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Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc
Electrochimica Acta 52 (2007) 4449–4456
Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc KeunWoo Cho, V. Shankar Rao, HyukSang Kwon ∗,1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 GuseongDong, YuSeongGu, Daejeon 305-701, Republic of Korea Received 13 September 2006; received in revised form 5 December 2006; accepted 12 December 2006 Available online 11 January 2007
Abstract A trivalent chromium based conversion coating (CCC), based on chromium nitrate solution with Co(II) ions, was developed on Zn substrate. The corrosion resistance of the trivalent CCC, measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution using anodic polarization and electrochemical impedance spectroscopy (EIS), was very sensitive to both immersion time and bath pH. Micro-cracks were found on the surface of the CCC. Besides, the density of micro-crack and the coating thickness also depended on immersion time and bath pH. With increasing the coating thickness its pitting potential increased and passive current density decreased. The trivalent CCC formed on Zn for 40 s in pH 1.7 bath showed the best corrosion resistance, and the pitting potential increased significantly from −355 mVSCE for Zn to 975 mVSCE for the trivalent CCC on Zn. To explain the corrosion behavior of the trivalent CCC using EIS analysis, a modified equivalent circuit, which considered the micro-cracks in the coating and chromium corrosion product (CCP) deposited in the micro-cracks, was designed and the variation of each electrical parameter was examined. Especially, its corrosion behavior was well described by the variation of the resistance of CCP (Rccp ). © 2007 Elsevier Ltd. All rights reserved. Keywords: Trivalent chromium based conversion coating; Microstructure; Corrosion resistance; EIS analysis; Equivalent circuit
1. Introduction Electrodeposited Zn coating has been used to protect ferrous substrates in automobile industries. However, the corrosion rate of the Zn coating increases rapidly in a humid air, therefore, various surface treatments have been developed to improve the corrosion resistance of the Zn coating: hexavalent chromium based conversion coating (CCC), generally called chromate conversion coating, phosphate conversion coating and organic painting, etc. [1–3]. Among them, CCC is widely used because of its superior corrosion resistance, good paint adhesion and low cost [4–6]. CCC is performed by immersing Zn coated articles in a chromic acid bath, which forms a passive layer containing Zn oxides and mixed Cr(III)–Cr(VI) oxides [7,8]. Because the Cr(VI) species present in the chromic acid bath and the coating are toxic and carcinogenic, nontoxic trivalent CCC has been
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Corresponding author. Tel.: +82 42 869 3326; fax: +82 42 869 3310. E-mail address: [email protected] (H. Kwon). ISE member.
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studied as an alternative to hexavalent CCC [8–12]. The corrosion resistance of the trivalent CCC is generally lower than that of the hexavalent CCC [8–10]. Thus, many recent studies have focused on the improvement in the corrosion resistance of the trivalent CCC by sealing treatments on the CCC and/or by additions of different additives in a Cr(III) bath. Bellezze et al. [8] reported the corrosion resistance of the trivalent CCC increased dramatically through the sealing treatment in Si based solution and was equivalent to that of the hexavalent CCC. According to Fonte et al. [11], the trivalent CCC formed in a Cr(III) bath containing transition metal ions such as Co(II), Ni(II) and Fe(II) showed superior corrosion resistance than that formed in the bath without transition metal ions. It is well known that the microstructure and the corrosion resistance of the hexavalent CCC largely depend on immersion time and bath pH as well as bath composition [13,14]. However, the influences of immersion time and bath pH on those of the trivalent CCC formed on Zn substrate are rarely studied. Furthermore, correlation between the microstructure and the corrosion resistance of the trivalent CCC has not been reported yet. We have developed a trivalent CCC bath based on chromium nitrate (Cr(NO3 )3 ) solution with Co(II) ions. In this paper, we
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examined the effects of immersion time and bath pH on the microstructure and corrosion resistance of the trivalent CCC. The corrosion behavior of the trivalent CCC was also investigated to explain the mechanism for the improvement in its corrosion resistance. 2. Experimental A commercially pure Zn plate was used as a substrate for the trivalent CCC. The surface of the Zn plate was polished with 2000 grit silicon carbide paper, degreased with acetone, ultrasonically cleaned in ethanol for 3 min, rinsed in distilled water, activated for 2 s in 3 wt.% nitric acid and rinsed in distilled water again prior to the trivalent CCC. The composition of the bath for the trivalent CCC was 50 mL/L Cr(NO3 )3 (40%), 20 g/L CoCl2 ·6H2 O and 3 mL/L H2 SO4 . Bath pH was adjusted to three different values (pH 1.1, 1.7 and 2.3) by adding NaOH to the bath. The trivalent CCCs were formed on Zn in the bath for 20–60 s at 30 ◦ C. Then these coatings were rinsed in distilled water, and dried in an oven for 4 min at 40 ◦ C, and in a desiccator for 48 h at an ambient temperature (22 ± 2 ◦ C). The surface morphology of the trivalent CCC was observed by scanning electron microscope (SEM). The coating thickness was measured using cross-sectional SEM image of the trivalent CCC and also by Rurherford backscattering spectrometry (RBS). X-ray photoelectron spectroscopy (XPS) was used to analyze the valence state of Cr in the trivalent CCC. The spectrum was obtained with VG Scientific, model ESCALAB 200R, using Mg K␣ X-ray source (hν = 15 kV, 20 mA) at 300 W. The base pressure in the spectrometer chamber was maintained at 5 × 10−10 Torr throughout the measurement. The binding energy scale of the spectrophotometer was calibrated using C 1s (284.8 eV) substrate. The specimen was analyzed by means of Ar-ion sputtering with primary beam energy of 5 keV, 0.30 A. The corrosion resistance of the trivalent CCC was evaluated by anodic polarization test and electrochemical impedance spectroscopy (EIS) in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at an ambient temperature (22 ± 2 ◦ C). Anodic polarization test was performed at a scan rate of 0.5 mV/s using a conventional three-electrode polarization cell: specimen as a working electrode, saturated calomel electrode (SCE) as a reference electrode and a platinum counter electrode. EIS was conducted by frequency sweep from 105 to 10−2 Hz using a superimposed sinusoidal signal of 5 mV rms at the open circuit potential. All the potentials in this work are referred to the SCE. 3. Results and discussion 3.1. Effects of immersion time on the microstructure and corrosion resistance of the trivalent CCC CCC is generally performed by immersing Zn coated articles in an acidic Cr bath within 60 s because the thickness of Zn coating decreases due to dissolution of Zn with increasing immersion time. Thus, it was examined the microstructures and
Fig. 1. SEM micrographs of the trivalent CCCs formed on Zn for (a) 20 s, (b) 40 s and (c) 60 s, in pH 1.7 bath at 30 ◦ C.
