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Effect of chromate on the electrochemical behavior of sintered Zn–Al coating in seawater
Surface & Coatings Technology 202 (2008) 5847–5852
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of chromate on the electrochemical behavior of sintered Zn–Al coating in seawater Huili Hu a, Ning Li a,b,⁎, Yongming Zhu b a b
Department of Applied Chemistry, Harbin Inst. Technol. Harbin 150001, PR China College of Ocean, Harbin Inst. Technol. at Weihai, Weihai 264209, PR China
A R T I C L E
I N F O
Article history: Received 3 March 2008 Accepted in revised form 7 June 2008 Available online 16 June 2008 Keywords: Electrochemical impedance spectroscopy X-ray photoelectron spectroscopy Corrosion Zinc–aluminum–chromium coating Chromium-free
A B S T R A C T To improve the anticorrosion resistance of chromium-free Zn–Al sintered coating, characteristics of chromate on zinc–aluminum–chromium were studied by electrochemical methods. Depth profiles of elements were obtained by glow discharge spectrum (GDS) and X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical states of main elements in Zn–Al coatings. Corrosion behaviors of steel samples coated with chromium-free Zn–Al and zinc–aluminum–chromium exposed to seawater were investigated by using electrochemical impedance spectroscopy (EIS) measurement. Both in open circuit potential and EIS monitoring self-healing phenomena of chromate were obviously observed. The self-healing function endows zinc–aluminum–chromium coating with better anticorrosion performance. Electrochemical analysis show that self-healing action of coatings is missing when replace chromate with molybdate and silane. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Zinc–aluminum–chromium coating, also named as Dacromet [1–3], is the leading inorganic coating specified by automotive companies worldwide and is a proven coating system in many industries. As a water-based and VOC (volatile organic compound) compliant coating, zinc–aluminum–chromium (ZAC) is comprised mainly of overlapping flakes zinc and aluminum in an inorganic binder. Time to first appearance of red rusts in neutral salt spray (NSS) test for steel samples coated with 10 μm ZAC is longer than 1000 h. Coating application process does not require acid pickling or involve electroplating, hereby ZAC is hydrogen embrittlement free and widely applied to fasteners. ZAC coating can also serve as an undercoat in marine protection when matched with organic topcoat. But there are strong carcinogenic chromium compounds in the ZAC coating. Recently, some corporations such as MCII (Metal Coatings International Inc.) in USA [4], NDS (Nippon Dacro Shamrock) in Japan [5], Dacral in French and MCB (Metal Coatings Brazil) in Brazil, developed the next generation of water-based coating technology, chromium-free Dacromet or Geomet [6]. But the detailed investigation about chromium-free Dacromet has not been reported recently. In order to study the effects of the substitutes for Cr(VI), the electrochemical behaviors of steel coated with ZAC and chromiumfree zinc–aluminum (ZA) in seawater are discussed. Distribution and chemical states of main elements were also researched.
The ZAC and ZA slurries in this study were consisting of two components A and B. A of ZAC and ZA is the same and composed of metal zinc powders (ECKART Co. Germany), aluminum powders (ECKART Co. Germany), acetone, butylene glycol and polyoxyethylene octylphenyl ether (OP-10). B1 of ZAC was prepared by dissolving analytical grade ZnO, CrO3 and H3BO3 in deionized water. In the composition B2 of ZA, CrO3 was replaced by ammonium molybdate tetrahydrate [(NH4)6Mo7O24 · 4H2O] and organosilane (gamma-aminopropyltriethoxysilane). Table 1 lists the compositions of the slurries employed. All coatings involved in this research were painted on the mild carbon steel. Samples (5 cm × 8 cm × 0.2 cm) were ground with SiC
⁎ Corresponding author. College of Ocean, HarBin Institute of Technology at Weihai, Weihai 264209, PR China. Tel./fax: +86 631 5687232. E-mail addresses: [email protected], [email protected] (N. Li). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.084
Table 1 Chemical compositions of ZAC and ZA slurries Compositions
Concentration in ZAC (g L− 1)
Concentration in ZA (g L− 1)
Flake zinc powder Flake aluminum powder Ethylene alcohol Polyethylene alcohol Polyoxyethylene octylphenyl ether (OP-10) Chromic acid anhydride Ammonium molybdate tetrahydrate Gamma-aminopropyltriethoxysilane Boracic acid ZnO
250 50 150 8 12
250 50 150 8 12
80
20 2
20 10 20 20
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H. Hu et al. / Surface & Coatings Technology 202 (2008) 5847–5852
Fig. 3. Zn LMM Auger spectra of ZA/steel.
