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Corrosion behavior of a zirconium-titanium based phosphonic acid conversion coating on AA6061 aluminium alloy
Acta Metall. Sin.(Engl. Lett.)Vol.21 No.4 pp269-274 Aug. 2008
CORROSION BEHAVIOR OF A ZIRCONIUM-TITANIUM BASED PHOSPHONIC ACID CONVERSION COATING ON AA6061 ALUMINIUM ALLOY S.H. Wang1)∗ , C.S. Liu1) and F.J. Shan1,2) 1) Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China 2) Material & Chemical Engineering College, Liaoning University of Technology, Jinzhou 121001, China Manuscript received 29 September 2007; in revised form 28 November 2007
The conversion coating was formed by dipping AA6061 in a fluorotitanate/zirconate acid and amino trimethylene phosphonic acid (ATMP) solution at room temperature. The formation process and the anti-corrosion performance of the conversion coating were investigated using electrochemical test and salt spray test (SST), respectively. The electrochemical test shows that the Zr/Ti and ATMP coating improves the corrosion resistance of AA6061 as good as the chromate (VI) coating. But the results of SST show that the corrosion resistance of Zr/Ti and ATMP coating is not as good as the chromate (VI) coating. The corrosion area is less than 2% after 72 h. KEY WORDS Corrosion resistance; Fluorotitanate/zirconate acid; Phosphonic acid; Aluminium alloy; Conversion coating
1. Introduction Traditionally, conversion coatings on aluminium alloys are based on chromium chemistry. Notwithstanding the very good corrosion protection performance of chromate conversion coatings, hexavalent chromium is both toxic and carcinogenic[1] . Due to environmental and health concerns[2] , various efforts have been performed to abolish it from all manufacturing processes, including conversion coating. In the past years, zirconium and titanium-based coatings have found[3,4] some applications in certain market niches, but they have failed to replace chromating as a pretreatment before painting. Recently, a novel surface conversion coating technique namely “in situ phosphatizing coatings (ISPC)” has been developed[5−8] . The chemical bonds linked to the paint polymers to seal the pores of metal phosphate in situ should enhance coating adhesion and suppress metal corrosion. Azusa[8] indicated that self-assembled zirconium-phosphate derivative films effectively protected aluminum substrates. In particular, the multi-layered zirconium-phosphate film fabricated by the adsorption of 1,12-dodecyldiphosphonic acid has shown superior anticorrosion properties to Al1100. In this article, we describe a chromium-free conversion treatment based on fluorotiCorresponding author. Tel.: +86 24 81648618. E-mail address: [email protected] (S.H. Wang)
∗
· 270 · tanate/zirconate acid and amino trimethylene phosphonic acid (ATMP), to replace chromate treatments for the protection of aluminium alloys. 2. Experimental 2.1 Pretreatment and preparation of substrate The AA6061 sheets (20 mm×40 mm×2 mm) were taken from aluminium wheels. The sheets surface was further processed by milling to a bright surface with sand paper (250# –800# ), and cleaned with ethanol and water before treatment. The conversion coating of the alloy was carried out by using the following methods: Step (1) Degreased in alkaline solution for 2 min at 45◦ C and rinsed in tap water. Step (2) Etched for 1 min by immersing in acid picking solution (H2 SO4 and HF) at room temperature and then thoroughly rinsed with distilled water. Step (3) The formed coating was immersed in conversion solution for 30 s–180 s at room temperature. The conversion solution mainly contained H2 TiF6 , H2 ZrF6 and ATMP. The pH value of solution was adjusted to 3–3.5 with ammonia. To contrast, chromate sheets were prepared using the cleaning procedure previously described, followed by immersion in a treating solution (6.4% H3 PO4 , 0.9% CrO3 and 0.3% NaF). Step (4) Rinsed in distilled water, followed by drying for 30 min in an air circulation furnace at 100◦ C. 2.2 Electrochemical analysis To follow the deposition of the conversion coating, open circuit potential (OCP) was recorded as a function of