corrosion resistances of the trivalent CCCs formed on Zn in the Cr(III) bath from 20 to 60 s. Fig. 1 shows the surface morphologies of the trivalent CCCs formed on Zn with immersion time of 20, 40 and 60 s in the pH 1.7 bath. After 20 s immersion, trivalent CCC with microcracks was formed on Zn, which was similar to the surface morphology of hexavalent CCC [6,10,15]. According to previous reports [6,15,16], the hexavalent CCC deposited on Zn with gel-like structure in an acidic Cr bath, during drying process the CCC would shrink due to an internal tensile stress in the coating, and finally micro-cracks were formed on the surface of the coating. Moreover, with the coating thickness the tensile stress increased, resulting in an increase in the density of micro-
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Fig. 2. Anodic polarization curves of Zn with and without the trivalent CCC in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCCs were formed on Zn substrate for 20, 40 and 60 s in pH 1.7 bath at 30 ◦ C.
crack. With increasing immersion time to 40 s the Zn surface was uniformly covered with the trivalent CCC, and the density of micro-crack on the surface increased. However, the density of micro-crack decreased significantly with further increasing immersion time to 60 s. The coating thickness also decreased from 1 to 0.35 m with increasing immersion time from 40 to 60 s. Gigandet et al. [17] suggested that both the growth of CCC and the dissolution of the deposited CCC progressed at the same time during immersion of Zn in an acidic Cr bath. Thus, it is considered that the thickness of trivalent CCC increases until 40 s immersion because the growth rate of the trivalent CCC is faster than its dissolution rate. However, with further increasing immersion time its growth rate becomes less than dissolution rate, and finally the coating thickness decreases. The anodic polarization curves for the Zn substrate and the trivalent CCCs formed on Zn in the pH 1.7 bath for 20, 40 and 60 s, obtained in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution, are presented in Fig. 2. After trivalent CCC, the pitting corrosion resistance of Zn greatly improved as confirmed by an increase in the pitting potential from −355 mVSCE for Zn to 975 mVSCE for the trivalent CCC on Zn for 40 s. Moreover, the pitting potential increased with increasing immersion time to 40 s, but decreased with immersion time above 40 s. The passive current density decreased gradually with increasing immersion time to 40 s, but increased with further increasing immersion time. Thus, the pitting potential and passive current density of the trivalent CCC largely depended on its thickness. With the coating thickness, the pitting potential of the trivalent CCC increased, and the passive current density decreased. The best corrosion resistance of the trivalent CCC with maximum coating thickness was obtained for immersion time of 40 s. 3.2. Effects of bath pH on the microstructure and corrosion resistance of the trivalent CCC Fig. 3 shows the surface morphologies of the trivalent CCCs formed on Zn for 40 s in the bath of pH 1.1, 1.7 and 2.3. For
Fig. 3. SEM micrographs of the trivalent CCCs on Zn for 40 s at 30 ◦ C in (a) pH 1.1, (b) pH 1.7 and (c) pH 2.3 bath.
the coating formed in the pH 1.1 bath surface was very rough with high defect density, whereas for the coating formed in the pH 1.7 bath micro-cracks were found on the surface and the its surface roughness decreased compared to the CCC formed in the pH 1.1 bath. The surface of the CCC formed in the pH 2.3 bath was very smooth and no micro-crack was observed on the surface. These results indicate that with the bath pH the surface roughness of the trivalent CCC decreases. Further, the uniform growth of the trivalent CCC becomes difficult in low pH bath due to the high dissolution rate of both the Zn substrate and the trivalent CCC. Fig. 4 shows the variation of the trivalent CCC thickness as a function of bath pH. The thickness of the CCC increased with
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Fig. 4. Effects of bath pH on the thickness of the trivalent CCC. The CCCs were formed on Zn for 40 s in pH 1.1, 1.7 and 2.3 bath at 30 ◦ C.
increasing bath pH to 1.7, and then decreased above pH 1.7. It was reported that bath pH affected the dissolution rate of Zn, which provided electrons for deposition of CCC, as well as that of the deposited CCC [14]. Therefore, it is considered that the trivalent CCC formed in the pH 1.7 bath shows the maximum thickness because at pH less than 1.7 the dissolution rate of the deposited CCC is very fast and at pH more than 1.7 the dissolution rate of Zn providing electrons for the deposition of the CCC is very slow. The anodic polarization curves of the trivalent CCC on Zn for 40 s in the pH 1.1, 1.7 and 2.3 bath, which were measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution, are presented in Fig. 5. The pitting potential increased with increasing bath pH from 1.1 to 1.7, but decreased with further increasing pH to 2.3. The passive current density decreased with increasing bath pH to 1.7, but increased with bath pH above pH 1.7. From Figs. 4 and 5, it was found that the thickness and corrosion resistance of the trivalent CCC increased with increasing pH to 1.7, and with a further increase in pH they decreased.
Fig. 5. Anodic polarization curves of the trivalent CCCs in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCCs were formed on Zn for 40 s in pH 1.1, 1.7 and 2.3 bath at 30 ◦ C.