Fig. 1. Normalized main composition distribution in the depth by GDS of the ZAC (a) and ZA (b) coating.
Fig. 4. High-resolution XPS spectra of the Al 2p regions for ZA/steel after 5 min sputtering.
abrasive papers from 240 to 800 grits, and then degreased with acetone. The polished samples were cleaned in distilled water and dried by a cold air stream. After dip-spin in ZAC or ZA slurries, the samples were solidified and sintered in an oven. ZAC was cured at 300 °C for 15 min, while ZA could be done for 15 min at 280 °C. Undergoing four cycles of the dip-spin and solidification processes, silvery samples can be well prepared. The results by magnetic thickness tester indicated that thickness of the coatings were 8 μm around. For ZAC or ZA coatings, surface roughnesses are almost same, Ra is 0.28 μm and Rz 1.9 μm. A 75OA type glow discharge spectrum (GDS) (Leco Co. America) was employed to measure the components of ZAC/steel and ZA/steel.
X-ray photo electron spectroscopy (XPS) was used to analyze chemical composition of zinc–aluminum coatings. A PHI ESCA 5700 instrument with an Al Ka (1476.6 eV) X-ray source operating at a pressure of 10− 7 Pa was used. XPS spectra with 5 min Ar+ etching were obtained. The etching rate was about 2 nm/min. All the spectra were calibrated to the binding energy of C 1s peak at 285 eV. A scanning electron microscope (SEM, Tescan Vega TS 5136 SB) coupled with Sutw–Sapphire energy dispersive X-ray spectroscope (EDX) was used to study the morphologies and the local chemical compositions of run-out coatings. Before SEM-EDX, the samples were coated with 2–3 nm gold by evaporation.
Fig. 2. XPS spectrum of ZA coating after 5 min sputtering.
Fig. 5. Evolution of open circuit potentials of ZAC/steel and ZA/steel in seawater.
H. Hu et al. / Surface & Coatings Technology 202 (2008) 5847–5852
Coated specimens were immersed in aerated natural seawater and held at 25 °C. Electrochemical measurements were carried out after samples were immersed for different time. A conventional threeelectrode cell was utilized, consisting of a saturated calomel reference electrode (SCE), a platinum foil counter electrode and the exposed sample working electrode with a circular 1.13 cm2 surface area. The electrochemical measurements were performed using a Gamry potentiostat PCI4-750 (Gamry Instruments Inc. USA). Open
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circuit potentials were measured with 4 Hz sampling frequency and take the arithmetic average within 2 min as the final value. The frequency range of electrochemical impedance spectroscopy (EIS) measurements is from 105 Hz to 10− 2 Hz with 10 points per decade for the impedance measurements. The amplitude of the sinusoidal perturbation was 5 mV. Three identical specimens were prepared for all tests in this study.
Fig. 6. Impedance spectra of ZAC/steel (a) and ZA/steel (b) at different exposure times to seawater, (a1) and (b1) are Nyquist plots; whereas (a2), (b2) and (a3), (b3) are the corresponding |Z|mod plots and phase plots.
