Comparative electrochemical studies of zinc chromate and zinc phosphate as corrosion inhibitors for zinc

Progress in Organic Coatings 52 (2005) 339–350

Comparative electrochemical studies of zinc chromate and zinc phosphate as corrosion inhibitors for zinc A.C. Bastosa , M.G.S. Ferreiraa,b , A.M. Sim˜oesa,∗ a

Chemical Engineering Department, Instituto Superior T´ecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Departamento de Engenharia de Cerˆ amica e do Vidro, Universidade de Aveiro, Aveiro, Portugal Received 28 May 2004; received in revised form 9 September 2004; accepted 9 September 2004

Abstract The anticorrosive performance of two inhibitive pigments, zinc chromate and zinc phosphate, was compared using electrochemical impedance spectroscopy (EIS) and the scanning vibrating electrode technique (SVET) in pigment extracts in 0.1 M NaCl. It was observed that zinc was protected from corrosion in both extracts. In tests using hot dip galvanised steel painted with an epoxy primer incorporating the pigments, the SVET detected the anodic and cathodic distribution along the scribes, although no significant differences were observed among the various primers. On the contrary, EIS was able to distinguish processes occurring on the metal surface exposed by the scribe in different samples. For primers with anticorrosive pigment, a time constant at high frequencies was attributed to a layer of protective nature, probably formed by metal ions from the substrate and inhibitive ions leached from the anticorrosive pigments. © 2004 Elsevier B.V. All rights reserved. Keywords: Anticorrosive pigments; Corrosion; EIS; SVET; Zinc; Phosphate; Chromate

1. Introduction Decades of industrial practice have lead to the selection of a few pigments with excellent anticorrosive properties in a wide range of situations. Chromates have long been the first choice for many applications, but they represent environmental and health threats that are leading to their replacement. Among the new non-toxic anticorrosive pigments that today line up as substitutes for traditional pigments, zinc phosphate is probably the most widely accepted, and it is incorporated in many paint formulations. Although not fully understood, the inhibitive action of chromate pigments is now well accepted to be based on the chromate ions that leach out to solution, where they act in the same way as other chromate soluble inhibitors. In solution, chromate ions are present in the hexavalent form and become reduced to the trivalent form to counterbalance the anodic oxidation of the metal substrate. It is the trivalent chromium ∗

Corresponding author. E-mail address: [email protected] (A.M. Sim˜oes).

0300-9440/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2004.09.009

that is usually found in the passive layer. On iron, films of oxide spinels with Cr3+ and Fe3+ have been reported [1–3]. The Cr/Fe atomic ratio seems to depend on the pH, oxygen and chromate content of the solution. Other authors describe the film composition as a mixture of oxides and hydroxides of iron and chromium [4–6]. Hydroxylated layers may be present [2] and adsorbed Cr(VI) has also been observed [7]. On aluminium alloys, the action of chromate has also been studied and an inhibiting effect on the oxygen reduction reaction at the copper precipitates was observed [8]. Studies of chromate passive layers on zinc are scarce but a recent study reports a passive film of Cr(III) with the absence of zinc [9]. Zinc yellow, being a mixed salt of zinc chromate, potassium chromate and zinc hydroxide, has, in addition to CrO4 2− , two more ionic species of inhibitive nature, Zn2+ and OH− . It has been referred that for the same concentration of chromate, zinc yellow has a high inhibitive power compared to other simple chromate salts [10]. OH− leads to a rise in pH, decreasing the corrosivity, particularly at the anodic sites, whereas Zn2+ precipitates as Zn(OH)2 at the cathodic sites. The protective layers formed on the metal prevent either the

