solartron potentiostat and driven by Fracom 2.1 software. EIS spectra were acquired at the open circuit potential in a frequency range from 80 kHz to 10 mHz, with an ac excitation amplitude of 10 mV. Results are interpreted in terms of Nyquist and Bode plots after: 0.5, 1.5, 4, 6, 8, 24, 32 and 48 h of immersion in saline solution.
3. Results and discussion 3.1. Morphology and structure of as-sputtered Al based alloys Fig. 1 shows the surface topography of aluminium coatings deposited onto glass (a) and steel (b) substrates. The topography is smoother when coatings are deposited on steel substrates. The coating deposited on the glass substrate grows under conditions at low-energy ion bombardment, due to a low influence of electrical bias on the insulating substrate. According to previous studies (see [2] for example), Al – Cr coatings deposited from large size targets with a chromium content of approximately 18 (Al – Cr 18) and 25 at.% (Al – Cr 25) present a compact morphology and a single phased amorphous microstructure (Fig. 2). It has also been shown previously that coating Al/Cr compositions ratio decreases as the inlet volume fraction of nitrogen increases [1]. This result is consistent with EDS analysis, combined with SEM observations, which reveal that introduction of nitrogen in Al –Cr coating reduces the Al content. Moreover, nitrogen incorporation into the coating stabilises the amorphous phase and favours a dense morphology [1]. Nevertheless, few surface defects like growth defects or small cavities are observed on coatings. 3.2. Intrinsic corrosion behaviour of Al– Cr coatings Polarisation curves of Al and Al– Cr alloys deposited onto glass substrates, measured after 1 h of immersion in a 3% NaCl aqueous solution, are shown in Fig. 3. The electrochemical characteristics determined from these curves are reported in Table 2.
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Fig. 3. Cyclic polarisation curves of Al and Al – Cr coatings deposited onto glass substrate after 1 h of immersion in saline solution.
Fig. 1. AFM surface topography of Al coatings on (a) glass and (b) AISI steel substrates.
Fig. 2. Scanning electron micrograph of fracture cross-section of Al – Cr 18 at.% coating deposited on glass with the associated X-ray diffraction pattern.
The open circuit potential of the ‘pure’ Al coating rapidly increases and becomes stable at approximately 730 m/ ECS due to the formation of a protective corrosion product film on the aluminium surface. The polarisation curve of aluminium reveals a drastic increase in current density for a small anodic overpotential. This arises from the fact that the pitting potential of aluminium is very close to the corrosion potential in saline solution [10,11]. Comparison of initial open circuit potentials (E0) of Al – Cr alloy coatings to Al, shows that the incorporation of chromium leads to a shift of the open circuit potential towards positive values, which is in agreement with the literature [6,9]. During 1 h of immersion, the potential of Al – Cr18 coating is quite stable at approximately 850 mV/ECS, whereas the potential of the ‘pure’ Al coating rapidly increases from 1170 to 730 mV/ECS. Thus, the corrosion potential of Al – Cr18 coating, which was calculated after 1 h of immersion by using Tafel extrapolation is more negative than Al. The polarisation curve shape of an Al –Cr alloy coating containing 18 at.% Cr is close to that of aluminium, however the pitting potential is shifted to 690 mV/SCE and a passivation level appears. In the sweep reverse curve, we notice a pitting repassivation, which is absent in the aluminium coating. The incorporation of chromium at 25 at.% induces a spontaneous ennoblement of potential which reaches 565 mV/SCE after 1 h of immersion in saline solution. On the polarisation curve, we notice that the current density slightly increases for anodic overpotential and then reaches a level at higher current density than Al – Cr 18 at.%. At approximately 400 mV/SCE an increase of current density associated to pitting formation occurs. During the immersion, a galvanic corrosion occurs between chromium, the more noble element, and Al which is preferentially attacked. A film, rich in aluminium corrosion products, is formed on the binary amorphous alloys. Katoh [9] emphasises that no Cr compounds were detected in such