time using a CHI660B electrochemistry workstation during dipping in conversion solution. The OCP measurements were performed using a three electrode pyrex glass cell with a saturated calomel electrode (SCE) as reference and a platinum counter electrode as assistant. The OCP was recorded for an immersion time of 300 s in the conversion solution. The solution conditions were the same as in the case of conversion coatings. To assess the corrosion behavior of the coatings, electrochemical impedance spectroscopy and Tafel polarization measurements were tested in 5% NaCl solution (open to air). The sheets were clamped to the cell, sealed by a scotch tape. The exposed area of the electrode was 1 cm2 . The scan rate was 1 mV/s. The sheets were immersed in the electrolyte for 5 min before measurement. The EIS frequency was ranged from 1 kHz to 10 mHz with 12 steps per decade. 2.3 Salt spray testing To evaluate the corrosion protection of the conversion coating without topcoats, according to GB/T 10125-1997 (salt spray test, SST), 5% NaCl was atomized in a salt spray chamber at 35◦ C with a solution of pH approximately 7. The tested sheets were placed at an angle of 20◦ ±5◦ in the chamber, exposed to the salt fog for a certain period. 3. Results and Discussion 3.1 Open circuit potential during conversion coating The evolution of open circuit potential (OCP) during the whole process of immersion in fluorotitanate/zirconate and ATMP solution is given in Fig.1. The general trend of the
· 271 · -0.7 -0.8
E/V
-0.9 -1.0 -1.1 -1.2 -1.3 -1.4
0
30
60
90
120
150
180
210
240
270
300
Time / s
Fig.1 Evolution of open circuit potential of AA6061 during the whole process. -2 3 5 2 4 1
-4 -2
lg(i / A cm )
OCP consisted of a decrease during the first seconds and afterwards maintained a constant value (about −1.36 V vs. SCE) until the end of the conversion treatment. The initial fast decrease of OCP was related to the dissolution of aluminium and its oxide caused by the presence of fluoride compounds in the conversion bath. However, after seconds, the increase in the cathodic current due to the reduction of titanate/zirconate partially balanced the decrease in OCP caused by the anodic dissolution of aluminium, giving rise to a slower decrease in OCP. After 60 s, the sheet surface was completely covered by a relatively thick conversion coating, but it contained pores and defects, and the OCP reached a constant value. This was caused by the thickening of the conversion layer that lowered the possibility of both the electrons tunneling through the coating itself and access of the solution to the aluminium surface and therefore educed the growth rate of coating, i.e. the anodic and cathodic currents. The decrease in anodic current caused an increase in OCP, whereas the decrease in cathodic current caused a decrease in OCP, then the two effects compensated each other and the OCP remained constant until the end of the conversion treatment.
3 5
4
-6
-8 -1.0
-0.9
-0.8
-0.7
1
30 s
2
60 s
3
90 s
4
120 s
5
180 s
-0.6
-0.5
E/V
Fig.2 Polarization curves for Zr/Ti and ATMP for different times.
3.2 Electrochemical behavior Fig.2 shows the potentiodynamic polarization curves in 5% NaCl for AA6061 after 30–180 s passivation in the fluorotiFig.3 Electrochemical impedance spectanate/zirconate and ATMP acid solution. troscopy of Zr/Ti and ATMP for difCorresponding fitting results are shown in ferent times. Table 1. The corrosion potential moved toward negative direction with the increase in passivation time. This suggested the tendency of an oxide coating to inhibit the cathodic reactions. After 90 s, the corrosion potential became unchanged. The highest corrosion potential was −0.72 V vs. SCE. Fitting results in Table 1 show that the corrosion current density was the same order from 30 s to 180 s and current density increased after decreased first, the lowest was 120 s and the highest was 180 s. EIS of different time (Fig.3) indicated that the difference of inductance arc size was not high except 180 s. This was attributed to the further dissolution of newly formed coating in acidic solution after the treatment for long time.
