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New approaches for sealing anodic coatings
New Approaches for Sealing Anodic Coatings Aluminum substrate can be sealed with sol-gel method after anodic oxidation in sulfuric acid. By M. Zemanov~ and M. Chovancov~, Department of Inorganic Technology, Slovak University of Technology in Bratislava
ealing of anodic alumina enhances the corrosion resistance of p r e p a r e d coatings and increases the UV-light resistance of dyes in anodic coatings. Traditional sealing processes, such as hot water sealing, steam sealing, and cold nickel sealing 1,2,3 are well established while other sealing options are being investigated. Among the most promising new sealing methods are those focused on antismutting agents and electrochemical sealing. 1 The new approaches, as well as the established alternatives, address the same requirements--corrosion resistance, abrasion resistance, and hardness w i t h o u t posing environmental problems. Another promising approach for sealing of anodic coatings is to form a protective layer with a sol-gel method. The glass-type layer is especially good for corrosion resistance of anodically oxidized aluminum.
S
PRINCIPLE Sealing follows immediately after the anodic oxidation of aluminum. The principle of the sol-gel process is to first prepare a sol (i.e., liquid colloidal dispersion) by hydrolysis of organometallic compounds and then have the coating gel (solidify) to form the hard coating. The sintering process finishes the production of the hard seal coat. The characteristics of the coating developed is a function of both the starting material and conditions of hydrolysis reaction. Our process of creating the coating uses a colloidal method and a dip technique is used. By "colloidal method" of sol-gel coating, preparation we mean the hydrolysis of organometallic compound ( a l u m i n u m butoxide), which is obtained with an excess stoichometric water. The thermal behavior of three components should be taken into account. Aluminum substrate, anodic a l u m i n a coating, and sol-gel a l u m i n a layer each have different coefficients of thermal expansion. The strategy in producing a crack-free coating is to optimize all of the parameters influencing the process. 4 The regulation of anodic alumina thickness, sol-gel alumina thickness, and the thermal regime were each investigated and optimization of each facet contributed to the successful sealing of porous alumina oxide. The investigation of the anticorrosive properties of the material follows. N~-tnl-~r ~N0~
TECHNOLOGICAL PROCESS The steps in the process include degreasing, water wash, alkali etch, water wash, weak acid etch, water wash, anodic oxidation, water wash, second water wash, and sealing. The water wash uses tap water or treated water. The second water wash uses distilled or deionized water. The alkaline etch is composed of 100 g/L NaOH used at 70 to 80°C, for i to 5 rain. to remove the natural oxide film on the aluminum. The acid corrosion etch consists of 50% nitric acid at 70°C for 3 to 5 rain. It neutralizes the remains of alkali and cleans out the smut on the surface of aluminum from the alkali solution. Anodizing 180 to 220 g/L H2SO 4 with an aluminum cathode at 18 to 20°C is carried out in 1.5 A/din 2 (14-16 V) for 20 min. (thickness of layer 10 ram). The sealing is conducted at an operating temperature of 270°C for 15 min. at controlled heating and cooling of 10°C/min. The samples were dipped into AI203 sol before the sealing process. 5 RESULTS A N D D I S C U S S I O N A n o d i c o x i d a t i o n : It is very i m p o r t a n t to work with an acceptable layer thickness of the anodic alumina for this type of sealing. The time of anodic oxidation was the parameter controlling the thickness of the anodic layer. The current density remains constant at 1.5 A/din 2 and time of anodic oxidation was varied from 10 to 40 minutes. The aim was to reach a thickness to keep the protective properties of the anodic alumina layer but also to prepare it for sealing by the sol-gel method to avoid cracks. A 20 minute anodic oxidation to reach a thickness of 10 ~m is proper for the following sol-gel type of sealing. Sol-gel sealing: The thickness of sol-gel coatings is dependent on sol viscosity and the velocity of withdrawing. 6 In our case the velocity of withdrawing of the sample from the sol was constant, at 11.7 cm/min, and the molar ratio r [n (alkoxide)/n (water), where n is number of moles] was varied (Figure 1.). The thickness d was calculated with usage of a gravimetric method and under the assumption of the alumina density according to equation: (l) d = ( m 2 - m 1) / ( S x p) where m 2 is the mass of the sample after sol-gel alumi33
