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Corrosion behaviour of molybdate–phosphate–silicate coatings on galvanized steel
Corrosion Science 51 (2009) 2455–2462
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Corrosion behaviour of molybdate–phosphate–silicate coatings on galvanized steel Y. Hamlaoui a,c, L. Tifouti b, F. Pedraza c,* a b c
Institut des Sciences et Sciences de l’Ingénieur, Centre Universitaire de Souk-Ahras, BP-1553, 41000 Souk-Ahras, Algeria Laboratoire de Génie de l’Environnement, Université Badji Mokhtar, BP-1223, 23020 El Hadjar-Annaba, Algeria Laboratoire d’Etude des Matériaux en Milieux Agressifs (LEMMA), Pôle Sciences et Technologie, Université de La Rochelle, Avenue Michel Crépeau, 17042 La Rochelle Cedex 1, France
a r t i c l e
i n f o
Article history: Received 3 March 2009 Accepted 18 June 2009 Available online 25 June 2009 Keywords: A. Chromate layer A. Molybdate–phosphate–silicate (MPS) A. Galvanized steel B. Tafel B. EIS
a b s t r a c t In this work, a Cr-free conversion layer based on molybdate–phosphate–silicate (MPS) was synthesised on a galvanized steel by simple immersion and its corrosion behaviour was compared to that of a typical chromate layer. Stationary electrochemical techniques and electrochemical impedance spectroscopy (EIS) were employed to highlight the corrosion mechanisms of both coatings in different NaCl concentrations, immersion times and pH. Contrary to the chromate layer, the MPS coating showed good electrochemical stability even in concentrated NaCl solutions and remarkable electrochemical efficiency. With increasing time, two corrosion stages were associated with the two likely sublayers of the MPS coating. Furthermore, the MPS coating behaved better than the chromate layer in acidic and alkaline pH, especially the latter as a compact corrosion product layer formed. Finally, each coating/electrolyte interface was characterised by an electrical equivalent circuit giving a satisfactory correlation between the experimental and the calculated impedance. It derived that the MPS could be an environmentally friendly alternative to chromating. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction For years, chromating has been applied to produce corrosion resistant conversion layers onto different substrates. The processes and mechanisms of chromating galvanized steel were thoroughly studied by different authors (e.g. [1–3]) and two corrosion models were proposed depending on whether this conversion layer is uniform and perfect or whether it contains inhomogeneities [4]. Further insight on the degradation mechanisms of these layers was obtained by combining DC polarisation measurements and electrochemical impedance spectroscopy (EIS) [5]. Using these techniques, Mertens and Temmerman [6] clearly underlined the protection behaviour conferred by this treatment as the double layer capacity (Cdl) of the chromate layer was seven times lower than that obtained on the untreated galvanized steel. However, because of the carcinogenic and toxicity of Cr(VI), the European Union banned its use in the automotive industry in 2000 [7]. Therefore, several alternative conversion treatments were investigated to develop non hazardous processes for hot dipped galvanized steel (HDG). For instance, the use of Na3PO4 as a passivating compound on zinc electrode was studied by Aramaki [8], who reported that the resulting conversion layer suppressed the cathodic process and inhibited the anodic zones of zinc. In addition, Ogle et al. [9] showed that the surface was covered with deposits of Zn(OH)2, ZnO and Zn3(PO4) after long immersion times in a NaCl * Corresponding author. Tel.: +33 (0) 5 46 45 82 97; fax: +33 (0) 5 46 45 72 72. E-mail address: [email protected] (F. Pedraza). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.06.037
solution. In the case of molybdate conversion coatings, the morphology and anticorrosive properties strongly depended on the acid employed to acidify the bath [10]. A combination of phosphate–molybdate was also reported to confer a significant anticorrosive effect in rain solution compared to the untreated galvanised steel [11]. In addition, electrodeposited silicate coatings were also quoted to provide a higher barrier resistance and better stability than the chromium passivates [12]. Therefore, any combination of the above [molybdate–phosphate–silicate (MPS)] could result in a significant improvement of the corrosion resistance. The elaboration of MPS coatings was first reported by Tang et al. [13] and upgraded by Song and Mansfeld [14] with the addition of silane and nitric acid to improve the coating process. In the latter, resistance values similar to those of chromate conversion coatings were found [14]. Previous studies [12,15,16] had shown that the presence of silicates allowed the formation of a physical barrier preventing the penetration of aggressive ions because of the complex layer resulting from polymerisation reactions with Zn. Moreover, the reactions of the film formation were reported incomplete during the drying period and continued during immersion in NaCl solution resulting in a decrease of the corrosion rate [17]. This phenomenon is of great importance because the corrosion of galvanized steel often appears during storage, transport and its use in not very aggressive environments. Taking into account the beneficial effects of phosphates, molybdates and silicates, this work intends to provide further insight on the effects of a MPS conversion treatment on the corrosion behaviour of the HDG steel in different NaCl concentrations, pH and
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immersion times. The results are compared with the electrochemical response (Tafel, cyclic voltammetry and EIS) of a conventional chromated HDG steel.
3. Results and discussion
2. Experimental conditions
The chromated HDG surface [Fig. 1(a)] exhibited a rough aspect with some initiation of pits. The EDS microanalyses revealed the major presence of Cr and O although Zn and S were also present, supporting the works of Sandberg et al. [20], who reported a complex mixture of compounds like Cr2(SO4)3, Cr2O3, Cr(OH)3, ZnSO4 and ZnO. Very few cracks due to dehydration were observed in contrast to the MPS coating [Fig. 1(b)], whose microstructure showed a ‘‘dry mud” appearance suggesting that the coating shrank upon drying. Nevertheless, the surface was fully covered with no sign of spallation. The EDS analyses revealed homogenous contents of Si, Mo, Zn, O and P throughout the surface. In spite of these differences, the preliminary results from standard salt spray tests [21] were rather promising as the first signs of white corrosion appeared after 24, 72 and 96 h for the untreated galvanised steel, the MPS and the chromate coating, respectively. This encouraged us to study this new Cr-free film under various conditions, namely different NaCl concentrations, under immersion and different pHs.
2.1. Sample preparation and coatings The samples consisted of rectangular plates (4 4 cm) of carbon steel that was galvanized at industrial scale (Mittal Steel, Algeria) to obtain 15 lm thick layers. Then the samples were ultrasonically cleaned in an acetone bath, rinsed with distilled water and dried with hot air prior to chromating. This was undertaken by immersion in an acidic bath containing 200 g L1 Na2Cr2O7 + 10 g L1 H2SO4 (pH 1.2) during 10 s at ambient temperature [18]. The chromate layers were thinner than 1 lm thick, as verified by scanning electron microscopy (SEM) cross sections. For the MPS coatings, the samples were first immersed in an alkaline bath to increase the formation of superficial hydroxides Zn(OH)2, rinsed thoroughly in water and then immersed in a mixture of Na2(MoO4)2H2O:Na3(PO4)12H2O:Na2Si3O73H2O (1:1:2) ratio during 15 min. The ratio (1:1:2) was retained after several optimisation tests all by maintaining a higher amount of silicates. Right after, the samples were subjected to a tangential hot spray air to eliminate the excess of the solution and finally dried in air to consolidate the coating. 2.2. Experimental set-up and corrosion tests The experimental set-up was composed of a classic three electrodes cell using a Pt grid as counter electrode and a saturated calomel electrode (SCE) as the reference one, the coated samples being connected to the working electrode. The measurements were carried out using a potentiostat/galvanostat EGG 273A coupled to a frequency response analyser (FRA) EGG 1025 allowing to scan frequencies between 100 kHz and 100 mHz with 10 mV as the applied sinusoidal perturbation. Prior to any electrochemical test, the time to stabilisation of the open circuit potential was 30 min. The Tafel polarisation curves were obtained at a scanning rate of 10 mV/min between ±250 mV with respect the corrosion potential (Ecorr). The polarization resistance Rp measurement was carried out in the vicinity of the corrosion potential (±10 mV at the same scanning rate). The experiments were monitored using the software EGG M352 and Powersine software and the EIS results were fitted using the EQUIVCRT software designed by Holland Researcher Boukamp software [19]. All values reported here, including electrochemical data, are averages of two or three measurements. The morphology and composition of the coatings were characterised by scanning electron microscopy (SEM) in a Philips XL 20 operating at 20 kV coupled to energy dispersive spectrometry (EDS). The corrosive media consisted of 0.1, 0.5 and 1 M NaCl solutions.