Fig. 6. SEM micrograph showing cross-section of the trivalent CCC formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
From the results until now, it was revealed that the corrosion resistance of the trivalent CCC was proportional to the coating thickness. The trivalent CCC formed on Zn for 40 s in the pH 1.7 bath showed the best corrosion resistance with maximum thickness about 1 m (Fig. 6). 3.3. XPS analysis of the trivalent CCC XPS analysis of the trivalent CCC was performed to examine the valence state of Cr present in the coating. Fig. 7 shows the XPS spectrum of Cr in the trivalent CCC formed on Zn for 40 s in the pH 1.7 bath, which showed the best corrosion resistance. This spectrum was obtained after sputtering for 2 min from the top surface by Ar-ion. In Fig. 7, the two peaks appeared at 586.0 and 576.2 eV correspond to Cr2p1/2 and Cr2p3/2 , respectively [18]. For the hexavalent CCC, it is well known that the Cr2p3/2 peak can be resolved into more than two peaks representing the Cr compound with the Cr(III) and Cr(VI) states [6,15,19]. However, the Cr2p3/2 peak, obtained from the trivalent CCC, was composed of only one peak corresponding to the Cr2p3/2 peak of Cr2 O3 [6,9,10]. From XPS analysis, it was confirmed that Cr in the trivalent CCC was in the form of Cr2 O3 and there was no Cr(VI) component in the coating.
Fig. 7. XPS spectrum of Cr present in the trivalent CCC formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C (after Ar sputtering for 2 min).
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Fig. 10. Equivalent circuit models of porous layer (a) without and (b) with corrosion product. Fig. 8. Effect of immersion time on the Nyquist plots of the trivalent CCC in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
3.4. The corrosion behavior of the trivalent CCC The polarization results in Sections 3.1 and 3.2 show that the trivalent CCC formed on Zn for 40 s in the pH 1.7 bath exhibits the best corrosion resistance. To examine its corrosion behavior, impedance analysis of the trivalent CCC on Zn was performed in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution (exposure area: 0.32 cm2 ). Nyquist plots of the trivalent CCC obtained after immersion for 10–80 h showed a distorted semi-circle at a low frequency region (Fig. 8). It is suggested that the distortion of the low frequency region is caused by a diffusion control reaction and affected by Warburg impedance. The radius of this semi-circle increased with immersion time to 50 h, and then decreased. The increase in the radius of the semi-circle can be interpreted to an increase in the surface resistance caused by surface passivation. However, the decrease in the radius of the semi-circle with increasing immersion time above 50 h cannot be explained by only the surface passivation. Therefore, in order to analyze the corrosion behavior of the trivalent CCC clearly, it is necessary to find an equivalent circuit for the trivalent CCC and measure electrical parameters of the equivalent circuit.
Fig. 9. Bode plot of the trivalent CCC after 50 h in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
Fig. 11. Equivalent circuit of the trivalent CCC formed on Zn for 40 s in deaerated pH 1.7 bath at 30 ◦ C.
Fig. 9 shows Bode plot of the trivalent CCC obtained after 50 h immersion. Three different time constants were observed from the Bode plot of the trivalent CCC, indicating that the equivalent circuit of the trivalent CCC is composed with more than three capacitive components. Pang et al. [20] has proposed a simple circuit, with two time constants (Fig. 10(a)), to describe the impedance behavior of a porous coating. In this case, Rs corresponds to the electrolyte resistance, Rcoat and Qcoat represent the resistive and capacitive responses of the pores and defects of the layer, respectively. Rct models the charge transfer resistance and Qdl describes double layer capacitance. However, this model cannot explain the three time constants observed in the Bode plot of the trivalent CCC (Fig. 9). Bonora et al. [21] suggested that corrosion products could deposit in a porous coating and affect
Fig. 12. Spectrum fitting of the trivalent CCC after 50 h in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
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the impedance spectrum of the coating, when the coating was exposed to corrosive environment, and they proposed an equivalent circuit model shown in Fig. 10(b). In this model, Rcp and Qcp represent the resistive and capacitive responses of the corrosion products, respectively. The equivalent circuit of Fig. 10(b) can explain three capacitive components observed from the Bode plot of the trivalent CCC. However, it was impossible to fit the impedance spectrum of the trivalent CCC into the equivalent circuit of Fig. 10(b). We could fit the impedance data using a
modified equivalent circuit shown in Fig. 11, which considered Warburg impedance observed in Fig. 8 and the deposition of chromium corrosion product (CCP) in the micro-cracks of the trivalent CCC. Definitions of the electrical parameters in the modified equivalent circuit are presented in Table 1. Fig. 12 shows the impedance data of the trivalent CCC obtained after 50 h immersion and the fitting results, demonstrating that the experimental results are well described by the equivalent circuit proposed for the trivalent CCC in Fig. 11.
Fig. 13. Variation of (a) coating resistance (Rcoat ), (b) admittance (Yo ) of coating capacitance (Qcoat ), (c) charge transfer resistance (Rct ), (d) Yo of double layer capacitance (Qdl ), (e) Yo of Warburg impedance (W), (f) CCP resistance (Rccp ) and (g) Yo of CCP capacitance (Qccp ) of the trivalent CCC with immersion time.