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3. Results and discussion Fig. 1 shows the normalized compositional depth profiles in the samples, which were prepared after two solidification cycles in ZAC or ZA slurries. The transition layer of elements is caused by the roughness of coating and the unflatness of sputtering craters by GDS. From Fig. 1, it can be seen that the enrichment of zinc relative to other elements at about 0.5 μm. Al, Cr, B, Si and Mo are distributed evenly through the total coatings. The ratio of Zn/Al in coating is corresponding to that in the slurries. Similar results were obtained by Wang [7]. Due to normalization treatment and higher concentration of Cr in ZAC than the sum of Mo and Si in ZA, the relative concentrations of Zn and Al in ZAC are lower than in ZA. Element Cr simultaneously acts as the passivator and binder. As binders, silane coupling agent has more optimized molecular structure than Cr(III). Accordingly much more Cr in ZAC than Si in ZA is essential for good adhesion of coatings. Fig. 2 presents the XPS spectra of the ZA coating on mild carbon steel after 5 min Ar+ sputtering. It can be seen that the photoelectron peak for Zn 2p appears clearly at a binding energy (BE) of 1022 eV, O 1s at BE = 531 eV and C 1s at BE = 285 eV, respectively. The binding energies of the Zn 2p3/2 XPS peaks observed on the surface of ZA coating is 1022.1 ± 0.5 eV, which cannot distinguish between the Zn2+ in ZnO (1021.80, 1022.5 eV) and metallic Zn (1021.80, 1021.90 eV) [8]. The difference in kinetic energies of Zn LMM Auger spectra between ZnO and the metallic Zn were reported to be more than 4.5 eV [9,10]. Thus, the Zn LMM Auger line was used to analyze the presence of metallic Zn in the films. The Zn LMM Auger line for ZA/ steel samples is shown in Fig. 3. In this case, the kinetic energy (KE) of Zn2+ in ZnO (987.6 eV) is quite different from the KE of Zn0 in Zn (990.7 eV). The majority of zinc exists in ZA coating in form of Zn0 and only a minor quantity of zinc are oxided during the sintering process, which endows ZA and ZAC coatings with sacrificial anode protection for steel. Fig. 4 provides high-resolution XPS spectra of the Al 2p regions for the ZA coating. Deconvolution of the Al 2p spectrum shows two components corresponding to Al in form of Al3+ at 74.8 eV and Al0 at 72.1 eV. The ratio of Al3+ and Al0 to total aluminum is 32.3% and 67.7% respectively. Compared with hot-dip galvanized Zn–Al alloy coatings [11], the quantity of Al3+ in sintered zinc–aluminum coating is much lower. Fig. 5 shows the evolution of open circuit potentials of ZAC/steel and ZA/steel in seawater. For mild carbon steel substrates, ZA or ZAC coatings act effectively as sacrificial anodes only when their potentials are more negative than −0.85 V (vs SCE) [12]. It is observed that OCP of ZA/steel drastically shifts positively to −0.72 V (vs SCE) after 5 days of exposure. After that, the ZA/steel OCP shifts to more positive values gradually. Finally, OCP of ZA varies around −0.6 V (vs SCE). Compared with ZA/steel, the general trend of ZAC/steel OCP shifts positively evenly and some “fall points” distribute at intervals. The potential values of these “fall points” are around −1.0 V, which is near to the initial potential of ZAC/steel in seawater. Taking a view of the OCP evolution, the self-healing of ZAC endowed by chromate make the sacrificial anode protection of ZAC “slower” and “longer”, that will improve the anticorrosion property of ZAC/steel. Fig. 6 represents the impedance spectra of ZAC/steel and ZA/steel in seawater. It should be stated that the Nyquist or Bode plots corresponding those “fall points” of ZAC/steel were not included. The results show that the modulus of Z of ZAC/steel is larger than that of ZA/steel. Diversities can be seen between the impedance spectra of ZAC/steel and ZA/steel. The depressed arcs of high-frequency domain in Nyquist plots of ZAC/steel disappear after 1 day of exposure and turn to the straight line gradually. Within the first 0–3 days of immersion, two faint relaxation time constants are observed on the
Fig. 7. SEM micrograph and EDX spectrum of the rust stains for ZA/steel immersion in seawater after 50 days.