3.1. Coatings and preliminary corrosion tests
3.2. Electrochemical behaviour in NaCl solutions From the shape of the polarization curves obtained on chromate and MPS coatings, it was readily observed that the corrosion rate in the aerated NaCl medium was under cathodic control (oxygen diffusion) (Fig. 2). Therefore, the corrosion current icorr was calculated using the Stern and Geary equation (icorr ¼ ba =2:3Rp ) [22–24], with ba being the anodic Tafel slope. In the chromate coatings, the corrosion potentials (Ecorr) were observed to shift slightly towards more cathodic values with the increase of NaCl concentration. However, the current density (Icorr) increased significantly (Table 1) and was even 3-folded when passing from 0.1 to 1 M, indicating that the layer was not resistant in such medium. In addition, the anodic slope changed for the highest concentrations whereas the cathodic slopes became progressively more polarised suggesting that the coating acted as a physical barrier. In contrast, the MPS coatings (Fig. 2b and Table 2) showed basically the same anodic slopes regardless of the concentration but the cathodic branches were far more polarised than in the chromate coatings regardless of the concentration. The Ecorr were also slightly more anodic for the 0.1 and 0.5 M solutions and shifted towards similar values than those of the chromate coatings in the 1 M baths. However, only very small increases of the Icorr values were recorded with the increase of concentration. As an indicative measure, the surface passivation [24,25] given by equation:
ðI0corr Icorr Þ I0corr
100
Fig. 1. SEM surface morphology of the (a) chromate and (b) MPS coatings on galvanized steel.
ð1Þ
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Fig. 2. Polarisation curves obtained in different NaCl concentrations on (a) the chromate coatings and (b) the MPS coatings.
Table 1 Electrochemical parameters obtained on the chromated layer at different NaCl concentrations. Tests conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
0.1 M (pH 6.7) 0.5 M (pH 6.7) 1 M (pH 7) 0.1 M on untreated HDG
1030 1035 1040 1038
65.35 70.21 36.81 146.97
515.3 425.3 98.20 313
55.17 71.78 163.00 300.00
0.205 0.267 0.607 4.472
56.09 63.88 97.45 37.77
1.016 0.890 0.598 0.12
Table 2 Electrochemical parameters obtained on MPS coatings at different conditions in NaCl solutions. Tests conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
0.1 M (pH 6.7) 0.5 M (pH 6.7) 1 M (pH 7) 0.1 M after 3 days 0.1 M after 10 days
980 970 1042 955 970
39.59 40.20 39.00 30.1 36.30
249.00 247.78 221.10 403.29 174.68
69.32 70.46 76.69 34.21 90.60
0.257 0.262 0.285 0.127 0.332
22.09 16.47 24.89 35.52 22.46
0.32 0.23 0.32 1.04 0.25
where Icorr is the corrosion current of untreated HDG steel and Icorr the corrosion current of coated HDG steel (see Table 1) is of about 82% and 77% for the chromated and MPS HDG steel in 0.1 M NaCl solution, respectively. The cyclic polarization curves of the passivated galvanised coating showed significant differences as depicted in Fig. 3. The chromate coatings (Fig. 3a) exhibited a large hysteresis regardless of the concentration, hence the surface evolved significantly upon the polarization. In addition, a peak located at about 400 mV/ ECS at 0.1 M was observed, which suggested that reversible redox reactions were taking place, probably in relation with the chromium oxide species. With the increase in concentration, the oxidation peak was still present but was less marked and had moved towards more anodic values. This suggested that a relatively thick corrosion product layer was formed. In contrast, the cyclic polarization curves of the MPS coating (Fig. 3b) did not show any oxidation peak and very low hysteresis because the surface remained practically unchanged after polarisation. This situation was very much alike to that of the untreated HDG steel, where the existence of a simple layer is usually ascribed to Zn and Zn oxide/hydroxide [24,26]. These results were confirmed through the specific surface evolution (Q) and compactness of the corrosion layer (C = Q/Icorr) [27], whose values are given in Tables 1 and 2 for the chromate and MPS coatings, respectively. The compactness of the layer represents the amount of the adherent corrosion product or the surface area covered with the coating and with the corrosion products per unit of corrosion current. The higher the Q value, the more significant the degradation of the coating. This was the
case of the chromate coatings for all concentrations compared to the MPS layers. In addition, despite the compactness of the corrosion layer was higher in the chromate coatings, it decreased with the increase of NaCl concentration whereas it remained practically constant in the MPS coatings. All these DC results therefore suggested that the MPS films were stable, and behaved as a physical barrier inhibiting to some extent the cathodic reaction through the formation of a corrosion product layer. Additional information to explain these mechanisms was sought through electrochemical impedance spectroscopy (EIS) between 30 kHz and 0.1 Hz at the Ecorr. Fig. 4 shows the EIS diagrams for the chromate coatings. The physical values were fitted with the EQUIVCRT software through appropriate equivalent circuits (see later) and the results are gathered in Table 3. The chromate layer resistance ‘‘Rc” decreased with the increase of NaCl concentration and when passing from 0.1 to 0.5 and 1 M, the same ratios of about 0.3 and 0.6, respectively, were found. The CPEc and CPEdl (constant phase element) are the non ideal (dispersive) layer capacitance and double layer capacitance, respectively; Rct is the charge transfer resistance, and Rs corresponds to the resistance of the electrolyte. The EIS Nyquist diagrams in NaCl 0.1 M revealed the existence of an inductive loop in the capacitive plan between two capacitive loops. The loop at HF (higher frequencies) is typically related to the layer resistance, and the one at LF (lower frequencies) to the corrosion process itself [5]. However, by increasing the concentration, the information related to the slowest phenomena was gradually lost and only one loop was observed at 1 M NaCl. The increase of concentration also brought about more depressed Nyquist
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Fig. 3. Cyclic polarisation curves obtained in different NaCl concentrations on the (a) chromate coatings and (b) MPS coatings.
Fig. 4. EIS (a) Nyquist and (b) Bode diagrams obtained at different NaCl concentrations on the chromate coatings.