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release of metal ions from the surface towards the solution or the oxygen or proton reduction, by, for instance, impeding the flow of electrons needed for such reactions. A hydrophobic character has also been proposed for the Cr(III) oxide layers, as well as a lower zeta potential of oxide layers when chromate is adsorbed, hindering the adsorption of aggressive anions such as chloride [11]. Phosphate has been used for decades as soluble inhibitor and also in conversion coatings. The use of zinc phosphate as pigment was proposed for the first time in 1965 [12]. Several modes of action have been proposed, including the formation of soaps of zinc [13], in addition to an improvement of the binder barrier properties [14]. Studies with soluble phosphates report phosphate layers at both the anodic [15] and cathodic regions [16]. Tertiary phosphates precipitate with iron and also with zinc, forming a protective layer that hinders the access of oxygen or the oxidation of the base metal. A neutralizing ability, capable of inhibiting corrosion by controlling the pH, was also suggested [10]. Real exposure of paints with zinc phosphate has led to very good results, contrasting with accelerated tests where the results were quite poor, possibly due to the slow kinetics of the reactions or to the low solubility of the pigment [17]. The pH of the solution seems to play a major role, since the solubility of zinc phosphate is higher in acidic media [18]. In this work, two anticorrosive pigments were studied: the classical basic zinc potassium chromate (zinc yellow) and the new alternative, zinc phosphate. The pigments were tested either incorporated in an epoxy primer or by preparing aqueous extracts. A two-component epoxy primer was selected and three variations were studied: a commercial zinc phosphate primer, a modification of the former by replacing the amount of phosphate by chromate, and a clear coat, based on the primer formulation but without any pigments or extenders. Comparative electrochemical tests were performed using electrochemical impedance spectroscopy (EIS) and the scanning vibrating electrode technique (SVET). EIS is a wellestablished technique in corrosion research; it can work with either coated or uncoated samples and its capability to differentiate the processes occurring in the electrochemical system makes it suitable for this kind of study. The SVET measures potential differences in solution, created by uneven ionic distributions. By a prior calibration, the measured potential values are converted to currents. The SVET technique was introduced to the field of corrosion in the early 1970s [19] and some work has been published since then [20–25].

mounted (Fig. 1a), whereas for the SVET measurements, zinc samples were epoxy mounted, leaving exposed a rectangle of ∼1 mm2 (Fig. 1b). The specimens were polished with SiC grit papers of grades 220, 500, 800 and 1000, washed in deionized water (Millipore), degreased with ethanol and dried with compressed air. 2.2. Coated samples Three primers were used: a two-component epoxy primer with zinc phosphate (commercial product), an epoxy primer with addition of chromate (modification of the commercial primer by replacing the phosphate pigment by zinc yellow) and the corresponding epoxy clear coat. The primers were applied to hot-dip galvanised steel (HDG) by dipping the substrates into the paint for ∼1 s and leaving them to dry. The tests were performed 1 month after application of the coating. Dry film thickness measured using an Elcometer 355 digital thickness gauge was ∼40 ␮m for EIS measurements and ∼20 ␮m for SVET measurements. The smaller thickness in the samples for the SVET study was chosen in order to allow a closer approach of the tip to the metal surface at the scribe. For the SVET measurements, squares of 2 cm × 2 cm were cut, scribed and glued to an epoxy sample holder and the edges and sides isolated with epoxy adhesive, as shown in Fig. 1c. For EIS, a scribe of 1 cm long was made using a scalpel, across the coating and down to the substrate; a plastic cylinder was then glued to the surface, exposing an area of 3.80 cm2 , as shown in Fig. 1d. 2.3. Exposure media Pigment extracts were made based on existing literature [26], by magnetic stirring 1 g of pigment in 500 mL of 0.1 M NaCl for 24 h, followed by filtering twice. The pigments were the same as used in the paints, i.e., zinc chromate known as zinc yellow (K2 CrO4 ·3ZnCrO4 ·Zn(OH)2 ·2H2 O) and zinc phosphate (Zn3 (PO4 )2 ·2H2 O). Table 1 shows some details of the solutions used. 2.4. Electrochemical techniques SVET measurements were made using Applicable Electronics Inc., equipment. Tape glued around the sample holdTable 1 Characterization of the exposure media (T = 20.0 ◦ C)

2. Experimental 2.1. Bare metal samples Zinc 99.95% pure, in the shape of 1-mm thick foil (produced by Goodfellow Ltd., UK) was used. For EIS, 1 cm × 1 cm samples were electrically connected via a multithread copper wire with silver conductive adhesive and epoxy

0.1 M NaCl Chromate extract Phosphate extract a b c d

pHa

Conductivityb (mS cm−1 )

Concentration

6.20 6.55 6.43

9.44 10.90 9.46

– |Cr|total = 1.1 × 10−2 Mc |PO4 3− | = 4.8 × 10−5 Md

Metrohm 632 pH meter, electrode 6.0220.100. Crison GLP 31 conductimeter. Inductively coupled plasma. Molecular absorption spectrometry.