· 272 · Table 1 Electrochemical experiment results of Zr/Ti and ATMP for different times Time/s
Ecorr /V
Icorr /(µA/cm2 )
ba /V
bc /V
30 60 90 120 180
−0.698 −0.708 −0.721 −0.721 −0.722
2.292 2.205 2.474 1.603 9.127
21.855 23.240 17.351 30.130 24.352
1.857 2.371 2.171 2.931 2.725
Table 2 Electrochemical experiment results of bare, chromate and Zr/Ti and ATMP Sample
Ecorr /V
Icorr /(µA/cm2 )
ba /mV
bc /mV
η/%
Bare Chromate Zr/Ti and ATMP
−0.683 −0.697 −0.721
345.6 2.184 1.603
1.049 17.183 30.130
1.209 1.733 2.931
0 99.37 99.53
-1 -2
-2
lg(i / A cm )
Fig.4 shows that the potentiodynamic polarization curves of Zr/Ti and ATMP treated for 120 s are in contrast to that of chromate and the bare. Corresponding fitting results are shown in Table 2. Both the chromate and the Zr/Ti and ATMP treatments had a significant passivation effect on the electrochemical behavior of the bare. The corrosion potentials were displaced in the cathodic direction. The negative shift of the corrosion of the Zr/Ti and ATMP treatments was higher than those of the chromate treatment. However, the cathodic current density of the sheets treated with Zr/Ti and ATMP was lower compared with chromate. This suggested that cathodic reactions of Zr/Ti and ATMP coatings like reduction of oxygen or evolution of hydrogen was comparatively complicated. The Nyquist diagram of AA6061 sheets treated with Zr/Ti and ATMP can be seen in Fig.5. The treatments with both Zr/Ti and ATMP and the chromate resulted in higher corrosion resistance protection, compared with the bare. The Zr/Ti and ATMP treatment showed the same diameter as the chromate. That showed the trend of corrosion reaction on the Zr/Ti and ATMP coating was much weaker. The inductance arc in low frequency section indicated the occurrence of the pitting process.
-3
1
Zr/Ti and ATMP,120 s
2
Bare
3
Chromate
-4
2 1
-5
3 -6 -7 -8 -9 -1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
E/V
Fig.4 Polarization curves for bare, chromate and Zr/Ti and ATMP.
Fig.5 Electrochemical impedance spectroscopy for bare, chromate and Zr/Ti and ATMP.
· 273 ·
Fig.6 Photos of SST: (a) Bare, 72 h; (b) Chromate, 72 h; (c) Chromate, 96 h; (d) Chromate, 120 h; (e) Zr/ Ti and ATMP, 72 h; (f) Zr/Ti and ATMP, 96 h; (g) Zr/Ti and ATMP, 120 h.
In general, the electrochemical test showed that the Zr/Ti and ATMP coating improved the corrosion resistance of AA6061 similar to the chromate (VI) coating. 4. Salt Spray Testing Fig.6 shows the results of salt spray analysis for AA6061 sheets treated with Zr/Ti and ATMP solution for 120 s at room temperature without topcoats. After exposure for 72 h, for the bare sheet (Fig.6a), white stain and the pitting were observed on the surface and covered the entire surface of the sheet, white stain generated area exceeded 90%; for the chromate treated sheets (Fig.6b), no localized pitting and no white stain appeared; for the Zr/Ti and ATMP treated sheets (Fig.6e), no white stain appeared but localized pitting was observed and corrosion area was less than 2%. After 96 h, the localized pitting and white stain appeared on the Zr/Ti and ATMP coating and the corrosion area was approximately 10% (Fig.6f). No evidence of corrosion was observed on the chromate coating till 120 h (Fig.6c and Fig.6d). After 120 h, the white stain and pitting area on the Zr/Ti and ATMP coating was below 20% (Fig.6g). In general, the Zr/Ti and ATMP coating significantly improved the corrosion resistance of AA6061, but not as good as the chromate (VI) coating. The corrosion area was less than 2% after 72 h.
· 274 · 5. Conclusions The deposition of Zr/Ti and ATMP conversion coating on AA6061 aluminium alloy had been studied. The electrochemical test shows that the Zr/Ti and ATMP coating improves the corrosion resistance of AA6061 as good as the chromate (VI) coating. But the results of SST show that the corrosion resistance of Zr/Ti and ATMP coating is not as good as the chromate (VI) coating. The corrosion area is less than 2% after 72 h. Acknowledgements—This work was supported by the Science and Technology Plan Project of Liaoning Province, China (No. 2006221011). REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]
U.S. Public Health Service, Report No. ATDSR/TP-88/10 (Washington, DC, 1989). D.J. Mccoy, Proc. of the 2nd AESF/EPA Chromium Colloquium (Miami, Florida, 1990). O. Lunder, C. Simensen, Y. Yu and K. Nisancioglu, Surf. Coat. Tech. 184 (2004) 278. H. Vennschott, U. Karmashek and A. Roland, US Patent US5584946 (17 December 1996). T.L. Chhiu, Prog. Org. Coat. 42 (2001) 226. H. Neuder, S. Charles, K. Mark, C. Robert and T.L. Chhiu, Prog. Org. Coat. 47 (2003) 225. M.C. Whitten and T.L. Chhiu, Prog. Org. Coat. 38 (2000) 151. S. Azusa, S. Hiroyuki, F. Masanobu and T. Osamu, Surf. Coat. Tech. 169-170 (2003) 686.