Table 3 EIS parameters obtained on the chromated layer at different NaCl concentrations. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a))
a
R1 (X cm2)
C1 (lF cm2)
Rct (X cm2)
CPEdl (lF cm2 s(1a ))
a0
0.1 M 0.5 M 1M
20 17 12
140 96 53
1.33 3.11 0.83
0.735 0.695 0.620
7.19 6.64 –
0.106 0.035 –
19 17.25 21.27
92 115 165
0.85 0.84 0.79
semi-circles, thus lower non linearity coefficients (a), which implied the development of rougher surfaces. In addition, the Warburg diffusion of about r = 850 X s1/2 suggested that transport of a species towards the surface of the electrode had indeed occurred [28]. These results, coupled to the negative values of C1 and R1 associated with the inductive loop, could therefore suggest the growth of a pseudopassivating film, i.e., a corrosion products layer or to the competition between adsorption and dissolution processes [24,29,30]. This was supported by the presence of two well defined time constants in the Bode diagrams also quoted in the literature [31–33]. In the case of the MPS coatings, the EIS diagrams (Fig. 5 and Table 4) underlined some differences in comparison with the chromate coatings as the Nyquist diagrams showed only two capacitive loops but no inductive loop. The frequency dispersion of the impedance response at high frequencies (often observed as inductive loops) is usually attributed to non stationary (unsteady state) conditions caused by the experimental set-up (electrolyte resistance and conductivity, self-conductance of the connecting leads or/and the time necessary of the stabilization of the double
0
layer) as was reported in Refs. [5,34]. However, the inductive loop observed between high and intermediate frequencies was usually attributed to the relaxation process of adsorbed species or to surface modification [35,36]. Moreover, Bastos et al. [2] attributed the inductive loop in EIS diagrams at intermediate frequencies to the air formed over the oxide film that becomes dissolved in the chloride solution. Identical EIS spectra were also reported by Titz et al. [37] upon the oxygen diffusion controlled corrosion of a mild steel covered with a porous organic film. Nevertheless, and similar to the Cr-containing layers, all Bode diagrams were characterised by two well defined relaxation processes suggesting that the MPS coatings were composed of two layers. According to Kumaraguru et al. [16], the MPS films could have an external layer manly composed of Si–O and an internal one containing Si–O–Si and Mo–P–O–Zn compounds. The latter was reported to be nonporous and protective against corrosion. Therefore, the external layer could be corroded already at 0.1 M NaCl but at higher concentrations the corrosion products from the first layer seemed to prevent corrosion of the second layer immediately over the galvanized substrate. In addition, it should
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Fig. 5. EIS (a) Nyquist and (b) Bode diagrams obtained in different concentrations of NaCl on the MPS coatings.
Table 4 EIS parameters obtained on the MPS coatings at different conditions in NaCl solutions. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a))
a
Rct (X cm2)
Cdl (lF cm2 s(1a ))
a0
0.1 M (pH 6.7) 0.5 M (pH 6.7) 1 M (pH 7) 0.1 M after 3 days 0.1 M after 10 days
5 18 08 15 19
229 159 162 230 101
5.81 4.11 5.77 17.49 18.81
0.77 0.78 0.84 0.71 0.78
60 100 50 120 39
68 38 90 45 150
0.86 0.82 0.82 0.80 0.80
be noted that the layer resistance (Rc) and the charge transfer resistance (Rct) were by far higher in the case of the MPS coatings than in the chromate layers even in the most concentrated corrosive solutions and therefore confirm a more protective behaviour. 3.3. Evolution of the electrochemical behaviour with immersion time in 0.1 M NaCl solutions Because of the weak degradation of the coating in NaCl 0.1 M, several MPS samples were immersed in 0.1 M for 10 days in order to study the evolution of the coating with time. After 3 days of immersion (Fig. 6 and Table 2), the slope of the cathodic branch remained practically unchanged. In contrast, the anodic slope decreased by one order of decade. After 10 days of immersion, an increase in the slope of the cathodic branch and a reduction of the anodic one by half a decade was also observed; i.e., the corrosion process evolved towards a mixed control. According to the Icorr values, it was concluded that partial degradation of the layer oc-
0
curred with formation of a layer of corrosion products that was observed even at the naked eye. To confirm the DC polarization results, EIS measurements were carried out under the same experimental conditions. The results are summarised in Fig. 6(b) and Table 4. The layer resistance (Rc) showed some stability at the beginning of immersion but was halved after 10 days of immersion, in agreement with the behaviour of the cathodic branch of the polarization curves. By considering the above described structure of the MPS coatings [16], it was assumed that the chloride ions initially penetrated and degraded the external layer but the process was then slowed down by the presence of the second non porous internal layer. 3.4. Evolution of the electrochemical behaviour with pH in 0.1 M NaCl solutions The influence of the solution pH on the electrochemical behaviour of the modified galvanised coating was also studied in the
Fig. 6. DC polarisation curves (a) and Nyquist diagrams (b) obtained on the MPS coatings at different immersion times in 0.1 M NaCl solutions.
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0.1 M NaCl solutions. The adjustment of the pH was carried out by addition of maximum purity nitric acid or potassium hydroxide. Fig. 7 and Tables 5 and 6 gather the values obtained on the Cr-containing coatings. Compared to the initial neutral solution the corrosion current density increased in both the acidic and alkaline pH ranges and therefore the chromate layers could not confer adequate corrosion protection. The anodic slopes remained practically unchanged suggesting that the dissolution process of the films was not modified. In contrast, the cathodic slopes increased significantly in the acidic and alkaline pH because of a higher activity in the oxygen reduction. Indeed, Zeller and Savinell [5] showed that the resistance of chromate coatings in Na2SO4 was higher in deaerated medium that in an aerated one. This confirms the hypothesis by which the corrosion process is under cathodic control in this case. Furthermore, the EIS diagrams obtained at different pH showed the existence of two loops at LF, i.e., the pH did not have an effect on the number of relaxation processes. However, the values of the layer resistance agreed with the corrosion current density values. On the one hand, according to the morphology of the coating [Fig. 1(a)] and to the electrochemical results (presence of two relaxation processes at LF, corrosion under cathodic control); and on the other, when referring to previous works [36,38] and the two corrosion models proposed by the literature [4], it was concluded that the chromate coating contained surface porosity. The progressive disappearance of the second LF loop previously observed at 0.5 and 1 M corresponded therefore to the fast filling of the pores by corrosion products leading to an almost perfect layer. Fig. 8(a) and Table 7 show the influence of pH on the corrosion behaviour of the MPS coating. The Icorr values showed little stability
in the acid medium but good corrosion resistance of the coating in the basic medium. This was supported by the shift of Ecorr towards more cathodic values and the higher value of the compactness of the corrosion layer in the alkaline conditions. In contrast, the Ecorr shifted towards more anodic values in the acid medium hence suggesting that degradation of the coating was enhanced. The EIS diagrams were also plotted at the free corrosion potential [Fig. 8(b) and Table 8] to provide further insight on the corrosion mechanisms. The resistance at high frequency (Rc) values confirmed that the coating resisted poorly against corrosion in the acidic medium. However, good corrosion resistance was shown in alkaline conditions. Moreover, the layer resistance calculated for neutral and alkaline pH was 6 times higher than that obtained at acidic pH. This confirmed the assumption that the alkaline medium may support the formation of Si–O–Si, Si–O–Zn and Mo–P–O–Zn complex in the internal layer [16]. The EIS diagrams were also characterised by two time constants for the different pHs (4, 6.7 and 11). Therefore, it was concluded that neither the pH, nor the concentration or the immersion time influenced the number of relaxation processes. For Ce-containing systems, Mora et al. [30] proposed an equivalent circuit including two relaxation processes with diffusion at LF. Other authors [39] proposed an open boundary finite length diffusion (OFLD) in the electrical equivalent circuit instead of the diffusion impedance to simulate the corrosion process of a coating modified by thin layer of silane. Taking into account the different EIS behaviours of the Cr-containing and of the Cr-free treatments, different equivalent circuits were therefore derived as depicted in Fig. 9(a) and (b), respectively. For the Cr-containing coatings, the equivalent circuit giving a similar response to the interface in NaCl
Fig. 7. (a) Polarisation curves and (b) EIS diagrams obtained on the chromate coatings in 0.1 M NaCl solutions at different pH.