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Fig. 1. Samples used in the experiments. (a) Mounted bare zinc for EIS measurements in extracts, (b) mounting for SVET measurements of bare zinc in extracts, (c) coated sample glued with epoxy for SVET measurements (d) cell for EIS measurements on scribed coated samples.

ers worked as solution reservoir. The microelectrode had a platinum tip of 40 ␮m and was made to vibrate at an average distance of 200 ␮m above the surface, with 40 ␮m of amplitude. Each scan comprised 20 × 20 points and an optical image was acquired before each scan. Although the SVET equipment used can detect ionic currents in two directions, parallel and normal to the surface, only results from the flux normal to the surface will be presented. In general, for an active metal immersed in an electrolytic solution, the diffusion of metal cations from the metal oxidation in the anodic regions is detected as a positive current, whereas negative currents correspond to OH− emerging from the cathodic regions as a product from oxygen reduction. EIS measurements were made using Gamry instrumentation. The three-electrode cell consisted of the sample as working electrode, a platinum counter electrode and a Saturated Calomel Electrode (SCE) as reference. Measurements were made inside a Faraday cage, at room temperature and the solution was quiescent and exposed to air. Usually, a 50 kHz to 5 mHz frequency range was swept with a sinusoidal potential signal with an amplitude 10 mV rms, superimposed to the open circuit potential. Fitting was made using ZView 2.70 software and all the spectra were corrected to the geometrical area of the working electrode.

Measurements of the open circuit potential (OCP) were made with an AUTOLAB PGstat 20 apparatus and are plotted against the potential of the SCE reference.

3. Results 3.1. Bare metal electrodes SVET current mapping made on pure zinc immersed in sodium chloride solution shows significant differences. In the salt solution without pigment, activity occurred always with the development of one single anode and one single cathode (Fig. 2a). Each of these electrodes occupied nearly half of the surface in the first minutes of immersion. With time, the anodic area became more localized (Fig. 2b) until, after approximately 1 day, a pit was observed on the surface (Fig. 2c and d). When the NaCl solution contained the pigment extracts the current inhibition was obvious. The current values measured on the surface were significantly reduced and there was only weak indication of cathodic or anodic activity, with the pattern of the surface corresponding to scattered microscopic and weak activity, corresponding to currents below

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Fig. 2. SVET maps of the ionic currents measured 200 ␮m above pure zinc exposed to: 0.1 M NaCl (a–c); zinc phosphate extract in 0.1 M NaCl (e–g); zinc chromate extract in 0.1 M NaCl (i–k). Times of immersion are (top to bottom): ∼5 min, 1 h and 1 day. Scanned area: 1.25 mm × 1 mm; Scale units: ␮A cm−2 . Also shown are pictures of the surface after 1 day of immersion in each solution (d, h, l).

6 ␮A cm−2 with the phosphate (Fig. 2e–g) and 2 ␮A cm−2 with the chromate (Fig. 2i–k). Inhibition by the phosphate required a longer time before its full action was felt, i.e., before the phosphate film was formed, since some anodic activity was measured in the first few minutes of immersion. For longer exposure times the activity remained negligible with both extracts (Fig. 2g and k), as confirmed by the inspection of the surface at the end of the test (Fig. 2h and l). The open circuit potential of pure zinc in 0.1 M NaCl was practically constant and near the equilibrium potential of zinc (Fig. 3). With the phosphate, the potential was only slightly higher, although at the beginning it started from a more anodic value and then rose during a period of approximately 3 h, which probably corresponds to the growth of the protective film. In contrast, the chromate pigment has led to a significant anodic polarization, reaching potentials

Fig. 3. Open circuit potential of pure zinc in 0.1 M NaCl, either uninhibited or containing phosphate extract or chromate extract.

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Fig. 4. Impedance spectra obtained for pure zinc at different times of exposure to 0.1 M NaCl, without inhibitor (a), or containing phosphate extract (b) or chromate extract (c); (a1), (b1) and (c1) are Bode plots, whereas (a2), (b2) and (c2) are the corresponding Nyquist plots.

