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Studies on anodic oxide coating with low absorptance and high emittance on aluminum alloy 2024
Solar Energy Materials & Solar Cells 60 (2000) 51}57
Studies on anodic oxide coating with low absorptance and high emittance on aluminum alloy 2024 C. Siva Kumar , A.K. Sharma, K.N. Mahendra , S.M. Mayanna * Department of Post-graduate studies in Chemistry, Central College, Bangalore-560001, India Thermal Process Section, ISRO Satellite Centre, Vimanapura Post, Bangalore-560017, India Received 22 February 1999; received in revised form 12 April 1999; accepted 1 June 1999
Abstract Anodization of AA 2024 in sulfuric acid bath containing glycerol, lactic acid and ammonium metavenadate has been studied to develop white anodic oxide coating. Investigation on the in#uence of various operating parameters * coating thickness, current density and ammonium metavenadate concentration on the optical properties was carried out to optimize the process. Infrared, atomic absorption spectroscopic techniques and scanning electron micrograph were used to characterize the coating. The obtained oxide coating provides a ratio of solar absorptance (a) to infrared emittance (e), as low as 0.2. The optical properties and hardness values measured under optimum experimental conditions support its use as a thermal control coating. 2000 Elsevier Science B.V. All rights reserved. Keywords: AA 2024; Absorptance; Emittance; Thermal control
1. Introduction Anodization of aluminum and its alloys is a well-known electrochemical process with wide spread applications. The literature on the subject is exhaustive [1}3]. Recent review on the subject is noteworthy [4]. Modi"ed anodization processes have gained considerable importance in processing optical materials for aerospace applications [5,6]. In space missions, a white anodic oxide coating (low a; high e coating) on
* Corresponding author. 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 6 2 - 8
52
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
a surface functions as an important material in the thermal design of the spacecraft [7,8]. The external surface of the spacecraft, experiences extreme temperature changes. Under severe vacuum conditions with convention being the only mode for heat transfer, coating with low a/e ratios on these surfaces holds the key to maintaining equilibrium temperature [9]. An attempt has been made [10] to obtain anodic oxide coating with 70}80% re#ectivity. The present studies were undertaken to obtain oxide coating with better specular re#ectivity ('80%). A high-strength copper-containing aluminum alloy (AA2024) which often "nds a place on the external surfaces of spacecraft is used in the present investigations. Working parameters for a sulfuric acid anodization process with a few solution components were optimized to obtain white anodic oxide coating. The coating produced was thick enough to achieve high thermal emittance (e) with low solar absorptance (a) to suit thermal control applications.
2. Experiment Solutions were prepared by using AR/GR grade chemicals and distilled water. AA 2024 (4.5% Cu, 1.5% Mg, 0.8% Mn & the rest Al) sheets (45;45;2 mm) were degreased, alkali cleaned, de-smutted [2] and chemically polished [11] before anodizing. The pretreated samples were anodized in a bath solution containing: sulfuric acid * 200 ml/l, lactic acid * 28 ml/l, glycerol * 16 ml/l, ammonium metavenadate (AMV) * 5 g/l with working parameters: cd * 1.5 A/dm, time * 20 min. and temperature * 25$13C. Anodized samples were subjected to hydrothermal treatment (20 min) to e!ect surface sealing. Optical properties (a and e) of the samples were measured using solar re#ectometer version 50, model SSR-ER and emissometer model RD-I, respectively. Both these instruments were able to give an average value of the solar absorptance and IR emittance digitally over the entire solar or IR region. Micro-hardness of the coatings was measured with a Shimadzu michrohardness tester type M (Kyoto, Japan) using a diamond indentor with a load of 15 g for 15 s. Anodic oxide coatings obtained were removed from the substrate [12] and an infrared spectrum was recorded in KBr pellet. The coating composition was obtained using atomic absorption spectrophotometer (Shimadzu). The thickness of the coating was measured using a coating thickness tester (ISOSCOPE MP 2BEB, Helmut Fischer Germany).