Table 5 Electrochemical parameters obtained for the chromated layer as a function of pH in 0.1 M NaCl solutions. Test conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
pH 4 pH 6.7 pH 11
982 1030 975
47.65 65.35 52.88
101.35 515.3 135.00
204.40 55.17 170.30
0.760 0.205 0.633
102.3 56.09 96.0
0.500 1.016 0.564
Table 6 EIS parameters obtained on the chromated layer as function of pH in 0.1 M NaCl solutions. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a))
a
R1 (X cm2)
C1 (lF cm2)
Rct (X cm2)
Cdl (lF cm2 s(1a )
a0
pH 4 pH 6.7 pH 11
35 20 37
74.70 140 108.00
6.18 1.33 8.84
0.67 0.73 0.84
6.00 7.19 5.00
0.061 0.106 0.002
15.00 19 9.00
110 92 265
0.79 0.85 0.76
0
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Fig. 8. (a) Polarisation curves and (b) EIS diagrams obtained on the MPS coating in 0.1 M NaCl solutions at different pH.
Table 7 Electrochemical parameters obtained on the MPS coatings as a function of pH in 0.1 M NaCl solutions. Test conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
pH 4 pH 6.7 pH 11
958 980 1030
36.15 39.59 72.00
241.58 249.00 810.90
65.06 69.32 38.60
0.242 0.257 0.106
34.15 22.09 35.94
0.50 0.32 1.26
Table 8 EIS parameters obtained on the MPS coatings as a function of pH in 0.1 M NaCl solutions. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a)
a
Rct (X cm2)
Cdl (lF cm2 s(1a ))
a0
pH 4 pH 6.7 pH 11
20 5 03
38 229 420
5.98 5.81 17.47
0.79 0.77 0.65
16 60 321
1.75 68 8.5
0.83 0.86 0.89
(a)
(b)
CPEc
CPEc Rs
Rs
0
CPE
Zw Rct
CPEdl
C1
Rc R1
Rc Rct
Fig. 9. Equivalent circuits schematizing the coating/NaCl solution interface of (a) the chromate and (b) the MPS layers.
at various concentrations can be expressed with Rs: solution resistance, CPEc: constant phase element characterised by Cc (non uniform layer capacity) and a as the non linearity coefficient. Rc is the layer resistance related to the heterogeneous structure of the chromate layer. C1 and R1: capacity and resistance of relaxation time related to the electrode electrochemical processes. Rct: charge transfer resistance, CPEdl: non linear double layer capacity and Zw: diffusion impedance. The diffusion impedance equalled zero when the NaCl concentration was 0.1 and 0.5 and when the NaCl concentration was increased to 1 M the second electrochemical process disappeared. However, according to the previous results, the equivalent circuit giving the same impedance response for the solution/MPS coating interface shall be interpreted using two time constants (Fig. 9b). The electrochemical parameters (corrosion current density and layer resistance) obtained in NaCl solution at different concentrations clearly showed the stability and the effectiveness of MPS passivation as depicted in Fig. 10. Therefore, these preliminary studies suggested that the MPS coating could be an alternative treatment to replace Cr(VI) passivation treatments.
Fig. 10. Evolution of the corrosion rate as function of the NaCl concentration of untreated and treated HDG steel.
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4. Conclusions In this study, the corrosion behaviour of a chromated layer and a molybdate–phosphate–silicate (MPS) coating on an industrial galvanized steel was evaluated in different concentrations of NaCl solutions, immersion times and pH. As opposed to the chromate coatings and despite their cracked morphology, the results showed that the MPS coatings exhibited adequate corrosion behaviour even in concentrated NaCl solutions, acidic or alkaline pH. They also showed appropriate corrosion resistance after relatively long immersion times. The electrochemical mechanisms of degradation seemed to show that the MPS coating was composed of two layers, an external porous one that brings about the development of a compact corrosion products layer and an internal one that arrests further corrosion of the underlying galvanized steel. Overall, a similar corrosion resistance than of the chromate layers was observed. Therefore, it can be concluded that the MPS coatings could represent an environmentally friendly alternative to the use of Cr(VI) treatments. References [1] A.M. Rocco, T.M.C. Nogueira, R.A. Simao, W.C. Lima, Surf. Coat. Technol. 179 (2004) 135–144. [2] A.C. Bastos, M.G.S. Ferreira, A.M. Simoes, Prog. Org. Coat. 52 (2005) 339–350. [3] M. Tencer, Appl. Surf. Sci. 252 (2006) 8229–8234. [4] A. Pirnat, L. Meszaros, G. Meszaros, B. Lengyel, Corros. Sci. 34 (1993) 1147–1155. [5] R.L. Zeller, R.F. Savinell, Corros. Sci. 26 (1986) 389–399. [6] S.F.L. Mertens, E. Temmerman, Corros. Sci. 43 (2001) 301–316. [7] Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on end-of life vehicles, Official Journal of the European Communities, 21 October 2000. [8] K. Aramaki, Corros. Sci. 45 (2003) 451–464. [9] K. Ogle, A. Tomandl, N. Meddahi, M. Wolpers, Corros. Sci. 46 (2004) 979–995. [10] O. Magalhaes, I.C.P. Maragarit, O.R. Mattos, J. Electroanal. Chem. 572 (2004) 433–440. [11] I.M. Zin, S.B. Lyon, V.I. Poktmurskii, Corros. Sci. 45 (2003) 777–788.