of −0.945 V. This value is within the region of stability of chromium oxides/hydroxides, which are known to be the major components of the passivating film. The fluctuations observed in the phosphate curve, in opposition to the stable curve for the chromate, indicate differences in reactivity between the two protective films, and agree with the current density measurements with the SVET. After a stationary state is achieved, the order of the corrosion potential values agrees with the SVET observations and follows the expected order of relative protective efficiency of the pigments (chromate > phosphate > reference system). The impedance response of pure zinc exposed to the three solutions is presented in Fig. 4. In the absence of inhibitor, the spectrum has two relaxation constants, one well developed at high frequencies and a small one at lower frequencies. The low-frequency process was clearly resolved in the first minutes of immersion but disappeared after 3 h of exposure, which means that it may be associated with the presence of an air-formed film of oxides that becomes dissolved in the chloride solution. Meanwhile, the high frequency pro-

cess became shifted to lower frequencies, which corresponds to a rise of the double layer capacitance as corrosion progresses (possibly due to increasing active area). The spectra obtained for pure zinc in NaCl solution is identical to the spectrum modelled by Titz et al. [27] for a metal covered with a porous oxide layer, in which corrosion proceeds under oxygen diffusion control. In the presence of the extracts the spectra are simpler, with only one maximum in the phase angle plot, although the slope in the impedance modulus at the low frequencies suggests the presence of a second time constant. The capacitance is smaller than described above, revealing the inhibiting effect of the pigments, and does not change with time. The low frequency resistance, which was ∼1 k cm2 in the NaCl solution, increased in the presence of the extracts, confirming a slower corrosion rate. The Nyquist plots, included in Fig. 4 for a better visualization of the results, also show the difference in behaviour among the three solutions; in the absence of inhibitor the spectrum has two semi-circles, the first of which is well-defined and reveals the corrosion process, whereas in the presence of the extracts the

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Fig. 5. General equivalent circuit and fittings for pure zinc in 0.1 M NaCl without inhibitor (a) and with zinc chromate extract (b). Fitting parameters for (a) Rs = 34.4  cm2 ; Y0 (Q1 ) = 1.62 × 10−5 F cm−2 sn−1 ; n(Q1 ) = 0.872; R1 = 1.31 × 103  cm2 ; Y0 (Q2 ) = 6.06 × 10−4 F cm−2 sn−1 ; n(Q2 ) = 1; R2 = 631  cm2 ; χ2 = 3.9 × 10−4 . Fitting parameters for (b) Rs = 26.2  cm2 ; Y0 (Q1 ) = 3.31 × 10−6 F cm−2 sn−1 ; n(Q1 ) = 0.903; R1 = 1.74 × 105  cm2 ; Y0 (Q2 ) = 6.51 × 10−6 F cm−2 sn−1 ; n(Q2 ) = 0.500; R2 = 1.14 × 106  cm2 ; χ2 = 1.0 × 10−3 .

semi-circle is always incomplete and reaches much higher values in the scale. For the equivalent circuit model fitting of the spectra, as shown in Fig. 5, the capacitance was replaced by a constant phase element (CPE) with an impedance given by Z = 1/Y0 (jω)n . Provided the exponent n is close to 1, the values of Y0 can be taken as a measure of the capacitance. Although the spectra in Fig. 5 are apparently quite different, the fitting was made using the same equivalent circuit, generically having a high frequency time constant, described by R1 and Q1 and a low-frequency time constant (R2 , Q2 ). The values of these electrical components, as well as their physical meaning, change however between the two systems. In NaCl, the high-frequency time constant corresponds to the charge transfer resistance and double layer capacitance associated with the corrosion process, whereas the low-frequency process, with a capacitance of approximately 600 ␮F cm−2 , can be attributed to the mass transport across the porous air-formed film of oxides. As described above, this low-frequency process ceased to be detected a few hours after immersion. The spectrum in Fig. 5(b), which is representative of both the situations of chromate and of phosphate extracts, can be described by the same circuit, although only R1 and Q1 are clearly resolved; a closer observation shows in fact the presence of a secondary slope in the impedance modulus plot, below 10−1 Hz.