3. Results and discussion The various subsystems of the spacecraft work at their fullest e$ciency within the speci"ed temperature limits. In the absence of an atmosphere, heat exchanged in the spacecraft is limited to radiation. The equilibrium temperature of any subsystem is controlled by the ratio of solar absorptance to infrared (IR) emittance of its surfaces. The chemical coating applied to the spacecraft components play an important role in thermal control by providing suitable optical properties. The steady-state
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
53
temperature of any subsystem of the spacecraft may be expressed by the following equation [7], provided that there is no internal heat dissipation ¹"[SA a/pAe] , where ¹ is the absolute temperature of the spacecraft in K, S is the solar constant (mean value 1353 W/m), p is the Stefan}Boltzman constant (56.7;10\ W/m K\), A is the projected surface area (m) of the spacecraft perpendicular to the solar rays, A is the total surface area (m) of the spacecraft, a is the absorptance of the surface of projected area and e is the emittance of the exposed surface to space. Because S, A , A and p are constants, it clearly shows that the temperature of any given area is directly controlled by the a/e ratio. Here, the term absorptance refers to all solar radiation (X-ray, UV visible, IR, radio frequency, etc.), whereas the term emittance is restricted to the IR range, because thermal radiation occurs mainly in the IR region. The presence of copper content (4.5%) in the alloy makes the anodic coating on the surface dull and heavier, which creates great di$culty in formulating the bath parameters to get anodic oxide "lm with tailor-made speci"cations. Preliminary investigations with various additives in a sulfuric acid anodizing bath were carried out. The investigations revealed that a bath solution containing sulfuric acid, glycerol, lactic acid and ammonium metavenadate (AMV) is able to give anodic oxide coating with required optical properties. Hence, a detailed investigation was undertaken in sulfuric acid anodizing medium to optimize the process. Here, sulfuric acid is used as a conductivity component, glycerol to decrease the corrosive e!ect of the electrolyte on the copper content of the alloy, lactic acid as a voltage component and AMV as an additive to improve the specular re#ectance of the coating. 3.1. Process optimization Emittance of the coating depends on the substrate emittance and on the coating thickness, and is independent of change in the anodizing parameters [13]. Hence, it is necessary to increase the specular re#ectance (decrease in solar absorptance) of the coating to arrive at a better (low) (a/e) ratio. This could be achieved by maintaining high electrolyte concentration, lower cd and high temperature during operation [14,15]. However, these conditions render the coating soft with least resistance to abrasion. Hence, careful experimental studies were undertaken to optimize the working parameters: coating thickness, AMV concentration and cd to get oxide coating with su$cient hardness and low a/e ratio. 3.1.1. Inyuence of the anodic xlm thickness The e!ect of anodic "lm thickness on solar absorptance (a), infrared emittance (e) and (a/e) ratio is shown in Table 1 and Fig. 1, respectively. Slow increase in the solar absorptance value is observed (Table 1) with initial increase in thickness and then a sharp increase with the "lm growth. In the case of emittance, a sharp increase was observed (Table 1) in the initial stages; the variation becomes slow with the growth of the anodic "lm. It is very important therefore, to limit the anodic "lm thickness for
54
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
Table 1 In#uence of anodizing parameters on the optical properties of the coating Process condition
Solar absorptance (a)
Thermal emittance (e)
Ratio (a/e)
Coating Thickness (lm) 2.4 6.8 10.7 14.1 18.0
0.152 0.154 0.160 0.184 0.213
0.67 0.77 0.80 0.82 0.83
0.227 0.200 0.200 0.224 0.257
AMV concentration (g/L) 0 5 10 15 20
0.176 0.16 0.17 0.174 0.176
0.80 0.80 0.79 0.80 0.80
0.22 0.2 0.215 0.217 0.22
Current density (A/m) 0.5 1.0 1.5 2.0
0.168 0.16 0.16 0.181
0.79 0.80 0.80 0.80
0.212 0.2 0.2 0.226
Fig. 1. Variation of a/e ratio with the coating thickness.
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
55
Fig. 2. Variation of a/e ratio with cd (䢇) and AMV concentration (䉬).
optimum optical properties. The optimum coating thickness of the anodic "lm was found to be in the range of 9$2 lm (Fig. 1). No change in the optical properties of the anodic "lm was observed after pore sealing operation. 3.1.2. Inyuence of current density The cd determines the rate of "lm growth and the nature of deposit. To investigate the in#uence of applied cd on the optical behavior of the coating, experiments were carried out at various cd values from 0.5}2.0 A/dm. The in#uence of applied cd on the a/e ratio of anodic "lm at optimum thickness is represented in Fig. 2. It is apparent from the "gure that a low a/e value is obtained at an applied cd range of 1.0}1.5 A/dm. Hardness values for samples obtained at various cds were measured. The hardness value was found to increase from 118 to 212 VHN with increase in cd from 0.5 to 2.0 A/dm. These results indicate that cd 1.5 A/dm is optimum to get hard (180 VHN) anodized coating with low a/e ratio (0.2). 3.1.3. Inyuence of AMV concentration Venadate anion (VO\), which has good mobility characteristics was used in the bath solution in the form of ammonium metavenadate salt, in order to "nd out its e!ect on the absorptance of the coating. AMV was added to the bath solution in small amounts, up to 20 g/l. The Vanadium incorporation into the coating increases with an increase in AMV concentration, which in turn increases the solar absorptance values
56
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
Fig. 3. Scanning Electron Micrograph of the white anodic oxide coating at 4,000;.