0.5
15
0.4 10 E
=L
0.3
E
0.2
E
]
0.1 100
120
140
1 160
180
5
r / n w a t e r : n a l u m i n u m butoxide
na deposition [kg], m 1 is mass of the sample after anodic oxidation [kg], S is area of the treated sample [m2], P is the density of the sol-gel alumina coating [kg/m3]. The dependence of the thickness on the molar ratio is a parabolic curve with local minimum. The dependence can be described by the mathematical equation derived by Landau and LevichT: d = [0.94 x (~x ~) 2/3]/[7LV 1/6 (p xg)l/2 ] where q means viscosity of the sol [Pa.s ], g is liquidvapor surface tension [N/m2], and r means the density of the sol solution [kg/m3]. The thickness of sol-gel alumina coating in combination with the other material component thicknesses is a parameter influencing the crack formation and the corrosion resistance of the material. A mixture of transition aluminas was identified for sol-gel type of coating, especially the transition &A1203. The resistance of the samples against atmospheric corrosion was tested in the constant h u m i d i t y c h a m b e r ASTM D 2247 (100% air h u m i d i t y and 35°C). The corrosion resistance of the coating sealed with sol (n alkoxide/n water = 1/150, withdrawing velocity 11.7 cm/min) and t h e r m a l l y t r e a t e d was compared with hydrothermal sealing (30 min). Time of anodic oxidation was 20 minutes before dipping into sol and 40 minutes before hydrothermal sealing. Figure 2 contrasts the changes in the coatings produced by the sol-gel method and conventional hydrothermal method. It is clear that enough mass has been created to confirm the increased resistance against atmospheric corrosion for sol type sealing. Blooming appeared on samples sealed hydrothermally. The weight of the sample increases for both types of sealing, although in the case of sealing with sol-gel the weight change is stabilized at the value of 16 mg/dm 2.
15
20
25
30
t/day
200
Figure 1: Dependence of the sol-gel alumina thickness on the molar ratio of aluminum alkoxide to water on anodized aluminum.The values are averaged from six measurements.
34
10
Figure 2: Dependence of the mass changes on time for: (a) anodized aluminum sealed by sol-gel alumina coating; and (b) anodized aluminum sealed by hydrothermal way.
For hydrothermal treated anodized aluminum, the curve shows no tendency to stabilize. The common corrosion mechanism employed for this type of corrosion on anodized aluminum is an oxygen concentration cell. The mechanism is not applicable for the glassy type of coating. The formation of secondary products on the coatings is a reason for the weight growth. Thermal t r e a t m e n t doesn't influence either the aluminum substrate or the anodic alumina. CONCLUSION
A l u m i n u m s u b s t r a t e can be sealed w i t h sol-gel coating after anodic oxidation in sulfuric acid. Corrosion resistance a g a i n s t a t m o s p h e r i c corrosion is comparable with h y d r o t h e r m a l sealing and even higher. The disadvantage of this process is its high price and decreased abrasion resistance and hardness. In spite of this, for special purposes this type of sealing is acceptable. REFERENCES 1. Wernick, S. et al., "The Surface Treatment and Finishing of Aluminum and Its Alloys," 5th Ed., Vol. 2, Chapters 6 and 11, Finishing Publications Ltd., Teddington, England; 1996. 2. Brace, A.W. and Sheasby, P.G., "The Technology of Anodizing Aluminum," 2nd Ed., Chapter 16, Technicopy Limited, England; 1979. 3. Kalantary, M.R:et al., J. Appl. Electrochem., 22:268; 1992. 4. Zemanov~, M. et al., Chem. Papers, 50 (2):55; 1996. 6. Brinker, C.J. and Scherer, G.W., "Sol-Gel Science," 1st Ed., Chapter 13, Academic Press, Boston; 1990. 7. Landau, L.D. and Levich, B.G.,Acta Physiochirn., 17:42; 1942.