phase angle spectra [Fig. 6(a3)]. But from Fig. 6(a2), only in the first minutes of immersion for ZAC/steel, the slope in the impedance modulus suggests the clear resolution of the second time constant at low frequency-domain. The semi-circles are all incomplete and the diameters become larger and larger [Fig. 6(a1)]. For existing of Cr(III) and Cr(VI) in ZAC/steel [13], metallic zinc activated by aggressive seawater has been re-passivated by Cr. There are a small amount of compact corrosion products covering on the surface of metal flakes in ZAC coating after a long time exposure to seawater. The highfrequency time constant corresponds to the charge transfer resistance and double layer capacitance associated with the corrosion process, whereas the low-frequency process can be attributed to the mass transport across the coating filled with loose and porous corrosion products [14]. After few days immersion this low-frequency process shifts to higher frequency and amalgamates with the high-frequency process. Compared with ZAC/steel, arcs don't disappear in Nyquist plots throughout the total immersion of ZA/steel. Bode plots show that the electrochemical impedance spectroscopy spectrum of ZA/steel has three relaxation constants, except the spectrum obtained at the first moment of immersion. The first one is related to ZA coating, which is in the high-frequency domain and means the poor conductive coating, for much porous corrosion products forms in the ZA coating (see Fig. 7). The second one is a faradic process (Cdl and Rct) at the ZA/steel interface (in zinc–aluminum coating pores), followed by a finite-space diffusion process. Fig. 8 shows the Nyquist plots of ZAC/steel at some “fall points”, such as after 4 days, 10 days, 11 days, 20 days and 32 days of immersion, respectively. It is easily seen that the arcs of faradic
H. Hu et al. / Surface & Coatings Technology 202 (2008) 5847–5852
Fig. 8. Nyquist plots of ZAC/steel after immersion in seawater for different times.
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(2) The electrochemical data, which presents in this study, may be the direct evidence for the self-healing of Cr(VI). There are some “fall points” or “active points” in the evolutions of open circuit potential, which is exactly corresponding to the developments of impedance behaviors and impedance values for zinc– aluminum–chromium in seawater. (3) Combination of molybdate and silane may act as alternatives for chromate, but the self-healing action of coatings is missing. Acknowledgement Thanks to Prof. Sun for his assistances of XPS measurement. References Fig. 9. Evolution of |Z|0.1
Hz
for ZA/steel and ZAC/steel systems with immersion time.
process are more visible and the impedance values of “fall points” are much lower than others. So these “fall points” could be called “active points”, those may correspond to the time when the active metallic zinc or aluminum exposed to aggressive medium directly. The similar evolution laws in OCP were observed in Fig. 5. But the time for ZAC/ steel remaining “active” has not been measured exactly, which should be carried out by other methods such as electrochemical noise. This is the major project afterwards. It was proposed that the impedance at low frequencies (such as |Z|0.1 Hz) could serve as an evaluation parameter for a painted metal's barrier effect [15,16]. As shown in Fig. 9, the |Z|0.1 Hz of ZAC/steel is higher than that of ZA/steel during the whole immersion process. This indicates that the protective performance of ZAC coating is much higher than ZA coating. The evolutions of |Z|0.1 Hz of ZAC/steel and ZA/steel in Fig. 9 have the similar behaviors to those OCP in Fig. 5. Especially, at those “fall points”, |Z|0.1 Hz of ZAC/steel also reduces drastically. 4. Conclusions (1) Elements distribute evenly through the coating. Most of zinc and aluminum exist in form of metallic state in sintered ceramic coating. In chromium-free zinc–aluminum coating, the ratio of Al3+ and Al0 to the total aluminum is 32.2% and 67.7%.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