[12] B. Veeraraghavan, D. Slavkov, S. Prabhu, M. Nicholson, B. Haran, B. Popov, B. Heimann, Surf. Coat. Technol. 167 (2003) 41–51. [13] P.T. Tang, G.B. Nielsen, P. Moeller, Plat. Surf. Finish. 81 (1994) 20–23. [14] Y.K. Song, F. Mansfeld, Corros. Sci. 48 (2006) 154–164. [15] R.K. Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, John Wiley & Sons, NY, 1979. [16] S.P. Kumaraguru, B. Veeraraghavan, B.N. Popov, J. Electrochem. Soc. 153 (2006) B253–B259. [17] M.F. Montemor, A. Rosqvist, H. Fagerholm, M.G.S. Ferreira, Prog. Org. Coat. 51 (2004) 188–194. [18] X. Zhang, C. Bos, W.G. van den Sloof, H.A. Terryn, J. Corros. Sci. Eng. 6 (57) (2004) 1–11. [19] B. Boukamp, Manual AC-emittance Data Analysis Systems Equivalent Circuit, Twente University of Technology, Enschede, The Netherlands, 1989. [20] J. Sandberg, I. Odnevall Wallinder, C. Leygraf, N. Le Bozec, J. Electrochem. Soc. 154 (2007) C120–C131. [21] ASTM-B-117 Salt Spray (Fog) Testing, ASTM, Philadelphia, PA, 1990. [22] M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56–63. [23] K.G. Boto, L.F.G. Williams, J. Electrochem. Soc. 124 (1977) 656–661. [24] V. Barranco, S. Feliu Jr., S. Feliu, Corros. Sci. 46 (2004) 2203–2220. [25] S.B. Lalvani, J.-C. Kang, M. Murthy, Corros. Sci. 37 (1995) 1599–1604. [26] Y. Hamlaoui, F. Pedraza, L. Tifouti, Corros. Sci. 50 (2008) 1558–1566. [27] I. Suzuki, Corros. Sci. 25 (1985) 1029–1034. [28] E. Almeida, L. Fedrizzi, T.C. Diamantinio, Surf. Coat. Technol. 105 (1998) 97– 101. [29] R.F.A. Jargelius-Petterson, B.G. Pound, J. Electrochem. Soc. 145 (1998) 1462– 1469. [30] N. Mora, E. Cano, J.L. Polo, J.M. Puente, J.M. Bastidas, Corros. Sci. 46 (2004) 563– 578. [31] A.A.O. Magalhaes, I.C.P. Margarit, O.R. Mattos, Electrochim. Acta 44 (1999) 4281–4287. [32] F. Deflorian, S. Rossi, L. Fedrizzi, Prog. Org. Coat. 52 (2005) 271–279. [33] C. Gabrielli, M. Keddam, F. Minouflet-Laurent, Electrochim. Acta 48 (2003) 1483–1490. [34] D.E. Williams, J. Asher, Corros. Sci. 24 (1984) 185–196. [35] I. Epelbain, M. Keddam, H. Takenouti, J. Appl. Electrochem. 2 (1976) 71–79. [36] C. Cachet, F. Ganne, G. Maurin, J. Petitjean, V. Vivier, R. Wiart, Electrochim. Acta 47 (2001) 509–518. [37] J. Titz, G.H. Wagner, H. Spahn, M. Herbert, K. Juttner, W.J. Lorenz, Corrosion 46 (1990) 221–229. [38] C. Deslouis, M. Duprat, Chr. Tournillon, Corros. Sci. 29 (1989) 13–30. [39] F. Mansfeld, L.T. Han, C.C. Lee, G. Zhang, Electrochim. Acta 43 (1998) 2933– 2945.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behaviour of molybdate–phosphate–silicate coatings on galvanized steel Y. Hamlaoui a,c, L. Tifouti b, F. Pedraza c,* a b c
Institut des Sciences et Sciences de l’Ingénieur, Centre Universitaire de Souk-Ahras, BP-1553, 41000 Souk-Ahras, Algeria Laboratoire de Génie de l’Environnement, Université Badji Mokhtar, BP-1223, 23020 El Hadjar-Annaba, Algeria Laboratoire d’Etude des Matériaux en Milieux Agressifs (LEMMA), Pôle Sciences et Technologie, Université de La Rochelle, Avenue Michel Crépeau, 17042 La Rochelle Cedex 1, France
a r t i c l e
i n f o
Article history: Received 3 March 2009 Accepted 18 June 2009 Available online 25 June 2009 Keywords: A. Chromate layer A. Molybdate–phosphate–silicate (MPS) A. Galvanized steel B. Tafel B. EIS
a b s t r a c t In this work, a Cr-free conversion layer based on molybdate–phosphate–silicate (MPS) was synthesised on a galvanized steel by simple immersion and its corrosion behaviour was compared to that of a typical chromate layer. Stationary electrochemical techniques and electrochemical impedance spectroscopy (EIS) were employed to highlight the corrosion mechanisms of both coatings in different NaCl concentrations, immersion times and pH. Contrary to the chromate layer, the MPS coating showed good electrochemical stability even in concentrated NaCl solutions and remarkable electrochemical efficiency. With increasing time, two corrosion stages were associated with the two likely sublayers of the MPS coating. Furthermore, the MPS coating behaved better than the chromate layer in acidic and alkaline pH, especially the latter as a compact corrosion product layer formed. Finally, each coating/electrolyte interface was characterised by an electrical equivalent circuit giving a satisfactory correlation between the experimental and the calculated impedance. It derived that the MPS could be an environmentally friendly alternative to chromating. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction For years, chromating has been applied to produce corrosion resistant conversion layers onto different substrates. The processes and mechanisms of chromating galvanized steel were thoroughly studied by different authors (e.g. [1–3]) and two corrosion models were proposed depending on whether this conversion layer is uniform and perfect or whether it contains inhomogeneities [4]. Further insight on the degradation mechanisms of these layers was obtained by combining DC polarisation measurements and electrochemical impedance spectroscopy (EIS) [5]. Using these techniques, Mertens and Temmerman [6] clearly underlined the protection behaviour conferred by this treatment as the double layer capacity (Cdl) of the chromate layer was seven times lower than that obtained on the untreated galvanized steel. However, because of the carcinogenic and toxicity of Cr(VI), the European Union banned its use in the automotive industry in 2000 [7]. Therefore, several alternative conversion treatments were investigated to develop non hazardous processes for hot dipped galvanized steel (HDG). For instance, the use of Na3PO4 as a passivating compound on zinc electrode was studied by Aramaki [8], who reported that the resulting conversion layer suppressed the cathodic process and inhibited the anodic zones of zinc. In addition, Ogle et al. [9] showed that the surface was covered with deposits of Zn(OH)2, ZnO and Zn3(PO4) after long immersion times in a NaCl * Corresponding author. Tel.: +33 (0) 5 46 45 82 97; fax: +33 (0) 5 46 45 72 72. E-mail address: [email protected] (F. Pedraza). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.06.037
solution. In the case of molybdate conversion coatings, the morphology and anticorrosive properties strongly depended on the acid employed to acidify the bath [10]. A combination of phosphate–molybdate was also reported to confer a significant anticorrosive effect in rain solution compared to the untreated galvanised steel [11]. In addition, electrodeposited silicate coatings were also quoted to provide a higher barrier resistance and better stability than the chromium passivates [12]. Therefore, any combination of the above [molybdate–phosphate–silicate (MPS)] could result in a significant improvement of the corrosion resistance. The elaboration of MPS coatings was first reported by Tang et al. [13] and upgraded by Song and Mansfeld [14] with the addition of silane and nitric acid to improve the coating process. In the latter, resistance values similar to those of chromate conversion coatings were found [14]. Previous studies [12,15,16] had shown that the presence of silicates allowed the formation of a physical barrier preventing the penetration of aggressive ions because of the complex layer resulting from polymerisation reactions with Zn. Moreover, the reactions of the film formation were reported incomplete during the drying period and continued during immersion in NaCl solution resulting in a decrease of the corrosion rate [17]. This phenomenon is of great importance because the corrosion of galvanized steel often appears during storage, transport and its use in not very aggressive environments. Taking into account the beneficial effects of phosphates, molybdates and silicates, this work intends to provide further insight on the effects of a MPS conversion treatment on the corrosion behaviour of the HDG steel in different NaCl concentrations, pH and
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immersion times. The results are compared with the electrochemical response (Tafel, cyclic voltammetry and EIS) of a conventional chromated HDG steel.
3. Results and discussion
2. Experimental conditions
The chromated HDG surface [Fig. 1(a)] exhibited a rough aspect with some initiation of pits. The EDS microanalyses revealed the major presence of Cr and O although Zn and S were also present, supporting the works of Sandberg et al. [20], who reported a complex mixture of compounds like Cr2(SO4)3, Cr2O3, Cr(OH)3, ZnSO4 and ZnO. Very few cracks due to dehydration were observed in contrast to the MPS coating [Fig. 1(b)], whose microstructure showed a ‘‘dry mud” appearance suggesting that the coating shrank upon drying. Nevertheless, the surface was fully covered with no sign of spallation. The EDS analyses revealed homogenous contents of Si, Mo, Zn, O and P throughout the surface. In spite of these differences, the preliminary results from standard salt spray tests [21] were rather promising as the first signs of white corrosion appeared after 24, 72 and 96 h for the untreated galvanised steel, the MPS and the chromate coating, respectively. This encouraged us to study this new Cr-free film under various conditions, namely different NaCl concentrations, under immersion and different pHs.