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Table 2 Electrochemical characteristics of pure Al and Al – Cr alloy coatings deposited onto glass substrates after 1 h of immersion in a 3% NaCl solution Al Initial open circuit potential E0 (mV/SCE) Ecorr (mV/SCE) icorr (AA cm 2)
Al – Cr 18 1170 715 0.07
Al-rich films. One might expect that the increase of superficial Cr content in the coating could accelerate aluminium dissolution and may rapidly produce ennoblement of the coating potential. Further surface analyses are in progress to determine whether the observed potential ennoblement and modification of corrosion behaviour when Cr content increases, are due either to chromium enrichment of the coating surface or to formation of Al-rich passive film during preferential aluminium dissolution. Nyquist impedance spectra developed after extended immersion in a 3% NaCl solution are presented in Fig. 4 for pure Al and Al– Cr alloy coatings, respectively. The impedance spectra of the Al coating (Fig. 4a) usually exhibit a capacitive semicircle at intermediate frequencies, which is associated with aluminium corrosion, and exhibit an inductive semicircle at low frequencies, which is associated to corrosion product relaxation and removal [12]. During the first few hours of immersion, the open circuit potential of the Al coating evolves from 1170 to 735 mV/SCE and an increase in capacitive semicircle diameter is observed. This corresponds to the formation of a barrier corrosion product film on the aluminium surface. Then the open circuit potential oscillates around a fixed potential value of 730 mV/SCE, and a decrease of the capacitive semicircle diameter (Rt) occurs reaching a value of 14.5 kV cm2 after 48 h of immersion. This decrease is associated with pitting initiation and growth on the aluminium surface confirmed by surface optical observations after 48 h of immersion. Usually the impedance diagrams of Al –Cr coatings (Fig. 4 b and c) present a capacitive semicircle. According to the impedance diagram shape, it is assumed that several reactions may occur, without separation on the diagrams. Thus the overall evolution of the semicircle during the immersion is considered here. After 0.5 h of immersion, the semicircle diameter (Rt) is estimated at 440 kV cm2 and 120 kV cm2, respectively, for Al – Cr 18 and Al – Cr 25, and only at 18 kV cm2 for Al coating. Thus, the incorporation of chromium, and essentially the amorphous character of the coating [4,6] enhances the corrosion resistance. However, the transfer resistance of Al – Cr 25 is approximately four times lower than Al – Cr 18. This seems to be due to a surface preferential dissolution of aluminium accelerated by the higher Cr content provoking a passivation of the coating, which is rapidly covered by protective corrosion products. Therefore, the open circuit potential rapidly ennobles from 740 to 504 mV/SCE for 2 h of immersion. Then, when the surface is completely covered by the protective film, the potential remains virtu-
Al – Cr 25
840 855 0.09
740 565 0.07
ally constant at around 460 mV/SCE, and the transfer resistance slowly increases and then stabilises at 330 kV cm2 after 48 h of immersion.
Fig. 4. Nyquist impedance spectra evolution during 48 h immersion tests in 3% NaCl solution for (a) Al, (b) Al – Cr 18 at.% and (c) Al – Cr 25 at.% coatings deposited onto glass substrate. ( 0.5 h, D 8 h, w 24 h and 5 48 h of immersion in saline solution).