F. Gheno, Trans. IMF 74 (1996) 7. J.G. Liu, C.W. Yan, Surf. Coat. Technol. 200 (2006) 4976. J.G. Liu, G.P. Gong, C.W. Yan, Surf. Coat. Technol. 200 (2006) 4967. E.G. Maze, G.L. Lelong, T.E. Dorsett, D.J. Guhde, T. Nishikawa, US Patent # 7078076, 2006. Y. Endo, T. Sakai, K. Takase, US Patent # 6960247, 2005. http://www.geomet.net/, Cited 13 May. 2008. Z.L. Wang, Mater. Heat Treat. 36 (2007) 18 (in Chinese with English Abstract). J. Chastain, R.C. King, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Inc., 1995, p. 242. H.Y. Zheng, M.Z. An, J.F. Lu, Appl. Surf. Sci. 254 (2008) 1644. B. Mutel, A.B. Taleb, O. Dessaux, P. Goudmand, L. Gengembre, J. Grimblot, Thin Solid Film 266 (1995) 119. S. Feliu Jr., V. Barranco, Acta Mater. 51 (2003) 5413. A. Meroufel, S. Touzain, Prog. Org. Coat. 59 (2007) 197. S.M. Han, Y.Z. Zheng, S.X. Yu, J.M. Xu, J. Chin. Soc. Corros. Prot. 22 (2002) 269 (in Chinese with English Abstract). Liu Jianguo, Gong Gaoping, Yan Chuanwei, Electrochim. Acta 50 (2005) 3320. M. Chen, Q.S. Yu, C.M. Reddy, H.K. Yasuda, Corrosion 56 (2000) 709. E. Potvin, L. Brossard, G. Larochelle, Prog. Org. Coat. 31 (1997) 363.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of chromate on the electrochemical behavior of sintered Zn–Al coating in seawater Huili Hu a, Ning Li a,b,⁎, Yongming Zhu b a b
Department of Applied Chemistry, Harbin Inst. Technol. Harbin 150001, PR China College of Ocean, Harbin Inst. Technol. at Weihai, Weihai 264209, PR China
A R T I C L E
I N F O
Article history: Received 3 March 2008 Accepted in revised form 7 June 2008 Available online 16 June 2008 Keywords: Electrochemical impedance spectroscopy X-ray photoelectron spectroscopy Corrosion Zinc–aluminum–chromium coating Chromium-free
A B S T R A C T To improve the anticorrosion resistance of chromium-free Zn–Al sintered coating, characteristics of chromate on zinc–aluminum–chromium were studied by electrochemical methods. Depth profiles of elements were obtained by glow discharge spectrum (GDS) and X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical states of main elements in Zn–Al coatings. Corrosion behaviors of steel samples coated with chromium-free Zn–Al and zinc–aluminum–chromium exposed to seawater were investigated by using electrochemical impedance spectroscopy (EIS) measurement. Both in open circuit potential and EIS monitoring self-healing phenomena of chromate were obviously observed. The self-healing function endows zinc–aluminum–chromium coating with better anticorrosion performance. Electrochemical analysis show that self-healing action of coatings is missing when replace chromate with molybdate and silane. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Zinc–aluminum–chromium coating, also named as Dacromet [1–3], is the leading inorganic coating specified by automotive companies worldwide and is a proven coating system in many industries. As a water-based and VOC (volatile organic compound) compliant coating, zinc–aluminum–chromium (ZAC) is comprised mainly of overlapping flakes zinc and aluminum in an inorganic binder. Time to first appearance of red rusts in neutral salt spray (NSS) test for steel samples coated with 10 μm ZAC is longer than 1000 h. Coating application process does not require acid pickling or involve electroplating, hereby ZAC is hydrogen embrittlement free and widely applied to fasteners. ZAC coating can also serve as an undercoat in marine protection when matched with organic topcoat. But there are strong carcinogenic chromium compounds in the ZAC coating. Recently, some corporations such as MCII (Metal Coatings International Inc.) in USA [4], NDS (Nippon Dacro Shamrock) in Japan [5], Dacral in French and MCB (Metal Coatings Brazil) in Brazil, developed the next generation of water-based coating technology, chromium-free Dacromet or Geomet [6]. But the detailed investigation about chromium-free Dacromet has not been reported recently. In order to study the effects of the substitutes for Cr(VI), the electrochemical behaviors of steel coated with ZAC and chromiumfree zinc–aluminum (ZA) in seawater are discussed. Distribution and chemical states of main elements were also researched.