2.1. Sample preparation and coatings The samples consisted of rectangular plates (4 4 cm) of carbon steel that was galvanized at industrial scale (Mittal Steel, Algeria) to obtain 15 lm thick layers. Then the samples were ultrasonically cleaned in an acetone bath, rinsed with distilled water and dried with hot air prior to chromating. This was undertaken by immersion in an acidic bath containing 200 g L1 Na2Cr2O7 + 10 g L1 H2SO4 (pH 1.2) during 10 s at ambient temperature [18]. The chromate layers were thinner than 1 lm thick, as verified by scanning electron microscopy (SEM) cross sections. For the MPS coatings, the samples were first immersed in an alkaline bath to increase the formation of superficial hydroxides Zn(OH)2, rinsed thoroughly in water and then immersed in a mixture of Na2(MoO4)2H2O:Na3(PO4)12H2O:Na2Si3O73H2O (1:1:2) ratio during 15 min. The ratio (1:1:2) was retained after several optimisation tests all by maintaining a higher amount of silicates. Right after, the samples were subjected to a tangential hot spray air to eliminate the excess of the solution and finally dried in air to consolidate the coating. 2.2. Experimental set-up and corrosion tests The experimental set-up was composed of a classic three electrodes cell using a Pt grid as counter electrode and a saturated calomel electrode (SCE) as the reference one, the coated samples being connected to the working electrode. The measurements were carried out using a potentiostat/galvanostat EGG 273A coupled to a frequency response analyser (FRA) EGG 1025 allowing to scan frequencies between 100 kHz and 100 mHz with 10 mV as the applied sinusoidal perturbation. Prior to any electrochemical test, the time to stabilisation of the open circuit potential was 30 min. The Tafel polarisation curves were obtained at a scanning rate of 10 mV/min between ±250 mV with respect the corrosion potential (Ecorr). The polarization resistance Rp measurement was carried out in the vicinity of the corrosion potential (±10 mV at the same scanning rate). The experiments were monitored using the software EGG M352 and Powersine software and the EIS results were fitted using the EQUIVCRT software designed by Holland Researcher Boukamp software [19]. All values reported here, including electrochemical data, are averages of two or three measurements. The morphology and composition of the coatings were characterised by scanning electron microscopy (SEM) in a Philips XL 20 operating at 20 kV coupled to energy dispersive spectrometry (EDS). The corrosive media consisted of 0.1, 0.5 and 1 M NaCl solutions.
3.1. Coatings and preliminary corrosion tests
3.2. Electrochemical behaviour in NaCl solutions From the shape of the polarization curves obtained on chromate and MPS coatings, it was readily observed that the corrosion rate in the aerated NaCl medium was under cathodic control (oxygen diffusion) (Fig. 2). Therefore, the corrosion current icorr was calculated using the Stern and Geary equation (icorr ¼ ba =2:3Rp ) [22–24], with ba being the anodic Tafel slope. In the chromate coatings, the corrosion potentials (Ecorr) were observed to shift slightly towards more cathodic values with the increase of NaCl concentration. However, the current density (Icorr) increased significantly (Table 1) and was even 3-folded when passing from 0.1 to 1 M, indicating that the layer was not resistant in such medium. In addition, the anodic slope changed for the highest concentrations whereas the cathodic slopes became progressively more polarised suggesting that the coating acted as a physical barrier. In contrast, the MPS coatings (Fig. 2b and Table 2) showed basically the same anodic slopes regardless of the concentration but the cathodic branches were far more polarised than in the chromate coatings regardless of the concentration. The Ecorr were also slightly more anodic for the 0.1 and 0.5 M solutions and shifted towards similar values than those of the chromate coatings in the 1 M baths. However, only very small increases of the Icorr values were recorded with the increase of concentration. As an indicative measure, the surface passivation [24,25] given by equation:
ðI0corr Icorr Þ I0corr
100
Fig. 1. SEM surface morphology of the (a) chromate and (b) MPS coatings on galvanized steel.
ð1Þ
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Fig. 2. Polarisation curves obtained in different NaCl concentrations on (a) the chromate coatings and (b) the MPS coatings.
Table 1 Electrochemical parameters obtained on the chromated layer at different NaCl concentrations. Tests conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
0.1 M (pH 6.7) 0.5 M (pH 6.7) 1 M (pH 7) 0.1 M on untreated HDG
1030 1035 1040 1038
65.35 70.21 36.81 146.97
515.3 425.3 98.20 313
55.17 71.78 163.00 300.00
0.205 0.267 0.607 4.472
56.09 63.88 97.45 37.77
1.016 0.890 0.598 0.12
Table 2 Electrochemical parameters obtained on MPS coatings at different conditions in NaCl solutions. Tests conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
0.1 M (pH 6.7) 0.5 M (pH 6.7) 1 M (pH 7) 0.1 M after 3 days 0.1 M after 10 days
980 970 1042 955 970
39.59 40.20 39.00 30.1 36.30
249.00 247.78 221.10 403.29 174.68
69.32 70.46 76.69 34.21 90.60
0.257 0.262 0.285 0.127 0.332
22.09 16.47 24.89 35.52 22.46
0.32 0.23 0.32 1.04 0.25
where Icorr is the corrosion current of untreated HDG steel and Icorr the corrosion current of coated HDG steel (see Table 1) is of about 82% and 77% for the chromated and MPS HDG steel in 0.1 M NaCl solution, respectively. The cyclic polarization curves of the passivated galvanised coating showed significant differences as depicted in Fig. 3. The chromate coatings (Fig. 3a) exhibited a large hysteresis regardless of the concentration, hence the surface evolved significantly upon the polarization. In addition, a peak located at about 400 mV/ ECS at 0.1 M was observed, which suggested that reversible redox reactions were taking place, probably in relation with the chromium oxide species. With the increase in concentration, the oxidation peak was still present but was less marked and had moved towards more anodic values. This suggested that a relatively thick corrosion product layer was formed. In contrast, the cyclic polarization curves of the MPS coating (Fig. 3b) did not show any oxidation peak and very low hysteresis because the surface remained practically unchanged after polarisation. This situation was very much alike to that of the untreated HDG steel, where the existence of a simple layer is usually ascribed to Zn and Zn oxide/hydroxide [24,26]. These results were confirmed through the specific surface evolution (Q) and compactness of the corrosion layer (C = Q/Icorr) [27], whose values are given in Tables 1 and 2 for the chromate and MPS coatings, respectively. The compactness of the layer represents the amount of the adherent corrosion product or the surface area covered with the coating and with the corrosion products per unit of corrosion current. The higher the Q value, the more significant the degradation of the coating. This was the
case of the chromate coatings for all concentrations compared to the MPS layers. In addition, despite the compactness of the corrosion layer was higher in the chromate coatings, it decreased with the increase of NaCl concentration whereas it remained practically constant in the MPS coatings. All these DC results therefore suggested that the MPS films were stable, and behaved as a physical barrier inhibiting to some extent the cathodic reaction through the formation of a corrosion product layer. Additional information to explain these mechanisms was sought through electrochemical impedance spectroscopy (EIS) between 30 kHz and 0.1 Hz at the Ecorr. Fig. 4 shows the EIS diagrams for the chromate coatings. The physical values were fitted with the EQUIVCRT software through appropriate equivalent circuits (see later) and the results are gathered in Table 3. The chromate layer resistance ‘‘Rc” decreased with the increase of NaCl concentration and when passing from 0.1 to 0.5 and 1 M, the same ratios of about 0.3 and 0.6, respectively, were found. The CPEc and CPEdl (constant phase element) are the non ideal (dispersive) layer capacitance and double layer capacitance, respectively; Rct is the charge transfer resistance, and Rs corresponds to the resistance of the electrolyte. The EIS Nyquist diagrams in NaCl 0.1 M revealed the existence of an inductive loop in the capacitive plan between two capacitive loops. The loop at HF (higher frequencies) is typically related to the layer resistance, and the one at LF (lower frequencies) to the corrosion process itself [5]. However, by increasing the concentration, the information related to the slowest phenomena was gradually lost and only one loop was observed at 1 M NaCl. The increase of concentration also brought about more depressed Nyquist
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Fig. 3. Cyclic polarisation curves obtained in different NaCl concentrations on the (a) chromate coatings and (b) MPS coatings.