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Table 1 Definitions of the electrical parameters Rs Rcoat Qcoat Rct Qdl W Rccp Qccp
Solution resistance Resistance of trivalent chromium based conversion coating Capacitance of trivalent chromium based conversion coating Charge transfer resistance Double layer capacitance Warburg impedance Resistance of chromium corrosion product Capacitance of chromium corrosion product
Fig. 13 shows the trend of each component of the equivalent circuit as a function of immersion time. The coating resistance of the trivalent CCC (Rcoat ) increased with immersion time to 50 h, and then decreased (Fig. 13(a)). Rcoat varied about from 30 to 70 cm2 , which was very small in comparison to the charge transfer resistance of the trivalent CCC (Rct ) and the resistance of CCP (Rccp ). Thus, this indicates that the effect of Rcoat on the corrosion resistance of the trivalent CCC is negligible. The admittance Yo of the coating capacitance of the trivalent CCC (Qcoat ) increased with immersion time (Fig. 13(b)). According to previous reports [8,14], the increase in the admittance Yo of Qcoat can be interpreted such that the surface area of the CCC increases due to the corrosion of the coating and/or its dielectric constant increases because of water uptake. The charge transfer resistance of the trivalent CCC (Rct ) increased with increasing immersion time (Fig. 13(c)). This means that the charge transfer between the CCC and solution becomes more difficult due to the passivation property of the trivalent CCC. Thus, it is suggested that the trivalent CCC acts as a barrier, which prevents the contact of substrate Zn with corrosive environment. The admittance Yo of the double layer capacitance (Qdl ) increased with immersion time (Fig. 13(d)). The admittance Yo of Qdl is proportional to the area involved in the electrochemical reaction, i.e. surface area of micro-cracks in contact with the solution. Therefore, the increase in the admittance Yo of Qdl indicates that the corrosion inside the micro-cracks becomes more severe and the surface area of the micro-cracks increases with immersion time. The admittance Yo of Warburg impedance (W) decreased rapidly with increasing immersion time from 10 to 20 h, and then decreased gradually (Fig. 13(e)). W is inversely proportional to the Yo according to Eq. 1. Thus, it means that mass transfer through the micro-cracks in the trivalent CCC becomes more difficult with immersion time. This may be caused by the deposition of CCP in the microcracks. σ 1 W = σω−1/2 − jσω−1/2 = √ = 1/2 1/2 Y (jω) 2(jω) o √ 2 Yo = σ RT 1 1 √ σ= + Cred (Dred )1/2 n2 F 2 2 Cox (Dox )1/2
(1a)
(1b) (1c)
ω is the angular frequency; j = (−1)1/2 ; R the ideal gas constant; T the absolute temperature; n the number of electron transferred; F Faraday’s constant; Cox the concentration of oxidation species; Cred the concentration of reduction species; Dox the dif-
Fig. 14. SEM micrographs of the trivalent CCC (a) before immersion and (b) after immersion for 80 h in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at ambient 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
fusivity of oxidation species; Dred is the diffusivity of reduction species. The resistance of the CCP (Rccp ) increased with increasing immersion time to 50 h, and then decreased dramatically (Fig. 13(f)). Campestrini et al. [22] reported that dense CCPs, which were deposited inside the micro-cracks of the CCC during immersion in corrosive environment, affected the charge carrier diffusion through the CCC. Thus, the increase in Rccp is concerned with the increase in the amount of dense CCP, which acts as a diffusion barrier of ion such as chloride ion through the trivalent CCC. With further increasing immersion time, it can be expected that the corrosion inside the microcracks becomes more severe and the crack propagation occurs. Finally, the CCP deposited in the micro-cracks will be loosened and cannot act as the diffusion barrier after critical immersion time. Therefore, it is considered that the decrease in Rccp after 50 h immersion results from the crack propagation of the trivalent CCC. The admittance Yo of the capacitance of the CCP (Qccp ) did not change significantly with immersion time until 50 h (Fig. 13(g)). Then, however, the admittance Yo of Qccp increased abruptly, suggesting that the surface area of CCP contacting with the solution increased because of the crack propagation of CCC after 50 h immersion. Fig. 14 shows the surface morphologies of the trivalent CCC before and after the impedance test. After 80 h immersion, propagation of the micro-cracks and pitting initiation were observed. This is well
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Fig. 15. Schematic representation for corrosion behavior of the trivalent CCC formed on Zn: (a) initial stage, (b) formation of CCP, (c) dense CCP acting as a diffusion barrier and (d) crack propagation.
corresponded with the analysis of the impedance spectra of the trivalent CCC, especially with the decrease in Rccp after 50 h immersion. From EIS analysis, the impedance spectra for the trivalent CCC on Zn with immersion time in the buffer solution containing chloride ions were well described by the modified equivalent circuit considering both the micro-cracks in the coating and the CCP deposited in the micro-cracks. Besides, the corrosion behavior of the trivalent CCC was well explained by the variation of Rccp . A schematic diagram of the corrosion behavior of the trivalent CCC based on the above discussion is shown in Fig. 15. The trivalent CCC with micro-cracks is formed on Zn in the Cr(III) bath (Fig. 15(a)). After immersion of the trivalent CCC in corrosive environment, dense CCP is deposited in the micro-cracks and it acts as a diffusion barrier, which inhibits the movement of an ion through the CCC (Fig. 15(b)). With increasing immersion time, the amount of dense CCP increases, and hence the resistance of the CCP also increases (Fig. 15(c)). However, with further increasing immersion time, the CCP deposited in the micro-cracks is loosened due to crack propagation and pit growth, and finally the resistance of CCP to inhibit corrosion decreases. These results well corresponded with other researchers’ work for the hexavalent CCC [22].
2. The surface roughness and surface defect concentration of the trivalent CCC decreased with increasing bath pH. The trivalent CCC formed in the pH 1.7 bath showed the maximum thickness because at pH less than 1.7 the dissolution rate of the CCC was very fast and at pH more than 1.7 the dissolution rate of Zn, which provided electrons for the deposition of the CCC, was very slow. 3. The trivalent CCC formed for 40 s in the pH 1.7 bath showed the best corrosion resistance, as confirmed by the significant increase its pitting potential from −355 mVSCE for Zn to 975 mVSCE for the trivalent CCC formed on Zn. 4. The impedance spectra for the trivalent CCC on Zn with immersion time in the buffer solution containing chloride ions were well described by the modified equivalent circuit considering both the micro-cracks in the CCC and the CCP deposited in the micro-cracks. 5. The corrosion behavior of the trivalent CCC was explained by the variation of the resistance of CCP (Rccp ), which increased with immersion time to 50 h, and then decreased. The increase in Rccp indicates an increase in the amount of dense CCP in the micro-cracks, which acts as a diffusion barrier of chloride ion through the CCC. However, with further increasing immersion time, the corrosion inside the micro-cracks becomes more severe and the crack propagation occurs. Finally, Rccp decreases drastically because the CCP cannot act as the diffusion barrier. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusions A trivalent CCC was formed on Zn substrate in chromium nitrate solution with cobalt chloride and sulfuric acid. The microstructure and corrosion resistance of the trivalent CCC were found to largely depend on bath pH and immersion time. Conclusions drawn from this study are as follows: 1. In the Cr(III) bath of pH 1.7, the density of micro-crack and thickness of the trivalent CCC increased with increasing immersion time from 20 to 40 s, and thereafter decreased with further increase in the immersion time. This indicates that both the growth of the CCC on Zn and the dissolution of the deposited CCC progress competitively during the immersion of Zn in the bath.