The ZAC and ZA slurries in this study were consisting of two components A and B. A of ZAC and ZA is the same and composed of metal zinc powders (ECKART Co. Germany), aluminum powders (ECKART Co. Germany), acetone, butylene glycol and polyoxyethylene octylphenyl ether (OP-10). B1 of ZAC was prepared by dissolving analytical grade ZnO, CrO3 and H3BO3 in deionized water. In the composition B2 of ZA, CrO3 was replaced by ammonium molybdate tetrahydrate [(NH4)6Mo7O24 · 4H2O] and organosilane (gamma-aminopropyltriethoxysilane). Table 1 lists the compositions of the slurries employed. All coatings involved in this research were painted on the mild carbon steel. Samples (5 cm × 8 cm × 0.2 cm) were ground with SiC
⁎ Corresponding author. College of Ocean, HarBin Institute of Technology at Weihai, Weihai 264209, PR China. Tel./fax: +86 631 5687232. E-mail addresses: [email protected], [email protected] (N. Li). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.084
Table 1 Chemical compositions of ZAC and ZA slurries Compositions
Concentration in ZAC (g L− 1)
Concentration in ZA (g L− 1)
Flake zinc powder Flake aluminum powder Ethylene alcohol Polyethylene alcohol Polyoxyethylene octylphenyl ether (OP-10) Chromic acid anhydride Ammonium molybdate tetrahydrate Gamma-aminopropyltriethoxysilane Boracic acid ZnO
250 50 150 8 12
250 50 150 8 12
80
20 2
20 10 20 20
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H. Hu et al. / Surface & Coatings Technology 202 (2008) 5847–5852
Fig. 3. Zn LMM Auger spectra of ZA/steel.
Fig. 1. Normalized main composition distribution in the depth by GDS of the ZAC (a) and ZA (b) coating.
Fig. 4. High-resolution XPS spectra of the Al 2p regions for ZA/steel after 5 min sputtering.
abrasive papers from 240 to 800 grits, and then degreased with acetone. The polished samples were cleaned in distilled water and dried by a cold air stream. After dip-spin in ZAC or ZA slurries, the samples were solidified and sintered in an oven. ZAC was cured at 300 °C for 15 min, while ZA could be done for 15 min at 280 °C. Undergoing four cycles of the dip-spin and solidification processes, silvery samples can be well prepared. The results by magnetic thickness tester indicated that thickness of the coatings were 8 μm around. For ZAC or ZA coatings, surface roughnesses are almost same, Ra is 0.28 μm and Rz 1.9 μm. A 75OA type glow discharge spectrum (GDS) (Leco Co. America) was employed to measure the components of ZAC/steel and ZA/steel.
X-ray photo electron spectroscopy (XPS) was used to analyze chemical composition of zinc–aluminum coatings. A PHI ESCA 5700 instrument with an Al Ka (1476.6 eV) X-ray source operating at a pressure of 10− 7 Pa was used. XPS spectra with 5 min Ar+ etching were obtained. The etching rate was about 2 nm/min. All the spectra were calibrated to the binding energy of C 1s peak at 285 eV. A scanning electron microscope (SEM, Tescan Vega TS 5136 SB) coupled with Sutw–Sapphire energy dispersive X-ray spectroscope (EDX) was used to study the morphologies and the local chemical compositions of run-out coatings. Before SEM-EDX, the samples were coated with 2–3 nm gold by evaporation.
Fig. 2. XPS spectrum of ZA coating after 5 min sputtering.
Fig. 5. Evolution of open circuit potentials of ZAC/steel and ZA/steel in seawater.
H. Hu et al. / Surface & Coatings Technology 202 (2008) 5847–5852
Coated specimens were immersed in aerated natural seawater and held at 25 °C. Electrochemical measurements were carried out after samples were immersed for different time. A conventional threeelectrode cell was utilized, consisting of a saturated calomel reference electrode (SCE), a platinum foil counter electrode and the exposed sample working electrode with a circular 1.13 cm2 surface area. The electrochemical measurements were performed using a Gamry potentiostat PCI4-750 (Gamry Instruments Inc. USA). Open
5849
circuit potentials were measured with 4 Hz sampling frequency and take the arithmetic average within 2 min as the final value. The frequency range of electrochemical impedance spectroscopy (EIS) measurements is from 105 Hz to 10− 2 Hz with 10 points per decade for the impedance measurements. The amplitude of the sinusoidal perturbation was 5 mV. Three identical specimens were prepared for all tests in this study.