Fig. 4. EIS (a) Nyquist and (b) Bode diagrams obtained at different NaCl concentrations on the chromate coatings.
Table 3 EIS parameters obtained on the chromated layer at different NaCl concentrations. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a))
a
R1 (X cm2)
C1 (lF cm2)
Rct (X cm2)
CPEdl (lF cm2 s(1a ))
a0
0.1 M 0.5 M 1M
20 17 12
140 96 53
1.33 3.11 0.83
0.735 0.695 0.620
7.19 6.64 –
0.106 0.035 –
19 17.25 21.27
92 115 165
0.85 0.84 0.79
semi-circles, thus lower non linearity coefficients (a), which implied the development of rougher surfaces. In addition, the Warburg diffusion of about r = 850 X s1/2 suggested that transport of a species towards the surface of the electrode had indeed occurred [28]. These results, coupled to the negative values of C1 and R1 associated with the inductive loop, could therefore suggest the growth of a pseudopassivating film, i.e., a corrosion products layer or to the competition between adsorption and dissolution processes [24,29,30]. This was supported by the presence of two well defined time constants in the Bode diagrams also quoted in the literature [31–33]. In the case of the MPS coatings, the EIS diagrams (Fig. 5 and Table 4) underlined some differences in comparison with the chromate coatings as the Nyquist diagrams showed only two capacitive loops but no inductive loop. The frequency dispersion of the impedance response at high frequencies (often observed as inductive loops) is usually attributed to non stationary (unsteady state) conditions caused by the experimental set-up (electrolyte resistance and conductivity, self-conductance of the connecting leads or/and the time necessary of the stabilization of the double
0
layer) as was reported in Refs. [5,34]. However, the inductive loop observed between high and intermediate frequencies was usually attributed to the relaxation process of adsorbed species or to surface modification [35,36]. Moreover, Bastos et al. [2] attributed the inductive loop in EIS diagrams at intermediate frequencies to the air formed over the oxide film that becomes dissolved in the chloride solution. Identical EIS spectra were also reported by Titz et al. [37] upon the oxygen diffusion controlled corrosion of a mild steel covered with a porous organic film. Nevertheless, and similar to the Cr-containing layers, all Bode diagrams were characterised by two well defined relaxation processes suggesting that the MPS coatings were composed of two layers. According to Kumaraguru et al. [16], the MPS films could have an external layer manly composed of Si–O and an internal one containing Si–O–Si and Mo–P–O–Zn compounds. The latter was reported to be nonporous and protective against corrosion. Therefore, the external layer could be corroded already at 0.1 M NaCl but at higher concentrations the corrosion products from the first layer seemed to prevent corrosion of the second layer immediately over the galvanized substrate. In addition, it should
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Fig. 5. EIS (a) Nyquist and (b) Bode diagrams obtained in different concentrations of NaCl on the MPS coatings.
Table 4 EIS parameters obtained on the MPS coatings at different conditions in NaCl solutions. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a))
a
Rct (X cm2)
Cdl (lF cm2 s(1a ))
a0
0.1 M (pH 6.7) 0.5 M (pH 6.7) 1 M (pH 7) 0.1 M after 3 days 0.1 M after 10 days
5 18 08 15 19
229 159 162 230 101
5.81 4.11 5.77 17.49 18.81
0.77 0.78 0.84 0.71 0.78
60 100 50 120 39
68 38 90 45 150
0.86 0.82 0.82 0.80 0.80
be noted that the layer resistance (Rc) and the charge transfer resistance (Rct) were by far higher in the case of the MPS coatings than in the chromate layers even in the most concentrated corrosive solutions and therefore confirm a more protective behaviour. 3.3. Evolution of the electrochemical behaviour with immersion time in 0.1 M NaCl solutions Because of the weak degradation of the coating in NaCl 0.1 M, several MPS samples were immersed in 0.1 M for 10 days in order to study the evolution of the coating with time. After 3 days of immersion (Fig. 6 and Table 2), the slope of the cathodic branch remained practically unchanged. In contrast, the anodic slope decreased by one order of decade. After 10 days of immersion, an increase in the slope of the cathodic branch and a reduction of the anodic one by half a decade was also observed; i.e., the corrosion process evolved towards a mixed control. According to the Icorr values, it was concluded that partial degradation of the layer oc-
0
curred with formation of a layer of corrosion products that was observed even at the naked eye. To confirm the DC polarization results, EIS measurements were carried out under the same experimental conditions. The results are summarised in Fig. 6(b) and Table 4. The layer resistance (Rc) showed some stability at the beginning of immersion but was halved after 10 days of immersion, in agreement with the behaviour of the cathodic branch of the polarization curves. By considering the above described structure of the MPS coatings [16], it was assumed that the chloride ions initially penetrated and degraded the external layer but the process was then slowed down by the presence of the second non porous internal layer. 3.4. Evolution of the electrochemical behaviour with pH in 0.1 M NaCl solutions The influence of the solution pH on the electrochemical behaviour of the modified galvanised coating was also studied in the
Fig. 6. DC polarisation curves (a) and Nyquist diagrams (b) obtained on the MPS coatings at different immersion times in 0.1 M NaCl solutions.