The time evolution of the fitted parameters corresponding to the three systems is presented in Fig. 6. The charge transfer resistance, R1 , was in the range of 103  cm2 in 0.1 M NaCl, 104  cm2 in the phosphate extract and 105  cm2 in chromate. In the NaCl solution, the resistance decreased after the first hours, possibly due to the activation of the zinc surface as the air-formed film became dissolved. With the pigments R1 was higher from the first measurements, meaning that at least a monolayer of film was formed in the first instants of immersion; the slight growth of R1 with time reveals no change of the behaviour in the film, but rather the build-up of the film on the surface. The value of Y0 increased for zinc in 0.1 M NaCl to nearly 1 mF, a value that suggests some influence of mass transfer across the gel-like layer of corrosion products. The values of Y0 were almost constant both in phosphate and in the chromate extracts, with values below that of a typical double layer capacitance, and with a slight tendency to decrease with time. The high values of the exponent n reveal that the CPE corresponds to a nearly capacitive response, as expected. In physical terms, Y0 can be either associated with the capacitance of a compact dielectric oxide film or it may correspond to the double layer capacitance over the fraction of area left uncovered by the passivating film, as discussed by Zeller and Savinell [28]. According to this last interpretation, a fraction of coverage of ∼99% can be estimated from the capacitance

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Fig. 6. Fit parameters for the impedance spectra obtained for pure zinc at different times of exposure to 0.1 M NaCl only, to phosphate extract in 0.1 M NaCl and to chromate extract in 0.1 M NaCl.

values for any of the two pigments, after 25 h of immersion. Estimation of the inhibition efficiency by the charge transfer resistance, R1 , gives an inhibition of ∼90% with the phosphate and 99% for the chromate. 3.2. XPS inspection XPS analysis of pure zinc after immersion in the phosphate and chromate extracts revealed well-defined peaks of phosphorous and chromium, respectively. Cr was present as a mixture of Cr(III) and Cr(VI). Zinc was detected by a single Zn 2p3/2 peak, at 1022.7 ± 0.3 eV, corresponding to ZnO or Zn(OH)2 . The O 1s peak was measured at 532 eV and is probably due to OH− , which means that both the zinc and the chromium are present mainly as hydroxides. In the samples exposed to the phosphate extract, a P 2p3/2 peak appeared at ∼140.2 eV. 3.3. Scribed samples EIS measurements were performed on samples of Hot Dip Galvanized Steel (HDG), with the various paint formulations and with a scribe reaching the zinc layer. After 1 h of immersion, the response was strikingly similar for all the coatings (Fig. 7), and also identical to the spectra obtained for the bare metal exposed to plain NaCl solution (Fig. 4a). The main difference is related to the higher impedance values, explained by the smaller ratio of the area exposed to the total sample area, roughly estimated as 1:50–1:100. The properties of the primer film, typically observed at the high frequencies, were not detected, probably because of the high impedance of the film; being in parallel with the scribe and having a much higher impedance, the coating does not allow any significant flow of current, and therefore it does not contribute to the impedance spectrum in the range of frequencies measured. After 24 h of free corrosion, the impedance was lowest in the sample without pigment, which reveals the higher activity of this system, and another time constant appeared at the high

frequencies. Fitting of the spectra was made using the equivalent circuit in Fig. 8, where the high frequency impedance (1 k cm2 ) is due to the electrolytic solution at the scribe. RHF and QHF describe the high frequency process, and Rct and Qdl correspond to the charge transfer resistance and the double layer capacitance, respectively. Finally, a component describing the mass transfer impedance was considered in some cases in order to account for a small tail appearing below 0.1 Hz. The high frequency process had a resistance of 5–10 k cm2 , as shown in Fig. 9, whereas the Y0 for this process has a value of roughly 10−8 to 10−5 F cm−2 . This capacitance is too high for a polymer film, which generally has values in the range of 10−10 to 10−8 F cm−2 [27,29]. Further, in the presence of chromate Y0 decreases with time, a behaviour that is the opposite to the known evolution of coating capacitance values (which tend to increase due to water uptake). This high frequency time constant can however be explained by the formation of the passivating film, of either chromium hydroxide or zinc phosphate. The formation of this film starts in the first minutes of exposure, possibly with the precipitation of a monolayer, and grows afterwards, during the first 24–48 h. The low frequency process gives Rct = 20–80 k cm2 and is maximum for the chromate. The double layer CPE grows with time, revealing the growth of the active area underneath the coating, i.e., delamination. The ratio of Y0 with and without pigment is roughly 0.1, meaning that the loss of adhesion was weaker in samples with pigment. The delamination rate, estimated from the CPE, was minimum in the epoxychromate coating, followed by the epoxyphosphate and finally by the clear coat. This was the same rate that was determined by visual inspection of the samples at the end of the test. The SVET was also used to study the ionic current distribution above the scribed samples, for the three coatings in 0.1 M NaCl. The evolution of the current maps and the current density values have not show any significant differences among the specimens with the various coatings. As an example, Fig. 10 shows SVET measurements of the scribed HDG sample, coated with the clear epoxy paint. In the first

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Fig. 7. Bode and Nyquist impedance plots of scribed samples of HDG, coated with epoxy primer with pigment, after 1 h (a) and 1 day (b) of exposure in 0.1 M NaCl. (a1) and (b1): Bode plots; (a2) and (b2): corresponding Nyquist plots.