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For the Al –Cr 18, the preferential dissolution of aluminium and the formation of the corrosion product film also occur and explain the ennoblement of potential from 840 to 506 mV/SCE in 48 h of immersion. The kinetic of the ennoblement mechanism is reduced, and obviously linked to the Cr content. After 48 h of immersion, the transfer resistance of both Al –Cr coatings are quite similar at approximately 300 kV cm2. The surface optical observations do not reveal modification of coating morphology during the immersion. Ultimately, the incorporation of Cr allows, in a narrow range of Cr content to elaborate an amorphous alloy coating which presents better corrosion resistance than a pure Al coating in saline solution thanks to the shift of the pitting potential toward positive values. An ennoblement of coating potential occurs when Cr content increases. We notice that Cr content above 20 at.% leads to rapid passivation of the coating with a higher Al dissolution at the beginning of the immersion when Cr content increases. 3.3. Corrosion behaviour of Al and Al – Cr alloy coatings onto mild steel Fig. 5 presents the polarisation curves of Al – Cr alloy coatings deposited onto steel substrates after 1 h of immersion in saline solution compared to Al and Cr coatings. The characteristics determined from these curves are reported in Table 3. The open circuit potential evolution of coated steel in saline solution during a 50 h immersion test is measured. Impedance measurements were performed for different immersion durations and results in terms of Nyquist diagrams are presented in Fig. 6. The potential of chromium coated steel is slowly shifted during immersion towards negative potential, indicating that the steel substrate is corroding through the open porosity.
Fig. 5. Single sweeping of polarisation curves of Al, Cr and Al – Cr alloy coatings deposited onto AISI 4135 steel substrate after 1 h of immersion in saline solution.
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The low corrosion current density and EIS measurements (Fig. 6a) show that Cr coating is very dense, with a low porosity. This metallic coating offers good barrier protection against the solution and local rust formation seems to partially seal the open porosity. After extended immersion tests, only few rust points are detected on the Cr coating surface. Aluminium coated steel presents a corrosion behaviour in saline solution slightly different from the coating deposited on the glass substrate. The difference is associated with the coating morphology modification following the substrate nature. The potential evolution shows a slow ennoblement during the first 20 h of immersion, associated with the formation of a protective corrosion product film on the aluminium surface. The Al coated steel impedance diagrams (Fig. 6b) in saline solution are in general composed of two capacitive semicircles (intermediate and low frequency), associated, respectively, to Al dissolution and the formation of a corrosion product film on aluminium. During the first few hours of immersion, the transfer resistance of the two semicircles increases, which is in agreement with the formation of a protective film. After 24 h of immersion, the potential oscillates around a fixed value of 730 mV/SCE. In the meanwhile, the transfer resistance of both semicircles decreases, which indicates an instability of the protective film. After 48 h of immersion, few deep and blackish pitting are observed on aluminium surface (Fig. 7a). The incorporation of chromium in the Al deposit ennobles the corrosion potential of the coated steel. The potential is intermediary between the potentials of the Al and Cr coatings. It is shifted toward a positive value when Cr content increases. The corrosion current densities of Al – Cr alloy coated steel are very low and close to that of the pure Al coating. The polarisation curve of Al – Cr 18 coated steel is very similar to the one of Al coated steel, with a sufficiently negative corrosion potential value to ensure cathodic protection of the steel substrate. A modification of oxidation kinetics seems to occur around 600 mV/SCE. The possible mechanism at this potential corresponds either to pitting formation on the Al –Cr coating, or steel dissolution through open porosity. For Al– Cr 25, the corrosion potential is very close to that of bare steel. For anodic overpotential, the current density corresponds to the contribution of the steel substrate dissolution through open porosity and also to the coating oxidation reaction (the pitting corrosion occurs around 450 mV/SCE). During long immersion tests, we observe that the potential of Al– Cr 25 evolves from 710 to 560 mV/SCE in 2 h of immersion, whereas 24 h of immersion are necessary for Al– Cr 18 potential to evolve from 880 to 560 mV/ SCE. The impedance diagrams (Fig. 6c,d) after 0.5 h of immersion show a single capacitive semicircle which corresponds to the electrochemical contribution of Al – Cr coatings. We notice a transfer resistance of 1300 kV cm2
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Table 3 Electrochemical characteristics of Al, Cr and Al – Cr alloy coatings deposited onto AISI 4135 steel substrates after 1 h of immersion in a 3% NaCl solution Steel Ecorr (mV/SCE) icorr (AA.cm 2)
590 145
Al/steel 1072 0.3
and 530 kV cm2, respectively, for Al– Cr 18 and Al – Cr 25, which confirms an accelerated dissolution of the latter coating due to micro-galvanic cells between Cr and Al and then, a spontaneous passivation. After 30 h of immersion for Al –Cr 18, and only 10 h of immersion for Al –Cr 25, the impedance diagrams present two capacitive semicircles. The first semicircle at intermediate frequencies is associated with steel dissolution through open porosity. The high value of transfer resistance indicates that the coatings are very dense with negligible macroscopic porosity. After immersion, a maximum number of two rust points were observed on the different samples surface tested in saline solution. It appears then that the amorphous Al – Cr coating is dense enough to be an effective barrier between the steel substrate and the saline solution. The second semicircle is attributed to the formation of corrosion products composed of steel and aluminium compounds.