of the coating (Table 1). Fig. 2 represents the variation of a/e ratio with AMV concentration. From the plot, a characteristic low a/e ratio of 0.2 is observed at a concentration of 5 g/l. The VO\ anion is thought to provide adequate polishing e!ects on the surface during the process.
3.2. Coating characterization The infrared spectrum obtained shows absorption bands at wave numbers 3400}3600 cm\ and 1600}1700 cm\ indicating the involvement of water in the crystal lattice. The moiety of Al O in the coatings was con"rmed by the strong band at 1130 cm\, and the shoulder at 575 cm\. Another shoulder at 920 cm\ supports the presence of Al (SO ) in the coating. A strong band was also observed at 2370 cm\, which may be due to the asymmetric stretching mode of free carbon dioxide entrapped in the coating. In addition, Atomic Absorption Spectroscopic analysis of the coating obtained at optimum conditions revealed a trace incorporation of vanadium (12 ppm/g of coating). Scanning electron micrograph of the white anodic oxide coating obtained at optimum conditions is shown in Fig. 3. It characterizes an evenly distributed surface morphology, to support better re#ectivity with clusters of grains measuring few microns.
4. Conclusion White anodizing process on AA 2024 was developed by using a bath solution containing: sulfuric acid (200 ml/l), glycerol (16 ml/l), lactic acid (28 ml/l) and AMV (5 g/l) at a cd of 1.5 A/dm for 20 mins with a working temperature of 25$13C. The
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
57
process provides anodic oxide coatings with a characteristically low solar absorptance to thermal emittance ratio as low as 0.2, which critically suits thermal control application. Chemical polishing prior to the anodizing process improves (lowers) solar absorptance values by about 25%. In addition, trace quantity of cuprous ions (0.01%) added to the polishing bath solution improves the specular re#ectivity of the coating.
Acknowledgements The authors (CSK, KNM & SMM) are grateful to ISRO Satellite Centre, Bangalore for "nancial support to this project. The authors express their gratitude to H. Narayanamurthy, Head, Thermal Systems Group and A.V. Patki, Director, Mechanical Systems Division, ISRO Satellite Centre, Bangalore for their encouragement and guidance and for providing the necessary testing and evaluation facilities.
References [1] C.A. Grubbs, Met. Finish. Guidebook, 26 (1) (1998) 472. [2] A.W. Brace, P.G. Sheasby, The Technology of Anodizing Aluminum, 2nd Edition, Technicopy Limited, Stonehouse, 1979. [3] S. Wernick, R. Pinner, Surface Treatment and Finishing of Al and its Alloys, Vols. I & II, Robert Draper Ltd., New York, 1972. [4] Hirayama, Yoshio, Hyomen Gijutsu, 41 (12) (1990) 1297. [5] Powers, H.D. John, T. Hang, US 4737246 (CI 204-58) (IPC C250-011/08) 12 April 1988. [6] Le, Huang G. Smith, C.A. O'Brien, L. Dudley, Proc. SPIE-Int. Soc. Opt. Eng., (1989) 1118 (Space Opt. Mater. Space Qualif. Opt.), 59}72. [7] B.N. Agarwal, Design of Geo-Synchronous Spacecraft, Prentice-Hall, Englewood Cli!s, NJ, 1986, p. 281. [8] A.K. Sharma, H. Bhojraj, V.K. Kaila, H. Narayanamurthy, Met. Finish. 95 (12) (1997) 14. [9] T.M. Kelleher, Symposium on Anodizing Aluminum, Aluminum Federation, Birmingham, 1967, pp. 71. [10] S. Wernick, R. Pinner, Surface Treatment and Finishing of Al and its Alloys, Vol. II, Robert Draper Ltd., New York, 1972, pp. 545. NASA Washington, 1969, 54P.PB-184005. [11] J.B. Mohler, Finishing of Aluminum, Reinhold, New York, 1963, pp. 51. [12] V.F. Henley, Anodic Oxidation of Al and its Alloys, Pergamon Press, Oxford, 1982, p. 98. [13] T. Pavlovic, A. Ignatiev, Sol. Energy Mater. 16 (1987) 319. [14] B.A. Scott, Trans, Inst. Met. Fin., 39 (1) (1962) 29. [15] W.E. Cooke, Plating 49 (11) (1962) 1157.