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0.1 M NaCl solutions. The adjustment of the pH was carried out by addition of maximum purity nitric acid or potassium hydroxide. Fig. 7 and Tables 5 and 6 gather the values obtained on the Cr-containing coatings. Compared to the initial neutral solution the corrosion current density increased in both the acidic and alkaline pH ranges and therefore the chromate layers could not confer adequate corrosion protection. The anodic slopes remained practically unchanged suggesting that the dissolution process of the films was not modified. In contrast, the cathodic slopes increased significantly in the acidic and alkaline pH because of a higher activity in the oxygen reduction. Indeed, Zeller and Savinell [5] showed that the resistance of chromate coatings in Na2SO4 was higher in deaerated medium that in an aerated one. This confirms the hypothesis by which the corrosion process is under cathodic control in this case. Furthermore, the EIS diagrams obtained at different pH showed the existence of two loops at LF, i.e., the pH did not have an effect on the number of relaxation processes. However, the values of the layer resistance agreed with the corrosion current density values. On the one hand, according to the morphology of the coating [Fig. 1(a)] and to the electrochemical results (presence of two relaxation processes at LF, corrosion under cathodic control); and on the other, when referring to previous works [36,38] and the two corrosion models proposed by the literature [4], it was concluded that the chromate coating contained surface porosity. The progressive disappearance of the second LF loop previously observed at 0.5 and 1 M corresponded therefore to the fast filling of the pores by corrosion products leading to an almost perfect layer. Fig. 8(a) and Table 7 show the influence of pH on the corrosion behaviour of the MPS coating. The Icorr values showed little stability
in the acid medium but good corrosion resistance of the coating in the basic medium. This was supported by the shift of Ecorr towards more cathodic values and the higher value of the compactness of the corrosion layer in the alkaline conditions. In contrast, the Ecorr shifted towards more anodic values in the acid medium hence suggesting that degradation of the coating was enhanced. The EIS diagrams were also plotted at the free corrosion potential [Fig. 8(b) and Table 8] to provide further insight on the corrosion mechanisms. The resistance at high frequency (Rc) values confirmed that the coating resisted poorly against corrosion in the acidic medium. However, good corrosion resistance was shown in alkaline conditions. Moreover, the layer resistance calculated for neutral and alkaline pH was 6 times higher than that obtained at acidic pH. This confirmed the assumption that the alkaline medium may support the formation of Si–O–Si, Si–O–Zn and Mo–P–O–Zn complex in the internal layer [16]. The EIS diagrams were also characterised by two time constants for the different pHs (4, 6.7 and 11). Therefore, it was concluded that neither the pH, nor the concentration or the immersion time influenced the number of relaxation processes. For Ce-containing systems, Mora et al. [30] proposed an equivalent circuit including two relaxation processes with diffusion at LF. Other authors [39] proposed an open boundary finite length diffusion (OFLD) in the electrical equivalent circuit instead of the diffusion impedance to simulate the corrosion process of a coating modified by thin layer of silane. Taking into account the different EIS behaviours of the Cr-containing and of the Cr-free treatments, different equivalent circuits were therefore derived as depicted in Fig. 9(a) and (b), respectively. For the Cr-containing coatings, the equivalent circuit giving a similar response to the interface in NaCl
Fig. 7. (a) Polarisation curves and (b) EIS diagrams obtained on the chromate coatings in 0.1 M NaCl solutions at different pH.
Table 5 Electrochemical parameters obtained for the chromated layer as a function of pH in 0.1 M NaCl solutions. Test conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
pH 4 pH 6.7 pH 11
982 1030 975
47.65 65.35 52.88
101.35 515.3 135.00
204.40 55.17 170.30
0.760 0.205 0.633
102.3 56.09 96.0
0.500 1.016 0.564
Table 6 EIS parameters obtained on the chromated layer as function of pH in 0.1 M NaCl solutions. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a))
a
R1 (X cm2)
C1 (lF cm2)
Rct (X cm2)
Cdl (lF cm2 s(1a )
a0
pH 4 pH 6.7 pH 11
35 20 37
74.70 140 108.00
6.18 1.33 8.84
0.67 0.73 0.84
6.00 7.19 5.00
0.061 0.106 0.002
15.00 19 9.00
110 92 265
0.79 0.85 0.76
0
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Fig. 8. (a) Polarisation curves and (b) EIS diagrams obtained on the MPS coating in 0.1 M NaCl solutions at different pH.
Table 7 Electrochemical parameters obtained on the MPS coatings as a function of pH in 0.1 M NaCl solutions. Test conditions
Ecorr (mV/SCE)
ba (103 V/dec)
Rp (X cm2)
Icorr (lA/cm2)
Vcorr (mm/y)
Q (C)
C (S 106)
pH 4 pH 6.7 pH 11
958 980 1030
36.15 39.59 72.00
241.58 249.00 810.90
65.06 69.32 38.60
0.242 0.257 0.106
34.15 22.09 35.94
0.50 0.32 1.26
Table 8 EIS parameters obtained on the MPS coatings as a function of pH in 0.1 M NaCl solutions. Test conditions
Rs (X cm2)
Rc (X cm2)
CPEc (lF cm2 s(1a)
a
Rct (X cm2)
Cdl (lF cm2 s(1a ))
a0
pH 4 pH 6.7 pH 11
20 5 03
38 229 420
5.98 5.81 17.47
0.79 0.77 0.65
16 60 321
1.75 68 8.5
0.83 0.86 0.89
(a)
(b)
CPEc
CPEc Rs
Rs
0
CPE
Zw Rct
CPEdl
C1
Rc R1
Rc Rct
Fig. 9. Equivalent circuits schematizing the coating/NaCl solution interface of (a) the chromate and (b) the MPS layers.
at various concentrations can be expressed with Rs: solution resistance, CPEc: constant phase element characterised by Cc (non uniform layer capacity) and a as the non linearity coefficient. Rc is the layer resistance related to the heterogeneous structure of the chromate layer. C1 and R1: capacity and resistance of relaxation time related to the electrode electrochemical processes. Rct: charge transfer resistance, CPEdl: non linear double layer capacity and Zw: diffusion impedance. The diffusion impedance equalled zero when the NaCl concentration was 0.1 and 0.5 and when the NaCl concentration was increased to 1 M the second electrochemical process disappeared. However, according to the previous results, the equivalent circuit giving the same impedance response for the solution/MPS coating interface shall be interpreted using two time constants (Fig. 9b). The electrochemical parameters (corrosion current density and layer resistance) obtained in NaCl solution at different concentrations clearly showed the stability and the effectiveness of MPS passivation as depicted in Fig. 10. Therefore, these preliminary studies suggested that the MPS coating could be an alternative treatment to replace Cr(VI) passivation treatments.
Fig. 10. Evolution of the corrosion rate as function of the NaCl concentration of untreated and treated HDG steel.
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4. Conclusions In this study, the corrosion behaviour of a chromated layer and a molybdate–phosphate–silicate (MPS) coating on an industrial galvanized steel was evaluated in different concentrations of NaCl solutions, immersion times and pH. As opposed to the chromate coatings and despite their cracked morphology, the results showed that the MPS coatings exhibited adequate corrosion behaviour even in concentrated NaCl solutions, acidic or alkaline pH. They also showed appropriate corrosion resistance after relatively long immersion times. The electrochemical mechanisms of degradation seemed to show that the MPS coating was composed of two layers, an external porous one that brings about the development of a compact corrosion products layer and an internal one that arrests further corrosion of the underlying galvanized steel. Overall, a similar corrosion resistance than of the chromate layers was observed. Therefore, it can be concluded that the MPS coatings could represent an environmentally friendly alternative to the use of Cr(VI) treatments. References [1] A.M. Rocco, T.M.C. Nogueira, R.A. Simao, W.C. Lima, Surf. Coat. Technol. 179 (2004) 135–144. [2] A.C. Bastos, M.G.S. Ferreira, A.M. Simoes, Prog. Org. Coat. 52 (2005) 339–350. [3] M. Tencer, Appl. Surf. Sci. 252 (2006) 8229–8234. [4] A. Pirnat, L. Meszaros, G. Meszaros, B. Lengyel, Corros. Sci. 34 (1993) 1147–1155. [5] R.L. Zeller, R.F. Savinell, Corros. Sci. 26 (1986) 389–399. [6] S.F.L. Mertens, E. Temmerman, Corros. Sci. 43 (2001) 301–316. [7] Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on end-of life vehicles, Official Journal of the European Communities, 21 October 2000. [8] K. Aramaki, Corros. Sci. 45 (2003) 451–464. [9] K. Ogle, A. Tomandl, N. Meddahi, M. Wolpers, Corros. Sci. 46 (2004) 979–995. [10] O. Magalhaes, I.C.P. Maragarit, O.R. Mattos, J. Electroanal. Chem. 572 (2004) 433–440. [11] I.M. Zin, S.B. Lyon, V.I. Poktmurskii, Corros. Sci. 45 (2003) 777–788.
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