CORROSION BEHAVIOR OF A ZIRCONIUM-TITANIUM BASED PHOSPHONIC ACID CONVERSION COATING ON AA6061 ALUMINIUM ALLOY S.H. Wang1)∗ , C.S. Liu1) and F.J. Shan1,2) 1) Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China 2) Material & Chemical Engineering College, Liaoning University of Technology, Jinzhou 121001, China Manuscript received 29 September 2007; in revised form 28 November 2007
The conversion coating was formed by dipping AA6061 in a fluorotitanate/zirconate acid and amino trimethylene phosphonic acid (ATMP) solution at room temperature. The formation process and the anti-corrosion performance of the conversion coating were investigated using electrochemical test and salt spray test (SST), respectively. The electrochemical test shows that the Zr/Ti and ATMP coating improves the corrosion resistance of AA6061 as good as the chromate (VI) coating. But the results of SST show that the corrosion resistance of Zr/Ti and ATMP coating is not as good as the chromate (VI) coating. The corrosion area is less than 2% after 72 h. KEY WORDS Corrosion resistance; Fluorotitanate/zirconate acid; Phosphonic acid; Aluminium alloy; Conversion coating
1. Introduction Traditionally, conversion coatings on aluminium alloys are based on chromium chemistry. Notwithstanding the very good corrosion protection performance of chromate conversion coatings, hexavalent chromium is both toxic and carcinogenic[1] . Due to environmental and health concerns[2] , various efforts have been performed to abolish it from all manufacturing processes, including conversion coating. In the past years, zirconium and titanium-based coatings have found[3,4] some applications in certain market niches, but they have failed to replace chromating as a pretreatment before painting. Recently, a novel surface conversion coating technique namely “in situ phosphatizing coatings (ISPC)” has been developed[5−8] . The chemical bonds linked to the paint polymers to seal the pores of metal phosphate in situ should enhance coating adhesion and suppress metal corrosion. Azusa[8] indicated that self-assembled zirconium-phosphate derivative films effectively protected aluminum substrates. In particular, the multi-layered zirconium-phosphate film fabricated by the adsorption of 1,12-dodecyldiphosphonic acid has shown superior anticorrosion properties to Al1100. In this article, we describe a chromium-free conversion treatment based on fluorotiCorresponding author. Tel.: +86 24 81648618. E-mail address: [email protected] (S.H. Wang)
∗
· 270 · tanate/zirconate acid and amino trimethylene phosphonic acid (ATMP), to replace chromate treatments for the protection of aluminium alloys. 2. Experimental 2.1 Pretreatment and preparation of substrate The AA6061 sheets (20 mm×40 mm×2 mm) were taken from aluminium wheels. The sheets surface was further processed by milling to a bright surface with sand paper (250# –800# ), and cleaned with ethanol and water before treatment. The conversion coating of the alloy was carried out by using the following methods: Step (1) Degreased in alkaline solution for 2 min at 45◦ C and rinsed in tap water. Step (2) Etched for 1 min by immersing in acid picking solution (H2 SO4 and HF) at room temperature and then thoroughly rinsed with distilled water. Step (3) The formed coating was immersed in conversion solution for 30 s–180 s at room temperature. The conversion solution mainly contained H2 TiF6 , H2 ZrF6 and ATMP. The pH value of solution was adjusted to 3–3.5 with ammonia. To contrast, chromate sheets were prepared using the cleaning procedure previously described, followed by immersion in a treating solution (6.4% H3 PO4 , 0.9% CrO3 and 0.3% NaF). Step (4) Rinsed in distilled water, followed by drying for 30 min in an air circulation furnace at 100◦ C. 2.2 Electrochemical analysis To follow the deposition of the conversion coating, open circuit potential (OCP) was recorded as a function of time using a CHI660B electrochemistry workstation during dipping in conversion solution. The OCP measurements were performed using a three electrode pyrex glass cell with a saturated calomel electrode (SCE) as reference and a platinum counter electrode as assistant. The OCP was recorded for an immersion time of 300 s in the conversion solution. The solution conditions were the same as in the case of conversion coatings. To assess the corrosion behavior of the coatings, electrochemical impedance spectroscopy and Tafel polarization measurements were tested in 5% NaCl solution (open to air). The sheets were clamped to the cell, sealed by a scotch tape. The exposed area of the electrode was 1 cm2 . The scan rate was 1 mV/s. The sheets were immersed in the electrolyte for 5 min before measurement. The EIS frequency was ranged from 1 kHz to 10 mHz with 12 steps per decade. 2.3 Salt spray testing To evaluate the corrosion protection of the conversion coating without topcoats, according to GB/T 10125-1997 (salt spray test, SST), 5% NaCl was atomized in a salt spray chamber at 35◦ C with a solution of pH approximately 7. The tested sheets were placed at an angle of 20◦ ±5◦ in the chamber, exposed to the salt fog for a certain period. 3. Results and Discussion 3.1 Open circuit potential during conversion coating The evolution of open circuit potential (OCP) during the whole process of immersion in fluorotitanate/zirconate and ATMP solution is given in Fig.1. The general trend of the