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[19] [20] [21] [22]
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Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc KeunWoo Cho, V. Shankar Rao, HyukSang Kwon ∗,1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 GuseongDong, YuSeongGu, Daejeon 305-701, Republic of Korea Received 13 September 2006; received in revised form 5 December 2006; accepted 12 December 2006 Available online 11 January 2007
Abstract A trivalent chromium based conversion coating (CCC), based on chromium nitrate solution with Co(II) ions, was developed on Zn substrate. The corrosion resistance of the trivalent CCC, measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution using anodic polarization and electrochemical impedance spectroscopy (EIS), was very sensitive to both immersion time and bath pH. Micro-cracks were found on the surface of the CCC. Besides, the density of micro-crack and the coating thickness also depended on immersion time and bath pH. With increasing the coating thickness its pitting potential increased and passive current density decreased. The trivalent CCC formed on Zn for 40 s in pH 1.7 bath showed the best corrosion resistance, and the pitting potential increased significantly from −355 mVSCE for Zn to 975 mVSCE for the trivalent CCC on Zn. To explain the corrosion behavior of the trivalent CCC using EIS analysis, a modified equivalent circuit, which considered the micro-cracks in the coating and chromium corrosion product (CCP) deposited in the micro-cracks, was designed and the variation of each electrical parameter was examined. Especially, its corrosion behavior was well described by the variation of the resistance of CCP (Rccp ). © 2007 Elsevier Ltd. All rights reserved. Keywords: Trivalent chromium based conversion coating; Microstructure; Corrosion resistance; EIS analysis; Equivalent circuit
1. Introduction Electrodeposited Zn coating has been used to protect ferrous substrates in automobile industries. However, the corrosion rate of the Zn coating increases rapidly in a humid air, therefore, various surface treatments have been developed to improve the corrosion resistance of the Zn coating: hexavalent chromium based conversion coating (CCC), generally called chromate conversion coating, phosphate conversion coating and organic painting, etc. [1–3]. Among them, CCC is widely used because of its superior corrosion resistance, good paint adhesion and low cost [4–6]. CCC is performed by immersing Zn coated articles in a chromic acid bath, which forms a passive layer containing Zn oxides and mixed Cr(III)–Cr(VI) oxides [7,8]. Because the Cr(VI) species present in the chromic acid bath and the coating are toxic and carcinogenic, nontoxic trivalent CCC has been
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Corresponding author. Tel.: +82 42 869 3326; fax: +82 42 869 3310. E-mail address: [email protected] (H. Kwon). ISE member.
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.12.032
studied as an alternative to hexavalent CCC [8–12]. The corrosion resistance of the trivalent CCC is generally lower than that of the hexavalent CCC [8–10]. Thus, many recent studies have focused on the improvement in the corrosion resistance of the trivalent CCC by sealing treatments on the CCC and/or by additions of different additives in a Cr(III) bath. Bellezze et al. [8] reported the corrosion resistance of the trivalent CCC increased dramatically through the sealing treatment in Si based solution and was equivalent to that of the hexavalent CCC. According to Fonte et al. [11], the trivalent CCC formed in a Cr(III) bath containing transition metal ions such as Co(II), Ni(II) and Fe(II) showed superior corrosion resistance than that formed in the bath without transition metal ions. It is well known that the microstructure and the corrosion resistance of the hexavalent CCC largely depend on immersion time and bath pH as well as bath composition [13,14]. However, the influences of immersion time and bath pH on those of the trivalent CCC formed on Zn substrate are rarely studied. Furthermore, correlation between the microstructure and the corrosion resistance of the trivalent CCC has not been reported yet. We have developed a trivalent CCC bath based on chromium nitrate (Cr(NO3 )3 ) solution with Co(II) ions. In this paper, we
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examined the effects of immersion time and bath pH on the microstructure and corrosion resistance of the trivalent CCC. The corrosion behavior of the trivalent CCC was also investigated to explain the mechanism for the improvement in its corrosion resistance. 2. Experimental A commercially pure Zn plate was used as a substrate for the trivalent CCC. The surface of the Zn plate was polished with 2000 grit silicon carbide paper, degreased with acetone, ultrasonically cleaned in ethanol for 3 min, rinsed in distilled water, activated for 2 s in 3 wt.% nitric acid and rinsed in distilled water again prior to the trivalent CCC. The composition of the bath for the trivalent CCC was 50 mL/L Cr(NO3 )3 (40%), 20 g/L CoCl2 ·6H2 O and 3 mL/L H2 SO4 . Bath pH was adjusted to three different values (pH 1.1, 1.7 and 2.3) by adding NaOH to the bath. The trivalent CCCs were formed on Zn in the bath for 20–60 s at 30 ◦ C. Then these coatings were rinsed in distilled water, and dried in an oven for 4 min at 40 ◦ C, and in a desiccator for 48 h at an ambient temperature (22 ± 2 ◦ C). The surface morphology of the trivalent CCC was observed by scanning electron microscope (SEM). The coating thickness was measured using cross-sectional SEM image of the trivalent CCC and also by Rurherford backscattering spectrometry (RBS). X-ray photoelectron spectroscopy (XPS) was used to analyze the valence state of Cr in the trivalent CCC. The spectrum was obtained with VG Scientific, model ESCALAB 200R, using Mg K␣ X-ray source (hν = 15 kV, 20 mA) at 300 W. The base pressure in the spectrometer chamber was maintained at 5 × 10−10 Torr throughout the measurement. The binding energy scale of the spectrophotometer was calibrated using C 1s (284.8 eV) substrate. The specimen was analyzed by means of Ar-ion sputtering with primary beam energy of 5 keV, 0.30 A. The corrosion resistance of the trivalent CCC was evaluated by anodic polarization test and electrochemical impedance spectroscopy (EIS) in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at an ambient temperature (22 ± 2 ◦ C). Anodic polarization test was performed at a scan rate of 0.5 mV/s using a conventional three-electrode polarization cell: specimen as a working electrode, saturated calomel electrode (SCE) as a reference electrode and a platinum counter electrode. EIS was conducted by frequency sweep from 105 to 10−2 Hz using a superimposed sinusoidal signal of 5 mV rms at the open circuit potential. All the potentials in this work are referred to the SCE. 3. Results and discussion 3.1. Effects of immersion time on the microstructure and corrosion resistance of the trivalent CCC CCC is generally performed by immersing Zn coated articles in an acidic Cr bath within 60 s because the thickness of Zn coating decreases due to dissolution of Zn with increasing immersion time. Thus, it was examined the microstructures and
Fig. 1. SEM micrographs of the trivalent CCCs formed on Zn for (a) 20 s, (b) 40 s and (c) 60 s, in pH 1.7 bath at 30 ◦ C.