Fig. 6. Impedance spectra of ZAC/steel (a) and ZA/steel (b) at different exposure times to seawater, (a1) and (b1) are Nyquist plots; whereas (a2), (b2) and (a3), (b3) are the corresponding |Z|mod plots and phase plots.
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H. Hu et al. / Surface & Coatings Technology 202 (2008) 5847–5852
3. Results and discussion Fig. 1 shows the normalized compositional depth profiles in the samples, which were prepared after two solidification cycles in ZAC or ZA slurries. The transition layer of elements is caused by the roughness of coating and the unflatness of sputtering craters by GDS. From Fig. 1, it can be seen that the enrichment of zinc relative to other elements at about 0.5 μm. Al, Cr, B, Si and Mo are distributed evenly through the total coatings. The ratio of Zn/Al in coating is corresponding to that in the slurries. Similar results were obtained by Wang [7]. Due to normalization treatment and higher concentration of Cr in ZAC than the sum of Mo and Si in ZA, the relative concentrations of Zn and Al in ZAC are lower than in ZA. Element Cr simultaneously acts as the passivator and binder. As binders, silane coupling agent has more optimized molecular structure than Cr(III). Accordingly much more Cr in ZAC than Si in ZA is essential for good adhesion of coatings. Fig. 2 presents the XPS spectra of the ZA coating on mild carbon steel after 5 min Ar+ sputtering. It can be seen that the photoelectron peak for Zn 2p appears clearly at a binding energy (BE) of 1022 eV, O 1s at BE = 531 eV and C 1s at BE = 285 eV, respectively. The binding energies of the Zn 2p3/2 XPS peaks observed on the surface of ZA coating is 1022.1 ± 0.5 eV, which cannot distinguish between the Zn2+ in ZnO (1021.80, 1022.5 eV) and metallic Zn (1021.80, 1021.90 eV) [8]. The difference in kinetic energies of Zn LMM Auger spectra between ZnO and the metallic Zn were reported to be more than 4.5 eV [9,10]. Thus, the Zn LMM Auger line was used to analyze the presence of metallic Zn in the films. The Zn LMM Auger line for ZA/ steel samples is shown in Fig. 3. In this case, the kinetic energy (KE) of Zn2+ in ZnO (987.6 eV) is quite different from the KE of Zn0 in Zn (990.7 eV). The majority of zinc exists in ZA coating in form of Zn0 and only a minor quantity of zinc are oxided during the sintering process, which endows ZA and ZAC coatings with sacrificial anode protection for steel. Fig. 4 provides high-resolution XPS spectra of the Al 2p regions for the ZA coating. Deconvolution of the Al 2p spectrum shows two components corresponding to Al in form of Al3+ at 74.8 eV and Al0 at 72.1 eV. The ratio of Al3+ and Al0 to total aluminum is 32.3% and 67.7% respectively. Compared with hot-dip galvanized Zn–Al alloy coatings [11], the quantity of Al3+ in sintered zinc–aluminum coating is much lower. Fig. 5 shows the evolution of open circuit potentials of ZAC/steel and ZA/steel in seawater. For mild carbon steel substrates, ZA or ZAC coatings act effectively as sacrificial anodes only when their potentials are more negative than −0.85 V (vs SCE) [12]. It is observed that OCP of ZA/steel drastically shifts positively to −0.72 V (vs SCE) after 5 days of exposure. After that, the ZA/steel OCP shifts to more positive values gradually. Finally, OCP of ZA varies around −0.6 V (vs SCE). Compared with ZA/steel, the general trend of ZAC/steel OCP shifts positively evenly and some “fall points” distribute at intervals. The potential values of these “fall points” are around −1.0 V, which is near to the initial potential of ZAC/steel in seawater. Taking a view of the OCP evolution, the self-healing of ZAC endowed by chromate make the sacrificial anode protection of ZAC “slower” and “longer”, that will improve the anticorrosion property of ZAC/steel. Fig. 6 represents the impedance spectra of ZAC/steel and ZA/steel in seawater. It should be stated that the Nyquist or Bode plots corresponding those “fall points” of ZAC/steel were not included. The results show that the modulus of Z of ZAC/steel is larger than that of ZA/steel. Diversities can be seen between the impedance spectra of ZAC/steel and ZA/steel. The depressed arcs of high-frequency domain in Nyquist plots of ZAC/steel disappear after 1 day of exposure and turn to the straight line gradually. Within the first 0–3 days of immersion, two faint relaxation time constants are observed on the
Fig. 7. SEM micrograph and EDX spectrum of the rust stains for ZA/steel immersion in seawater after 50 days.