· 271 · -0.7 -0.8
E/V
-0.9 -1.0 -1.1 -1.2 -1.3 -1.4
0
30
60
90
120
150
180
210
240
270
300
Time / s
Fig.1 Evolution of open circuit potential of AA6061 during the whole process. -2 3 5 2 4 1
-4 -2
lg(i / A cm )
OCP consisted of a decrease during the first seconds and afterwards maintained a constant value (about −1.36 V vs. SCE) until the end of the conversion treatment. The initial fast decrease of OCP was related to the dissolution of aluminium and its oxide caused by the presence of fluoride compounds in the conversion bath. However, after seconds, the increase in the cathodic current due to the reduction of titanate/zirconate partially balanced the decrease in OCP caused by the anodic dissolution of aluminium, giving rise to a slower decrease in OCP. After 60 s, the sheet surface was completely covered by a relatively thick conversion coating, but it contained pores and defects, and the OCP reached a constant value. This was caused by the thickening of the conversion layer that lowered the possibility of both the electrons tunneling through the coating itself and access of the solution to the aluminium surface and therefore educed the growth rate of coating, i.e. the anodic and cathodic currents. The decrease in anodic current caused an increase in OCP, whereas the decrease in cathodic current caused a decrease in OCP, then the two effects compensated each other and the OCP remained constant until the end of the conversion treatment.
3 5
4
-6
-8 -1.0
-0.9
-0.8
-0.7
1
30 s
2
60 s
3
90 s
4
120 s
5
180 s
-0.6
-0.5
E/V
Fig.2 Polarization curves for Zr/Ti and ATMP for different times.
3.2 Electrochemical behavior Fig.2 shows the potentiodynamic polarization curves in 5% NaCl for AA6061 after 30–180 s passivation in the fluorotiFig.3 Electrochemical impedance spectanate/zirconate and ATMP acid solution. troscopy of Zr/Ti and ATMP for difCorresponding fitting results are shown in ferent times. Table 1. The corrosion potential moved toward negative direction with the increase in passivation time. This suggested the tendency of an oxide coating to inhibit the cathodic reactions. After 90 s, the corrosion potential became unchanged. The highest corrosion potential was −0.72 V vs. SCE. Fitting results in Table 1 show that the corrosion current density was the same order from 30 s to 180 s and current density increased after decreased first, the lowest was 120 s and the highest was 180 s. EIS of different time (Fig.3) indicated that the difference of inductance arc size was not high except 180 s. This was attributed to the further dissolution of newly formed coating in acidic solution after the treatment for long time.
· 272 · Table 1 Electrochemical experiment results of Zr/Ti and ATMP for different times Time/s
Ecorr /V
Icorr /(µA/cm2 )
ba /V
bc /V
30 60 90 120 180
−0.698 −0.708 −0.721 −0.721 −0.722
2.292 2.205 2.474 1.603 9.127
21.855 23.240 17.351 30.130 24.352
1.857 2.371 2.171 2.931 2.725
Table 2 Electrochemical experiment results of bare, chromate and Zr/Ti and ATMP Sample
Ecorr /V
Icorr /(µA/cm2 )
ba /mV
bc /mV
η/%
Bare Chromate Zr/Ti and ATMP
−0.683 −0.697 −0.721
345.6 2.184 1.603
1.049 17.183 30.130
1.209 1.733 2.931
0 99.37 99.53
-1 -2
-2
lg(i / A cm )
Fig.4 shows that the potentiodynamic polarization curves of Zr/Ti and ATMP treated for 120 s are in contrast to that of chromate and the bare. Corresponding fitting results are shown in Table 2. Both the chromate and the Zr/Ti and ATMP treatments had a significant passivation effect on the electrochemical behavior of the bare. The corrosion potentials were displaced in the cathodic direction. The negative shift of the corrosion of the Zr/Ti and ATMP treatments was higher than those of the chromate treatment. However, the cathodic current density of the sheets treated with Zr/Ti and ATMP was lower compared with chromate. This suggested that cathodic reactions of Zr/Ti and ATMP coatings like reduction of oxygen or evolution of hydrogen was comparatively complicated. The Nyquist diagram of AA6061 sheets treated with Zr/Ti and ATMP can be seen in Fig.5. The treatments with both Zr/Ti and ATMP and the chromate resulted in higher corrosion resistance protection, compared with the bare. The Zr/Ti and ATMP treatment showed the same diameter as the chromate. That showed the trend of corrosion reaction on the Zr/Ti and ATMP coating was much weaker. The inductance arc in low frequency section indicated the occurrence of the pitting process.