Cr/steel 385 25
Al – Cr 18/steel 1030 0.5
Al – Cr 25/steel 610 0.5
Optical observations after extended immersion tests show that Al – Cr coatings are characterised by good adhesion and a dense morphology. In general, the coating morphology is not affected by the immersion test, however, we observed some local blackish marks (Fig. 7b), which correspond to pitting formation. The pitting shape is changed compared to Al coating, in fact it seems that the Cr addition prevents the depth propagation of pitting, which only is localised at the surface. We have previously emphasized that a Cr content of 25 at.% induces a fast ennoblement of coating potential, which significantly reduces galvanic protection of the steel substrate. The corrosion resistance of coated steel is then reduced, even if the coating presents an excellent intrinsic corrosion resistance. The Al – Cr 18 coating deposited onto steel presents the best corrosion resistance because it combines long-term cathodic protection of the steel ensured (by non-localised sacrificial dissolution of the coating) with an
Fig. 6. Nyquist impedance spectra evolution during a 48 h immersion test in 3% NaCl solution for (a) Cr, (b) Al, (c) Al – Cr 18% and (d) Al – Cr 25% coatings deposited onto AISI 4135 steel. ( 0.5 h, D 8 h, w 24 h and 5 48 h of immersion in saline solution).
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Fig. 7. Optical observations of pitting formation on (a) Al and (b) Al – Cr 18% coatings deposited on steel after 48 h of immersion in saline solution.
effective low-porosity protective barrier against the saline solution. 3.4. Influence of nitrogen incorporation on corrosion behaviour of single phased amorphous Al – Cr18 at.% coated steel in saline solution The exact nitrogen content has not been measured. Al – Cr – N7 and Al – Cr – N14 correspond, respectively, to 7 sccm and 14 sccm flow rates of nitrogen in the gas mixture during coating deposition. Fig. 8 presents the polarisation curves of Al –Cr18– N with two nitrogen contents (compared to the standard Al – Cr 18 coating) deposited onto glass substrates. The electrochemical characteristics determined from these curves are reported in Table 4. The incorporation of nitrogen in Al – Cr 18 coating deposited onto glass substrates still shifts the potential
Fig. 8. Cyclic polarisation curves of Al – Cr18 – N coatings deposited onto glass substrate compared to Al – Cr 18 coating after 1 h of immersion in saline solution.