phase angle spectra [Fig. 6(a3)]. But from Fig. 6(a2), only in the first minutes of immersion for ZAC/steel, the slope in the impedance modulus suggests the clear resolution of the second time constant at low frequency-domain. The semi-circles are all incomplete and the diameters become larger and larger [Fig. 6(a1)]. For existing of Cr(III) and Cr(VI) in ZAC/steel [13], metallic zinc activated by aggressive seawater has been re-passivated by Cr. There are a small amount of compact corrosion products covering on the surface of metal flakes in ZAC coating after a long time exposure to seawater. The highfrequency time constant corresponds to the charge transfer resistance and double layer capacitance associated with the corrosion process, whereas the low-frequency process can be attributed to the mass transport across the coating filled with loose and porous corrosion products [14]. After few days immersion this low-frequency process shifts to higher frequency and amalgamates with the high-frequency process. Compared with ZAC/steel, arcs don't disappear in Nyquist plots throughout the total immersion of ZA/steel. Bode plots show that the electrochemical impedance spectroscopy spectrum of ZA/steel has three relaxation constants, except the spectrum obtained at the first moment of immersion. The first one is related to ZA coating, which is in the high-frequency domain and means the poor conductive coating, for much porous corrosion products forms in the ZA coating (see Fig. 7). The second one is a faradic process (Cdl and Rct) at the ZA/steel interface (in zinc–aluminum coating pores), followed by a finite-space diffusion process. Fig. 8 shows the Nyquist plots of ZAC/steel at some “fall points”, such as after 4 days, 10 days, 11 days, 20 days and 32 days of immersion, respectively. It is easily seen that the arcs of faradic
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Fig. 8. Nyquist plots of ZAC/steel after immersion in seawater for different times.
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(2) The electrochemical data, which presents in this study, may be the direct evidence for the self-healing of Cr(VI). There are some “fall points” or “active points” in the evolutions of open circuit potential, which is exactly corresponding to the developments of impedance behaviors and impedance values for zinc– aluminum–chromium in seawater. (3) Combination of molybdate and silane may act as alternatives for chromate, but the self-healing action of coatings is missing. Acknowledgement Thanks to Prof. Sun for his assistances of XPS measurement. References Fig. 9. Evolution of |Z|0.1
Hz
for ZA/steel and ZAC/steel systems with immersion time.
process are more visible and the impedance values of “fall points” are much lower than others. So these “fall points” could be called “active points”, those may correspond to the time when the active metallic zinc or aluminum exposed to aggressive medium directly. The similar evolution laws in OCP were observed in Fig. 5. But the time for ZAC/ steel remaining “active” has not been measured exactly, which should be carried out by other methods such as electrochemical noise. This is the major project afterwards. It was proposed that the impedance at low frequencies (such as |Z|0.1 Hz) could serve as an evaluation parameter for a painted metal's barrier effect [15,16]. As shown in Fig. 9, the |Z|0.1 Hz of ZAC/steel is higher than that of ZA/steel during the whole immersion process. This indicates that the protective performance of ZAC coating is much higher than ZA coating. The evolutions of |Z|0.1 Hz of ZAC/steel and ZA/steel in Fig. 9 have the similar behaviors to those OCP in Fig. 5. Especially, at those “fall points”, |Z|0.1 Hz of ZAC/steel also reduces drastically. 4. Conclusions (1) Elements distribute evenly through the coating. Most of zinc and aluminum exist in form of metallic state in sintered ceramic coating. In chromium-free zinc–aluminum coating, the ratio of Al3+ and Al0 to the total aluminum is 32.2% and 67.7%.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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