For more information, (e-mail) [email protected]. Editor's Note:
This article was previously published in Metal rl}f
Finishing, December 2003.
www.metalfinishing.com
ealing of anodic alumina enhances the corrosion resistance of p r e p a r e d coatings and increases the UV-light resistance of dyes in anodic coatings. Traditional sealing processes, such as hot water sealing, steam sealing, and cold nickel sealing 1,2,3 are well established while other sealing options are being investigated. Among the most promising new sealing methods are those focused on antismutting agents and electrochemical sealing. 1 The new approaches, as well as the established alternatives, address the same requirements--corrosion resistance, abrasion resistance, and hardness w i t h o u t posing environmental problems. Another promising approach for sealing of anodic coatings is to form a protective layer with a sol-gel method. The glass-type layer is especially good for corrosion resistance of anodically oxidized aluminum.
S
PRINCIPLE Sealing follows immediately after the anodic oxidation of aluminum. The principle of the sol-gel process is to first prepare a sol (i.e., liquid colloidal dispersion) by hydrolysis of organometallic compounds and then have the coating gel (solidify) to form the hard coating. The sintering process finishes the production of the hard seal coat. The characteristics of the coating developed is a function of both the starting material and conditions of hydrolysis reaction. Our process of creating the coating uses a colloidal method and a dip technique is used. By "colloidal method" of sol-gel coating, preparation we mean the hydrolysis of organometallic compound ( a l u m i n u m butoxide), which is obtained with an excess stoichometric water. The thermal behavior of three components should be taken into account. Aluminum substrate, anodic a l u m i n a coating, and sol-gel a l u m i n a layer each have different coefficients of thermal expansion. The strategy in producing a crack-free coating is to optimize all of the parameters influencing the process. 4 The regulation of anodic alumina thickness, sol-gel alumina thickness, and the thermal regime were each investigated and optimization of each facet contributed to the successful sealing of porous alumina oxide. The investigation of the anticorrosive properties of the material follows. N~-tnl-~r ~N0~
TECHNOLOGICAL PROCESS The steps in the process include degreasing, water wash, alkali etch, water wash, weak acid etch, water wash, anodic oxidation, water wash, second water wash, and sealing. The water wash uses tap water or treated water. The second water wash uses distilled or deionized water. The alkaline etch is composed of 100 g/L NaOH used at 70 to 80°C, for i to 5 rain. to remove the natural oxide film on the aluminum. The acid corrosion etch consists of 50% nitric acid at 70°C for 3 to 5 rain. It neutralizes the remains of alkali and cleans out the smut on the surface of aluminum from the alkali solution. Anodizing 180 to 220 g/L H2SO 4 with an aluminum cathode at 18 to 20°C is carried out in 1.5 A/din 2 (14-16 V) for 20 min. (thickness of layer 10 ram). The sealing is conducted at an operating temperature of 270°C for 15 min. at controlled heating and cooling of 10°C/min. The samples were dipped into AI203 sol before the sealing process. 5 RESULTS A N D D I S C U S S I O N A n o d i c o x i d a t i o n : It is very i m p o r t a n t to work with an acceptable layer thickness of the anodic alumina for this type of sealing. The time of anodic oxidation was the parameter controlling the thickness of the anodic layer. The current density remains constant at 1.5 A/din 2 and time of anodic oxidation was varied from 10 to 40 minutes. The aim was to reach a thickness to keep the protective properties of the anodic alumina layer but also to prepare it for sealing by the sol-gel method to avoid cracks. A 20 minute anodic oxidation to reach a thickness of 10 ~m is proper for the following sol-gel type of sealing. Sol-gel sealing: The thickness of sol-gel coatings is dependent on sol viscosity and the velocity of withdrawing. 6 In our case the velocity of withdrawing of the sample from the sol was constant, at 11.7 cm/min, and the molar ratio r [n (alkoxide)/n (water), where n is number of moles] was varied (Figure 1.). The thickness d was calculated with usage of a gravimetric method and under the assumption of the alumina density according to equation: (l) d = ( m 2 - m 1) / ( S x p) where m 2 is the mass of the sample after sol-gel alumi33