corrosion resistances of the trivalent CCCs formed on Zn in the Cr(III) bath from 20 to 60 s. Fig. 1 shows the surface morphologies of the trivalent CCCs formed on Zn with immersion time of 20, 40 and 60 s in the pH 1.7 bath. After 20 s immersion, trivalent CCC with microcracks was formed on Zn, which was similar to the surface morphology of hexavalent CCC [6,10,15]. According to previous reports [6,15,16], the hexavalent CCC deposited on Zn with gel-like structure in an acidic Cr bath, during drying process the CCC would shrink due to an internal tensile stress in the coating, and finally micro-cracks were formed on the surface of the coating. Moreover, with the coating thickness the tensile stress increased, resulting in an increase in the density of micro-
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Fig. 2. Anodic polarization curves of Zn with and without the trivalent CCC in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCCs were formed on Zn substrate for 20, 40 and 60 s in pH 1.7 bath at 30 ◦ C.
crack. With increasing immersion time to 40 s the Zn surface was uniformly covered with the trivalent CCC, and the density of micro-crack on the surface increased. However, the density of micro-crack decreased significantly with further increasing immersion time to 60 s. The coating thickness also decreased from 1 to 0.35 m with increasing immersion time from 40 to 60 s. Gigandet et al. [17] suggested that both the growth of CCC and the dissolution of the deposited CCC progressed at the same time during immersion of Zn in an acidic Cr bath. Thus, it is considered that the thickness of trivalent CCC increases until 40 s immersion because the growth rate of the trivalent CCC is faster than its dissolution rate. However, with further increasing immersion time its growth rate becomes less than dissolution rate, and finally the coating thickness decreases. The anodic polarization curves for the Zn substrate and the trivalent CCCs formed on Zn in the pH 1.7 bath for 20, 40 and 60 s, obtained in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution, are presented in Fig. 2. After trivalent CCC, the pitting corrosion resistance of Zn greatly improved as confirmed by an increase in the pitting potential from −355 mVSCE for Zn to 975 mVSCE for the trivalent CCC on Zn for 40 s. Moreover, the pitting potential increased with increasing immersion time to 40 s, but decreased with immersion time above 40 s. The passive current density decreased gradually with increasing immersion time to 40 s, but increased with further increasing immersion time. Thus, the pitting potential and passive current density of the trivalent CCC largely depended on its thickness. With the coating thickness, the pitting potential of the trivalent CCC increased, and the passive current density decreased. The best corrosion resistance of the trivalent CCC with maximum coating thickness was obtained for immersion time of 40 s. 3.2. Effects of bath pH on the microstructure and corrosion resistance of the trivalent CCC Fig. 3 shows the surface morphologies of the trivalent CCCs formed on Zn for 40 s in the bath of pH 1.1, 1.7 and 2.3. For
Fig. 3. SEM micrographs of the trivalent CCCs on Zn for 40 s at 30 ◦ C in (a) pH 1.1, (b) pH 1.7 and (c) pH 2.3 bath.
the coating formed in the pH 1.1 bath surface was very rough with high defect density, whereas for the coating formed in the pH 1.7 bath micro-cracks were found on the surface and the its surface roughness decreased compared to the CCC formed in the pH 1.1 bath. The surface of the CCC formed in the pH 2.3 bath was very smooth and no micro-crack was observed on the surface. These results indicate that with the bath pH the surface roughness of the trivalent CCC decreases. Further, the uniform growth of the trivalent CCC becomes difficult in low pH bath due to the high dissolution rate of both the Zn substrate and the trivalent CCC. Fig. 4 shows the variation of the trivalent CCC thickness as a function of bath pH. The thickness of the CCC increased with
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Fig. 4. Effects of bath pH on the thickness of the trivalent CCC. The CCCs were formed on Zn for 40 s in pH 1.1, 1.7 and 2.3 bath at 30 ◦ C.
increasing bath pH to 1.7, and then decreased above pH 1.7. It was reported that bath pH affected the dissolution rate of Zn, which provided electrons for deposition of CCC, as well as that of the deposited CCC [14]. Therefore, it is considered that the trivalent CCC formed in the pH 1.7 bath shows the maximum thickness because at pH less than 1.7 the dissolution rate of the deposited CCC is very fast and at pH more than 1.7 the dissolution rate of Zn providing electrons for the deposition of the CCC is very slow. The anodic polarization curves of the trivalent CCC on Zn for 40 s in the pH 1.1, 1.7 and 2.3 bath, which were measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution, are presented in Fig. 5. The pitting potential increased with increasing bath pH from 1.1 to 1.7, but decreased with further increasing pH to 2.3. The passive current density decreased with increasing bath pH to 1.7, but increased with bath pH above pH 1.7. From Figs. 4 and 5, it was found that the thickness and corrosion resistance of the trivalent CCC increased with increasing pH to 1.7, and with a further increase in pH they decreased.
Fig. 5. Anodic polarization curves of the trivalent CCCs in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCCs were formed on Zn for 40 s in pH 1.1, 1.7 and 2.3 bath at 30 ◦ C.