-3
1
Zr/Ti and ATMP,120 s
2
Bare
3
Chromate
-4
2 1
-5
3 -6 -7 -8 -9 -1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
E/V
Fig.4 Polarization curves for bare, chromate and Zr/Ti and ATMP.
Fig.5 Electrochemical impedance spectroscopy for bare, chromate and Zr/Ti and ATMP.
· 273 ·
Fig.6 Photos of SST: (a) Bare, 72 h; (b) Chromate, 72 h; (c) Chromate, 96 h; (d) Chromate, 120 h; (e) Zr/ Ti and ATMP, 72 h; (f) Zr/Ti and ATMP, 96 h; (g) Zr/Ti and ATMP, 120 h.
In general, the electrochemical test showed that the Zr/Ti and ATMP coating improved the corrosion resistance of AA6061 similar to the chromate (VI) coating. 4. Salt Spray Testing Fig.6 shows the results of salt spray analysis for AA6061 sheets treated with Zr/Ti and ATMP solution for 120 s at room temperature without topcoats. After exposure for 72 h, for the bare sheet (Fig.6a), white stain and the pitting were observed on the surface and covered the entire surface of the sheet, white stain generated area exceeded 90%; for the chromate treated sheets (Fig.6b), no localized pitting and no white stain appeared; for the Zr/Ti and ATMP treated sheets (Fig.6e), no white stain appeared but localized pitting was observed and corrosion area was less than 2%. After 96 h, the localized pitting and white stain appeared on the Zr/Ti and ATMP coating and the corrosion area was approximately 10% (Fig.6f). No evidence of corrosion was observed on the chromate coating till 120 h (Fig.6c and Fig.6d). After 120 h, the white stain and pitting area on the Zr/Ti and ATMP coating was below 20% (Fig.6g). In general, the Zr/Ti and ATMP coating significantly improved the corrosion resistance of AA6061, but not as good as the chromate (VI) coating. The corrosion area was less than 2% after 72 h.
· 274 · 5. Conclusions The deposition of Zr/Ti and ATMP conversion coating on AA6061 aluminium alloy had been studied. The electrochemical test shows that the Zr/Ti and ATMP coating improves the corrosion resistance of AA6061 as good as the chromate (VI) coating. But the results of SST show that the corrosion resistance of Zr/Ti and ATMP coating is not as good as the chromate (VI) coating. The corrosion area is less than 2% after 72 h. Acknowledgements—This work was supported by the Science and Technology Plan Project of Liaoning Province, China (No. 2006221011). REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]
U.S. Public Health Service, Report No. ATDSR/TP-88/10 (Washington, DC, 1989). D.J. Mccoy, Proc. of the 2nd AESF/EPA Chromium Colloquium (Miami, Florida, 1990). O. Lunder, C. Simensen, Y. Yu and K. Nisancioglu, Surf. Coat. Tech. 184 (2004) 278. H. Vennschott, U. Karmashek and A. Roland, US Patent US5584946 (17 December 1996). T.L. Chhiu, Prog. Org. Coat. 42 (2001) 226. H. Neuder, S. Charles, K. Mark, C. Robert and T.L. Chhiu, Prog. Org. Coat. 47 (2003) 225. M.C. Whitten and T.L. Chhiu, Prog. Org. Coat. 38 (2000) 151. S. Azusa, S. Hiroyuki, F. Masanobu and T. Osamu, Surf. Coat. Tech. 169-170 (2003) 686.