Studies on anodic oxide coating with low absorptance and high emittance on aluminum alloy 2024 C. Siva Kumar , A.K. Sharma, K.N. Mahendra , S.M. Mayanna * Department of Post-graduate studies in Chemistry, Central College, Bangalore-560001, India Thermal Process Section, ISRO Satellite Centre, Vimanapura Post, Bangalore-560017, India Received 22 February 1999; received in revised form 12 April 1999; accepted 1 June 1999
Abstract Anodization of AA 2024 in sulfuric acid bath containing glycerol, lactic acid and ammonium metavenadate has been studied to develop white anodic oxide coating. Investigation on the in#uence of various operating parameters * coating thickness, current density and ammonium metavenadate concentration on the optical properties was carried out to optimize the process. Infrared, atomic absorption spectroscopic techniques and scanning electron micrograph were used to characterize the coating. The obtained oxide coating provides a ratio of solar absorptance (a) to infrared emittance (e), as low as 0.2. The optical properties and hardness values measured under optimum experimental conditions support its use as a thermal control coating. 2000 Elsevier Science B.V. All rights reserved. Keywords: AA 2024; Absorptance; Emittance; Thermal control
1. Introduction Anodization of aluminum and its alloys is a well-known electrochemical process with wide spread applications. The literature on the subject is exhaustive [1}3]. Recent review on the subject is noteworthy [4]. Modi"ed anodization processes have gained considerable importance in processing optical materials for aerospace applications [5,6]. In space missions, a white anodic oxide coating (low a; high e coating) on
* Corresponding author. 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 6 2 - 8
52
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
a surface functions as an important material in the thermal design of the spacecraft [7,8]. The external surface of the spacecraft, experiences extreme temperature changes. Under severe vacuum conditions with convention being the only mode for heat transfer, coating with low a/e ratios on these surfaces holds the key to maintaining equilibrium temperature [9]. An attempt has been made [10] to obtain anodic oxide coating with 70}80% re#ectivity. The present studies were undertaken to obtain oxide coating with better specular re#ectivity ('80%). A high-strength copper-containing aluminum alloy (AA2024) which often "nds a place on the external surfaces of spacecraft is used in the present investigations. Working parameters for a sulfuric acid anodization process with a few solution components were optimized to obtain white anodic oxide coating. The coating produced was thick enough to achieve high thermal emittance (e) with low solar absorptance (a) to suit thermal control applications.
2. Experiment Solutions were prepared by using AR/GR grade chemicals and distilled water. AA 2024 (4.5% Cu, 1.5% Mg, 0.8% Mn & the rest Al) sheets (45;45;2 mm) were degreased, alkali cleaned, de-smutted [2] and chemically polished [11] before anodizing. The pretreated samples were anodized in a bath solution containing: sulfuric acid * 200 ml/l, lactic acid * 28 ml/l, glycerol * 16 ml/l, ammonium metavenadate (AMV) * 5 g/l with working parameters: cd * 1.5 A/dm, time * 20 min. and temperature * 25$13C. Anodized samples were subjected to hydrothermal treatment (20 min) to e!ect surface sealing. Optical properties (a and e) of the samples were measured using solar re#ectometer version 50, model SSR-ER and emissometer model RD-I, respectively. Both these instruments were able to give an average value of the solar absorptance and IR emittance digitally over the entire solar or IR region. Micro-hardness of the coatings was measured with a Shimadzu michrohardness tester type M (Kyoto, Japan) using a diamond indentor with a load of 15 g for 15 s. Anodic oxide coatings obtained were removed from the substrate [12] and an infrared spectrum was recorded in KBr pellet. The coating composition was obtained using atomic absorption spectrophotometer (Shimadzu). The thickness of the coating was measured using a coating thickness tester (ISOSCOPE MP 2BEB, Helmut Fischer Germany).