Fig. 6. SEM micrograph showing cross-section of the trivalent CCC formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
From the results until now, it was revealed that the corrosion resistance of the trivalent CCC was proportional to the coating thickness. The trivalent CCC formed on Zn for 40 s in the pH 1.7 bath showed the best corrosion resistance with maximum thickness about 1 m (Fig. 6). 3.3. XPS analysis of the trivalent CCC XPS analysis of the trivalent CCC was performed to examine the valence state of Cr present in the coating. Fig. 7 shows the XPS spectrum of Cr in the trivalent CCC formed on Zn for 40 s in the pH 1.7 bath, which showed the best corrosion resistance. This spectrum was obtained after sputtering for 2 min from the top surface by Ar-ion. In Fig. 7, the two peaks appeared at 586.0 and 576.2 eV correspond to Cr2p1/2 and Cr2p3/2 , respectively [18]. For the hexavalent CCC, it is well known that the Cr2p3/2 peak can be resolved into more than two peaks representing the Cr compound with the Cr(III) and Cr(VI) states [6,15,19]. However, the Cr2p3/2 peak, obtained from the trivalent CCC, was composed of only one peak corresponding to the Cr2p3/2 peak of Cr2 O3 [6,9,10]. From XPS analysis, it was confirmed that Cr in the trivalent CCC was in the form of Cr2 O3 and there was no Cr(VI) component in the coating.
Fig. 7. XPS spectrum of Cr present in the trivalent CCC formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C (after Ar sputtering for 2 min).
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Fig. 10. Equivalent circuit models of porous layer (a) without and (b) with corrosion product. Fig. 8. Effect of immersion time on the Nyquist plots of the trivalent CCC in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
3.4. The corrosion behavior of the trivalent CCC The polarization results in Sections 3.1 and 3.2 show that the trivalent CCC formed on Zn for 40 s in the pH 1.7 bath exhibits the best corrosion resistance. To examine its corrosion behavior, impedance analysis of the trivalent CCC on Zn was performed in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution (exposure area: 0.32 cm2 ). Nyquist plots of the trivalent CCC obtained after immersion for 10–80 h showed a distorted semi-circle at a low frequency region (Fig. 8). It is suggested that the distortion of the low frequency region is caused by a diffusion control reaction and affected by Warburg impedance. The radius of this semi-circle increased with immersion time to 50 h, and then decreased. The increase in the radius of the semi-circle can be interpreted to an increase in the surface resistance caused by surface passivation. However, the decrease in the radius of the semi-circle with increasing immersion time above 50 h cannot be explained by only the surface passivation. Therefore, in order to analyze the corrosion behavior of the trivalent CCC clearly, it is necessary to find an equivalent circuit for the trivalent CCC and measure electrical parameters of the equivalent circuit.
Fig. 9. Bode plot of the trivalent CCC after 50 h in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
Fig. 11. Equivalent circuit of the trivalent CCC formed on Zn for 40 s in deaerated pH 1.7 bath at 30 ◦ C.
Fig. 9 shows Bode plot of the trivalent CCC obtained after 50 h immersion. Three different time constants were observed from the Bode plot of the trivalent CCC, indicating that the equivalent circuit of the trivalent CCC is composed with more than three capacitive components. Pang et al. [20] has proposed a simple circuit, with two time constants (Fig. 10(a)), to describe the impedance behavior of a porous coating. In this case, Rs corresponds to the electrolyte resistance, Rcoat and Qcoat represent the resistive and capacitive responses of the pores and defects of the layer, respectively. Rct models the charge transfer resistance and Qdl describes double layer capacitance. However, this model cannot explain the three time constants observed in the Bode plot of the trivalent CCC (Fig. 9). Bonora et al. [21] suggested that corrosion products could deposit in a porous coating and affect
Fig. 12. Spectrum fitting of the trivalent CCC after 50 h in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
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the impedance spectrum of the coating, when the coating was exposed to corrosive environment, and they proposed an equivalent circuit model shown in Fig. 10(b). In this model, Rcp and Qcp represent the resistive and capacitive responses of the corrosion products, respectively. The equivalent circuit of Fig. 10(b) can explain three capacitive components observed from the Bode plot of the trivalent CCC. However, it was impossible to fit the impedance spectrum of the trivalent CCC into the equivalent circuit of Fig. 10(b). We could fit the impedance data using a
modified equivalent circuit shown in Fig. 11, which considered Warburg impedance observed in Fig. 8 and the deposition of chromium corrosion product (CCP) in the micro-cracks of the trivalent CCC. Definitions of the electrical parameters in the modified equivalent circuit are presented in Table 1. Fig. 12 shows the impedance data of the trivalent CCC obtained after 50 h immersion and the fitting results, demonstrating that the experimental results are well described by the equivalent circuit proposed for the trivalent CCC in Fig. 11.
Fig. 13. Variation of (a) coating resistance (Rcoat ), (b) admittance (Yo ) of coating capacitance (Qcoat ), (c) charge transfer resistance (Rct ), (d) Yo of double layer capacitance (Qdl ), (e) Yo of Warburg impedance (W), (f) CCP resistance (Rccp ) and (g) Yo of CCP capacitance (Qccp ) of the trivalent CCC with immersion time.
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Table 1 Definitions of the electrical parameters Rs Rcoat Qcoat Rct Qdl W Rccp Qccp
Solution resistance Resistance of trivalent chromium based conversion coating Capacitance of trivalent chromium based conversion coating Charge transfer resistance Double layer capacitance Warburg impedance Resistance of chromium corrosion product Capacitance of chromium corrosion product
Fig. 13 shows the trend of each component of the equivalent circuit as a function of immersion time. The coating resistance of the trivalent CCC (Rcoat ) increased with immersion time to 50 h, and then decreased (Fig. 13(a)). Rcoat varied about from 30 to 70 cm2 , which was very small in comparison to the charge transfer resistance of the trivalent CCC (Rct ) and the resistance of CCP (Rccp ). Thus, this indicates that the effect of Rcoat on the corrosion resistance of the trivalent CCC is negligible. The admittance Yo of the coating capacitance of the trivalent CCC (Qcoat ) increased with immersion time (Fig. 13(b)). According to previous reports [8,14], the increase in the admittance Yo of Qcoat can be interpreted such that the surface area of the CCC increases due to the corrosion of the coating and/or its dielectric constant increases because of water uptake. The charge transfer resistance of the trivalent CCC (Rct ) increased with increasing immersion time (Fig. 13(c)). This means that the charge transfer between the CCC and solution becomes more difficult due to the passivation property of the trivalent CCC. Thus, it is suggested that the trivalent CCC acts as a barrier, which prevents the contact of substrate Zn with corrosive environment. The admittance Yo of the double layer capacitance (Qdl ) increased with immersion time (Fig. 13(d)). The admittance Yo of Qdl is proportional to the area involved in the electrochemical reaction, i.e. surface area of micro-cracks in contact with the solution. Therefore, the increase in the admittance Yo of Qdl indicates that the corrosion inside the micro-cracks becomes more severe and the surface area of the micro-cracks increases with immersion time. The admittance Yo of Warburg impedance (W) decreased rapidly with increasing immersion time from 10 to 20 h, and then decreased gradually (Fig. 13(e)). W is inversely proportional to the Yo according to Eq. 1. Thus, it means that mass transfer through the micro-cracks in the trivalent CCC becomes more difficult with immersion time. This may be caused by the deposition of CCP in the microcracks. σ 1 W = σω−1/2 − jσω−1/2 = √ = 1/2 1/2 Y (jω) 2(jω) o √ 2 Yo = σ RT 1 1 √ σ= + Cred (Dred )1/2 n2 F 2 2 Cox (Dox )1/2
(1a)
(1b) (1c)
ω is the angular frequency; j = (−1)1/2 ; R the ideal gas constant; T the absolute temperature; n the number of electron transferred; F Faraday’s constant; Cox the concentration of oxidation species; Cred the concentration of reduction species; Dox the dif-
Fig. 14. SEM micrographs of the trivalent CCC (a) before immersion and (b) after immersion for 80 h in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution at ambient 22 ± 2 ◦ C. The CCC was formed on Zn for 40 s in pH 1.7 bath at 30 ◦ C.