0.5
15
0.4 10 E
=L
0.3
E
0.2
E
]
0.1 100
120
140
1 160
180
5
r / n w a t e r : n a l u m i n u m butoxide
na deposition [kg], m 1 is mass of the sample after anodic oxidation [kg], S is area of the treated sample [m2], P is the density of the sol-gel alumina coating [kg/m3]. The dependence of the thickness on the molar ratio is a parabolic curve with local minimum. The dependence can be described by the mathematical equation derived by Landau and LevichT: d = [0.94 x (~x ~) 2/3]/[7LV 1/6 (p xg)l/2 ] where q means viscosity of the sol [Pa.s ], g is liquidvapor surface tension [N/m2], and r means the density of the sol solution [kg/m3]. The thickness of sol-gel alumina coating in combination with the other material component thicknesses is a parameter influencing the crack formation and the corrosion resistance of the material. A mixture of transition aluminas was identified for sol-gel type of coating, especially the transition &A1203. The resistance of the samples against atmospheric corrosion was tested in the constant h u m i d i t y c h a m b e r ASTM D 2247 (100% air h u m i d i t y and 35°C). The corrosion resistance of the coating sealed with sol (n alkoxide/n water = 1/150, withdrawing velocity 11.7 cm/min) and t h e r m a l l y t r e a t e d was compared with hydrothermal sealing (30 min). Time of anodic oxidation was 20 minutes before dipping into sol and 40 minutes before hydrothermal sealing. Figure 2 contrasts the changes in the coatings produced by the sol-gel method and conventional hydrothermal method. It is clear that enough mass has been created to confirm the increased resistance against atmospheric corrosion for sol type sealing. Blooming appeared on samples sealed hydrothermally. The weight of the sample increases for both types of sealing, although in the case of sealing with sol-gel the weight change is stabilized at the value of 16 mg/dm 2.
15
20
25
30
t/day
200
Figure 1: Dependence of the sol-gel alumina thickness on the molar ratio of aluminum alkoxide to water on anodized aluminum.The values are averaged from six measurements.
34
10
Figure 2: Dependence of the mass changes on time for: (a) anodized aluminum sealed by sol-gel alumina coating; and (b) anodized aluminum sealed by hydrothermal way.
For hydrothermal treated anodized aluminum, the curve shows no tendency to stabilize. The common corrosion mechanism employed for this type of corrosion on anodized aluminum is an oxygen concentration cell. The mechanism is not applicable for the glassy type of coating. The formation of secondary products on the coatings is a reason for the weight growth. Thermal t r e a t m e n t doesn't influence either the aluminum substrate or the anodic alumina. CONCLUSION
A l u m i n u m s u b s t r a t e can be sealed w i t h sol-gel coating after anodic oxidation in sulfuric acid. Corrosion resistance a g a i n s t a t m o s p h e r i c corrosion is comparable with h y d r o t h e r m a l sealing and even higher. The disadvantage of this process is its high price and decreased abrasion resistance and hardness. In spite of this, for special purposes this type of sealing is acceptable. REFERENCES 1. Wernick, S. et al., "The Surface Treatment and Finishing of Aluminum and Its Alloys," 5th Ed., Vol. 2, Chapters 6 and 11, Finishing Publications Ltd., Teddington, England; 1996. 2. Brace, A.W. and Sheasby, P.G., "The Technology of Anodizing Aluminum," 2nd Ed., Chapter 16, Technicopy Limited, England; 1979. 3. Kalantary, M.R:et al., J. Appl. Electrochem., 22:268; 1992. 4. Zemanov~, M. et al., Chem. Papers, 50 (2):55; 1996. 6. Brinker, C.J. and Scherer, G.W., "Sol-Gel Science," 1st Ed., Chapter 13, Academic Press, Boston; 1990. 7. Landau, L.D. and Levich, B.G.,Acta Physiochirn., 17:42; 1942.
For more information, (e-mail) [email protected]. Editor's Note:
This article was previously published in Metal rl}f
Finishing, December 2003.
www.metalfinishing.com