3. Results and discussion The various subsystems of the spacecraft work at their fullest e$ciency within the speci"ed temperature limits. In the absence of an atmosphere, heat exchanged in the spacecraft is limited to radiation. The equilibrium temperature of any subsystem is controlled by the ratio of solar absorptance to infrared (IR) emittance of its surfaces. The chemical coating applied to the spacecraft components play an important role in thermal control by providing suitable optical properties. The steady-state
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
53
temperature of any subsystem of the spacecraft may be expressed by the following equation [7], provided that there is no internal heat dissipation ¹"[SA a/pAe] , where ¹ is the absolute temperature of the spacecraft in K, S is the solar constant (mean value 1353 W/m), p is the Stefan}Boltzman constant (56.7;10\ W/m K\), A is the projected surface area (m) of the spacecraft perpendicular to the solar rays, A is the total surface area (m) of the spacecraft, a is the absorptance of the surface of projected area and e is the emittance of the exposed surface to space. Because S, A , A and p are constants, it clearly shows that the temperature of any given area is directly controlled by the a/e ratio. Here, the term absorptance refers to all solar radiation (X-ray, UV visible, IR, radio frequency, etc.), whereas the term emittance is restricted to the IR range, because thermal radiation occurs mainly in the IR region. The presence of copper content (4.5%) in the alloy makes the anodic coating on the surface dull and heavier, which creates great di$culty in formulating the bath parameters to get anodic oxide "lm with tailor-made speci"cations. Preliminary investigations with various additives in a sulfuric acid anodizing bath were carried out. The investigations revealed that a bath solution containing sulfuric acid, glycerol, lactic acid and ammonium metavenadate (AMV) is able to give anodic oxide coating with required optical properties. Hence, a detailed investigation was undertaken in sulfuric acid anodizing medium to optimize the process. Here, sulfuric acid is used as a conductivity component, glycerol to decrease the corrosive e!ect of the electrolyte on the copper content of the alloy, lactic acid as a voltage component and AMV as an additive to improve the specular re#ectance of the coating. 3.1. Process optimization Emittance of the coating depends on the substrate emittance and on the coating thickness, and is independent of change in the anodizing parameters [13]. Hence, it is necessary to increase the specular re#ectance (decrease in solar absorptance) of the coating to arrive at a better (low) (a/e) ratio. This could be achieved by maintaining high electrolyte concentration, lower cd and high temperature during operation [14,15]. However, these conditions render the coating soft with least resistance to abrasion. Hence, careful experimental studies were undertaken to optimize the working parameters: coating thickness, AMV concentration and cd to get oxide coating with su$cient hardness and low a/e ratio. 3.1.1. Inyuence of the anodic xlm thickness The e!ect of anodic "lm thickness on solar absorptance (a), infrared emittance (e) and (a/e) ratio is shown in Table 1 and Fig. 1, respectively. Slow increase in the solar absorptance value is observed (Table 1) with initial increase in thickness and then a sharp increase with the "lm growth. In the case of emittance, a sharp increase was observed (Table 1) in the initial stages; the variation becomes slow with the growth of the anodic "lm. It is very important therefore, to limit the anodic "lm thickness for
54
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
Table 1 In#uence of anodizing parameters on the optical properties of the coating Process condition
Solar absorptance (a)
Thermal emittance (e)
Ratio (a/e)
Coating Thickness (lm) 2.4 6.8 10.7 14.1 18.0
0.152 0.154 0.160 0.184 0.213
0.67 0.77 0.80 0.82 0.83
0.227 0.200 0.200 0.224 0.257
AMV concentration (g/L) 0 5 10 15 20
0.176 0.16 0.17 0.174 0.176
0.80 0.80 0.79 0.80 0.80
0.22 0.2 0.215 0.217 0.22
Current density (A/m) 0.5 1.0 1.5 2.0
0.168 0.16 0.16 0.181
0.79 0.80 0.80 0.80
0.212 0.2 0.2 0.226
Fig. 1. Variation of a/e ratio with the coating thickness.
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
55
Fig. 2. Variation of a/e ratio with cd (䢇) and AMV concentration (䉬).