2–3 days of immersion there was a clear separation between the cathode and the anode along the scribe (Fig. 10a) and there were no signs of delamination in the micro-video image. For longer exposure times, however, the anodic activity along the scribe increased and the signs of cathodic activity disappeared from the map, as delamination progressed away from the scribe (Fig. 10b).

4. Discussion The inhibiting power of anti-corrosive pigments depends upon a number of parameters, namely the pigment solubility,

the pH, the properties of the substrate and the composition of the solution. The levels of inhibitor concentration achieved in the extracts were much lower for the phosphate than for the chromate, although in both cases the inhibiting effect has proved to be sufficient to provide high inhibition efficiency to zinc in 0.1 M NaCl. In both cases, inhibition was the result of the formation of a layer of inhibitor on the surface. In the chromate, the film was formed instantly after immersion. Inhibition by the phosphate was a slower process, as confirmed by the SVET and the OCP measurements. The chromate was also the more efficient inhibitor. On the bare metal, it led to the highest charge transfer resistance and also to the development of a protective layer characterized by a capacitance

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Fig. 8. Equivalent circuit and fitting of impedance spectra for the scribed samples of HDG in 0.1 M NaCl with chromate primer (a) and with phosphate primer (b). Fitting parameters for (a) Rscribe = 775  cm2 ; Y0 (QHF ) = 3.05 × 10−8 F cm−2 sn−1 ; n(QHF ) = 0.844; RHF = 9.04 × 103  cm2 ; Y0 (Qdl ) = 6.76 × 10−7 F cm−2 sn−1 ; n(Qdl ) = 0.630; Rct = 7.33 × 104  cm2 ; χ2 = 5.2 × 10−4 . Fitting parameters for (b) Rscribe = 990  cm2 ; Y0 (QHF ) = 1.20 × 10−5 F cm−2 sn−1 ; n(QHF ) = 0.551; RHF = 5.47 × 103  cm2 ; Y0 (Qdl ) = 6.79 × 10−6 F cm−2 sn−1 ; n(Qdl ) = 0.768; Rct = 2.73 × 105  cm2 ; χ2 = 2.2 × 10−4 .

decrease of approximately two orders of magnitude, when compared with the reference solution. When these pigments are used in a coating, leaching to the solution is the first step towards inhibition. Since this is a slow process, inhibition can only be detected after a sufficient concentration of inhibitor is achieved near the metal. The mechanisms of passivation by zinc chromate are usually described by an oxidizing action of CrO4 2− . In the process of film formation, Cr(VI) is reduced to Cr(III), while Zn is oxidized to Zn(OH)2 , according to the reaction scheme 3Zn + 2CrO4 2− + 5H2 O → 3Zn(OH)2 + Cr2 O3 + 4OH− Chromate is usually considered to act especially at the damaged areas of the coating, where the electropositive metals are left exposed to the aggressive medium, and are therefore available for providing free electrons for chromate reduction. For the conditions of our experiments and for the duration of the tests, we found none of the pigments had the power to inhibit the corrosion occurring at the defect. An interesting feature was related with the development of a time constant at the high frequencies. Unlike the typical spectra observed with defective painted samples, the high frequency process could not be attributed to the dielectric properties of the coating, due to its high capacitance. On the other hand, it cannot be simply due to the precipitation of corrosion products, or it would have to be even more relevant in

the clear coat. This process has then to be due to a passivating layer. The SVET, however, has shown that the anodic activity occurred at the scribe irrespective of the pigment. This passivating layer, therefore, can only be underneath the coating, where it inhibits the cathodic reaction. Chromate pigments are usually classified as oxidizing inhibitors, because of the strong thermodynamic tendency of Cr(VI) to become reduced to Cr(III). It is claimed to inhibit steel corrosion at concentrations above 10−4 M [30]. According to our results, the precipitation of this layer seems to occur preferentially underneath the coating, as water permeates across the coating and leaches some pigment, and then at the scribe, after diffusion of the dissolved pigment. At the pH values expected underneath the coating, chromate acts as mixed anodic–cathodic inhibitor, with an efficiency that is only slightly smaller than at neutral pH [26]. On the other hand, at this pH the thickening of Zn oxide layers is easier. Therefore, the film probably consists of a mixture of chromium and zinc oxides and hydroxides. Both zinc and chromium oxides are know to behave as semiconductors, for which the capacitance is inversely proportional to the thickness: C = εε0