towards positive values and decreases the corrosion current density. We notice that the anodic part of the polarisation curve is modified when nitrogen is incorporated into Al – Cr coating. This incorporation increases the intrinsic corrosion resistance of the Al –Cr coatings because pitting corrosion disappears with an increase of nitrogen content. According to previous studies [1– 3] the nitrogen addition seems to favour amorphisation of the Al –Cr alloy. Therefore, one might reasonably suppose that the corrosion product film formed on the amorphous coating is uniform, adherent and homogeneous. It remains to be seen, however, whether the corrosion product film composition is now a mixture of Al and Cr compounds. Fig. 9a,b presents the polarisation curves of Al – Cr –N with two nitrogen contents (compared to the standard Al – Cr 18 coating) deposited onto the AISI 4315 steel substrate. The polarisation curve shape of Al –Cr – N alloy coated steel is similar to the standard Al – Cr coating, the main difference being observed at high anodic overpotentials where no pitting corrosion of the Al – Cr– N alloy films occurs. The optical observations do not reveal pit formation on the coating surface. Indeed, a modification of the oxidation slope, which corresponds to a new dissolution mechanism, also occurs around 585 mV/SCE for both Al – Cr– N coatings. We observe, on the reverse polarisation curve, that the corrosion potential is similar for both samples. In fact the additional reaction should be associated to the aggressive accelerated corrosion test, which suppresses the protection ensured by the sacrificial coating. Therefore, on the reverse polarisation, the electrochemical behaviour should correspond to steel corrosion through the open porosity of the corroded coating. Potential evolution and EIS measurements after extended immersion times show a fast potential ennoblement and, after 35 h of immersion, the impedance diagrams for both nitrogen contents represent the steel electrochemical characteristics, for which the high and constant value of
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Table 4 Electrochemical characteristics of Al – Cr and Al – C – N alloy coatings deposited onto glass substrates after 1 h of immersion in a 3% NaCl solution Al – Cr 18 Ecorr (mV/SCE) icorr (AA cm 2) Ep (pitting potential mV/SCE)
857 0.09 667
transfer resistance lead us to conclude that porosity rate is negligible. In fact, the surface observations after 48 h of immersion show only a few rust points. The coating remains adherent and no localised degradation were detected.
4. Conclusions The corrosion behaviour of Al – Cr and Al –Cr – (N) alloy coatings deposited by d.c. magnetron sputtering on glass and mild steel substrates was studied and compared to the corrosion behaviour of pure Al and Cr coatings.
Al – Cr18 N7 635 0.02 540
Al – Cr18 N14 490 0.015 none
In a narrow range of Cr content, the incorporation of chromium in an aluminium coating allows an amorphous coating to be deposited. These coatings present excellent intrinsic corrosion resistance in saline solution, with low corrosion rate and a pitting potential shifted towards more positive values. The corrosion potential of Al – Cr alloys is also shifted when Cr content increases with potential ennoblement occurring during long immersion tests. The ennoblement mechanism seems to be associated to preferential aluminium dissolution, which leads either to formation of a protective corrosion product film, or to an enriched chromium surface coating. Thorough research using local electrochemical techniques would permit to clarify the mechanism involved in the corrosion of amorphous Al– Cr coatings. The corrosion behaviour of coated steel is strongly dependant on the Cr content in the Al based coating. All Al – Cr alloy coatings are compact and dense, and thus provide improved barrier protection (for a given thickness) against the aggressive media. For a Cr content of 18 at.%, the coating also provides cathodic protection of steel, ensured by evenly distributed sacrificial dissolution of the Al – Cr coating. The cathodic protection efficiency decreases with further increasing Cr content, due to spontaneous passivation of the coating. Thus an Al – Cr coating with 18 at.% of Cr presents the best corrosion resistance in saline solution, combining an effective barrier and uniform sacrificial properties which continue to protect steel substrates in contact with the solution once open porosity develops. Nitrogen incorporation in Al –Cr coatings enhances the pitting resistance in saline solution, and for high nitrogen content, pitting corrosion is suppressed. However, this incorporation induces a potential shift towards positive values. Thus, sacrificial protection of steel substrate is reduced. The barrier protection effect is, however conserved and perhaps enhanced since nitrogen incorporation is expected to stabilise the amorphous phase and improve coating densification. A thorough research using local electrochemical techniques would permit to determine the principal dissolution mechanism of coated steel when its potential is shifted towards the oxidation domain of steel.
Acknowledgements Fig. 9. (a and b): Cyclic polarisation curves of Al – Cr18 – N coatings deposited onto AISI 4135 steel substrates compared to Al – Cr 18 coating after 1 h of immersion in saline solution.
B. Peraudeau and C. Savall are gratefully acknowledged for their technical support during the project.
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