fusivity of oxidation species; Dred is the diffusivity of reduction species. The resistance of the CCP (Rccp ) increased with increasing immersion time to 50 h, and then decreased dramatically (Fig. 13(f)). Campestrini et al. [22] reported that dense CCPs, which were deposited inside the micro-cracks of the CCC during immersion in corrosive environment, affected the charge carrier diffusion through the CCC. Thus, the increase in Rccp is concerned with the increase in the amount of dense CCP, which acts as a diffusion barrier of ion such as chloride ion through the trivalent CCC. With further increasing immersion time, it can be expected that the corrosion inside the microcracks becomes more severe and the crack propagation occurs. Finally, the CCP deposited in the micro-cracks will be loosened and cannot act as the diffusion barrier after critical immersion time. Therefore, it is considered that the decrease in Rccp after 50 h immersion results from the crack propagation of the trivalent CCC. The admittance Yo of the capacitance of the CCP (Qccp ) did not change significantly with immersion time until 50 h (Fig. 13(g)). Then, however, the admittance Yo of Qccp increased abruptly, suggesting that the surface area of CCP contacting with the solution increased because of the crack propagation of CCC after 50 h immersion. Fig. 14 shows the surface morphologies of the trivalent CCC before and after the impedance test. After 80 h immersion, propagation of the micro-cracks and pitting initiation were observed. This is well
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Fig. 15. Schematic representation for corrosion behavior of the trivalent CCC formed on Zn: (a) initial stage, (b) formation of CCP, (c) dense CCP acting as a diffusion barrier and (d) crack propagation.
corresponded with the analysis of the impedance spectra of the trivalent CCC, especially with the decrease in Rccp after 50 h immersion. From EIS analysis, the impedance spectra for the trivalent CCC on Zn with immersion time in the buffer solution containing chloride ions were well described by the modified equivalent circuit considering both the micro-cracks in the coating and the CCP deposited in the micro-cracks. Besides, the corrosion behavior of the trivalent CCC was well explained by the variation of Rccp . A schematic diagram of the corrosion behavior of the trivalent CCC based on the above discussion is shown in Fig. 15. The trivalent CCC with micro-cracks is formed on Zn in the Cr(III) bath (Fig. 15(a)). After immersion of the trivalent CCC in corrosive environment, dense CCP is deposited in the micro-cracks and it acts as a diffusion barrier, which inhibits the movement of an ion through the CCC (Fig. 15(b)). With increasing immersion time, the amount of dense CCP increases, and hence the resistance of the CCP also increases (Fig. 15(c)). However, with further increasing immersion time, the CCP deposited in the micro-cracks is loosened due to crack propagation and pit growth, and finally the resistance of CCP to inhibit corrosion decreases. These results well corresponded with other researchers’ work for the hexavalent CCC [22].
2. The surface roughness and surface defect concentration of the trivalent CCC decreased with increasing bath pH. The trivalent CCC formed in the pH 1.7 bath showed the maximum thickness because at pH less than 1.7 the dissolution rate of the CCC was very fast and at pH more than 1.7 the dissolution rate of Zn, which provided electrons for the deposition of the CCC, was very slow. 3. The trivalent CCC formed for 40 s in the pH 1.7 bath showed the best corrosion resistance, as confirmed by the significant increase its pitting potential from −355 mVSCE for Zn to 975 mVSCE for the trivalent CCC formed on Zn. 4. The impedance spectra for the trivalent CCC on Zn with immersion time in the buffer solution containing chloride ions were well described by the modified equivalent circuit considering both the micro-cracks in the CCC and the CCP deposited in the micro-cracks. 5. The corrosion behavior of the trivalent CCC was explained by the variation of the resistance of CCP (Rccp ), which increased with immersion time to 50 h, and then decreased. The increase in Rccp indicates an increase in the amount of dense CCP in the micro-cracks, which acts as a diffusion barrier of chloride ion through the CCC. However, with further increasing immersion time, the corrosion inside the micro-cracks becomes more severe and the crack propagation occurs. Finally, Rccp decreases drastically because the CCP cannot act as the diffusion barrier. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusions A trivalent CCC was formed on Zn substrate in chromium nitrate solution with cobalt chloride and sulfuric acid. The microstructure and corrosion resistance of the trivalent CCC were found to largely depend on bath pH and immersion time. Conclusions drawn from this study are as follows: 1. In the Cr(III) bath of pH 1.7, the density of micro-crack and thickness of the trivalent CCC increased with increasing immersion time from 20 to 40 s, and thereafter decreased with further increase in the immersion time. This indicates that both the growth of the CCC on Zn and the dissolution of the deposited CCC progress competitively during the immersion of Zn in the bath.
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[19] [20] [21] [22]
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