A L

where ε is the relative permitivity, ε0 the permitivity of vacuum, A the area and L is the film thickness. Provided the dielectric properties of the layer remain constant, the drop

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Fig. 9. Fit parameters of the impedance spectra of scribed painted HDG at different times of exposure to 0.1 M NaCl.

in capacitance means that the thickening of the film is more important than its growth along the surface, i.e., as if a monolayer of Cr(OH)3 was formed as soon as the water permeated across the thin polymer coating, through leaching of a small amount of chromate. After that, the capacitance decreased as the result of film thickening. The low frequency semicircle in the impedance spectrum, which is actually related to the corrosion process, gives the actual rate of delamination, and the ranking is the same as observed with the pigments extracts, i.e., chromate > phosphate > clear coat. 4.1. SVET SVET mapping at the scribes showed one large anode and one large cathode at the scribe in the first days of immersion. The fact that both the anode and the cathode are located at the scribe shows that the scribe itself constitutes a complete corrosion cell, without the need for further electrochemical reactions to balance the electrons. As already observed by Bierwagen and co-workers [23], for longer immersion times the cathodic currents cease to be detected by the SVET. This reveals that the cathodic reaction takes place away from the scribe, i.e., underneath the coating, and is thus a distinct sign

of the delamination process. Since the cathodic and anodic reactions have to balance each other, then a large ratio between the cathodic and the anodic areas will lead to a small density of the cathodic current. The cathodic current is essentially due to the reduction reaction of oxygen: O2 + 2H2 O + 4e− → 4OH− and occurs at the metal–coating interface, thus leading to an increase of pH (responsible for the rupture of coating metal bonds and/or to saponification of the coating) but also to an accumulation of anions in a confined region. Compensation of this excess of anions can be achieved either by the outward flow of the hydroxyl anions themselves or, more easily by the inward flow of cations. As pointed out by Leidheiser and Wang [31], the high concentration of Na+ in the outer solution makes them the most likely cations to migrate to the front of the delamination. Migration of sodium cations to the delamination front was confirmed by Stratmann, who found an excess of sodium in the delaminated area but also in a thin area considered to be beyond the delamination front [32]. This migration of cations can occur in two ways: either across the coating or from the scribe. This last possibility, however, would mean that other cations would migrate from the bulk into the scribe, generating a significant ionic

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Fig. 10. SVET maps of ionic currents measured 200 ␮m above a scribe in HDG coated with clear epoxy paint, after (a) 2 days and (b) 7 days of exposure to 0.1 M NaCl. Also presented are micro-video images of the corresponding sample area.

flow above the scribe, equivalent to that of Zn2+ ions flowing away from the surface. Such a flow should be detected by the SVET, in the same way as anodic currents are detected. In contrast, for a flow of ions across the coating, the cross area for the flow is much larger, leading to very small potential gradients above the coating, which are necessarily difficult to detect.

5. Conclusions Protection of zinc by pigments of zinc chromate and zinc phosphate was monitored by EIS, SVET and OCP, with good agreement between the three techniques. Evaluation of the electrochemical parameters for the bare zinc in pigment extracts allowed the ranking of the decreasing corrosion resistance in the extracts in 0.1 M NaCl as: chromate > phosphate > plain salt solution. Interpretation of the impedance spectra of the scribed coatings has shown that the equivalent circuit is identical to that of the bare metal in a first stage, whereas the processes occurring underneath the coating start to appear in a second stage. A high frequency relaxation constant observed in the scribed, pigmented coatings, was assigned to the formation of a protective layer of pigment underneath the coating. The decreasing rate of delamination, taken as the rate of growth of the double layer capacitance, was the same, i.e., chromate > phosphate > clear coat.

Mapping of the local ionic fluxes above the metal surface has shown that zinc corrosion in NaCl occurs in a localized way, whereas the presence of any of the two pigment extracts inhibits the establishment of this local activity.

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