optimum optical properties. The optimum coating thickness of the anodic "lm was found to be in the range of 9$2 lm (Fig. 1). No change in the optical properties of the anodic "lm was observed after pore sealing operation. 3.1.2. Inyuence of current density The cd determines the rate of "lm growth and the nature of deposit. To investigate the in#uence of applied cd on the optical behavior of the coating, experiments were carried out at various cd values from 0.5}2.0 A/dm. The in#uence of applied cd on the a/e ratio of anodic "lm at optimum thickness is represented in Fig. 2. It is apparent from the "gure that a low a/e value is obtained at an applied cd range of 1.0}1.5 A/dm. Hardness values for samples obtained at various cds were measured. The hardness value was found to increase from 118 to 212 VHN with increase in cd from 0.5 to 2.0 A/dm. These results indicate that cd 1.5 A/dm is optimum to get hard (180 VHN) anodized coating with low a/e ratio (0.2). 3.1.3. Inyuence of AMV concentration Venadate anion (VO\), which has good mobility characteristics was used in the bath solution in the form of ammonium metavenadate salt, in order to "nd out its e!ect on the absorptance of the coating. AMV was added to the bath solution in small amounts, up to 20 g/l. The Vanadium incorporation into the coating increases with an increase in AMV concentration, which in turn increases the solar absorptance values
56
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
Fig. 3. Scanning Electron Micrograph of the white anodic oxide coating at 4,000;.
of the coating (Table 1). Fig. 2 represents the variation of a/e ratio with AMV concentration. From the plot, a characteristic low a/e ratio of 0.2 is observed at a concentration of 5 g/l. The VO\ anion is thought to provide adequate polishing e!ects on the surface during the process.
3.2. Coating characterization The infrared spectrum obtained shows absorption bands at wave numbers 3400}3600 cm\ and 1600}1700 cm\ indicating the involvement of water in the crystal lattice. The moiety of Al O in the coatings was con"rmed by the strong band at 1130 cm\, and the shoulder at 575 cm\. Another shoulder at 920 cm\ supports the presence of Al (SO ) in the coating. A strong band was also observed at 2370 cm\, which may be due to the asymmetric stretching mode of free carbon dioxide entrapped in the coating. In addition, Atomic Absorption Spectroscopic analysis of the coating obtained at optimum conditions revealed a trace incorporation of vanadium (12 ppm/g of coating). Scanning electron micrograph of the white anodic oxide coating obtained at optimum conditions is shown in Fig. 3. It characterizes an evenly distributed surface morphology, to support better re#ectivity with clusters of grains measuring few microns.
4. Conclusion White anodizing process on AA 2024 was developed by using a bath solution containing: sulfuric acid (200 ml/l), glycerol (16 ml/l), lactic acid (28 ml/l) and AMV (5 g/l) at a cd of 1.5 A/dm for 20 mins with a working temperature of 25$13C. The
C. Siva Kumar et al. / Solar Energy Materials & Solar Cells 60 (2000) 51}57
57
process provides anodic oxide coatings with a characteristically low solar absorptance to thermal emittance ratio as low as 0.2, which critically suits thermal control application. Chemical polishing prior to the anodizing process improves (lowers) solar absorptance values by about 25%. In addition, trace quantity of cuprous ions (0.01%) added to the polishing bath solution improves the specular re#ectivity of the coating.
Acknowledgements The authors (CSK, KNM & SMM) are grateful to ISRO Satellite Centre, Bangalore for "nancial support to this project. The authors express their gratitude to H. Narayanamurthy, Head, Thermal Systems Group and A.V. Patki, Director, Mechanical Systems Division, ISRO Satellite Centre, Bangalore for their encouragement and guidance and for providing the necessary testing and evaluation facilities.
References [1] C.A. Grubbs, Met. Finish. Guidebook, 26 (1) (1998) 472. [2] A.W. Brace, P.G. Sheasby, The Technology of Anodizing Aluminum, 2nd Edition, Technicopy Limited, Stonehouse, 1979. [3] S. Wernick, R. Pinner, Surface Treatment and Finishing of Al and its Alloys, Vols. I & II, Robert Draper Ltd., New York, 1972. [4] Hirayama, Yoshio, Hyomen Gijutsu, 41 (12) (1990) 1297. [5] Powers, H.D. John, T. Hang, US 4737246 (CI 204-58) (IPC C250-011/08) 12 April 1988. [6] Le, Huang G. Smith, C.A. O'Brien, L. Dudley, Proc. SPIE-Int. Soc. Opt. Eng., (1989) 1118 (Space Opt. Mater. Space Qualif. Opt.), 59}72. [7] B.N. Agarwal, Design of Geo-Synchronous Spacecraft, Prentice-Hall, Englewood Cli!s, NJ, 1986, p. 281. [8] A.K. Sharma, H. Bhojraj, V.K. Kaila, H. Narayanamurthy, Met. Finish. 95 (12) (1997) 14. [9] T.M. Kelleher, Symposium on Anodizing Aluminum, Aluminum Federation, Birmingham, 1967, pp. 71. [10] S. Wernick, R. Pinner, Surface Treatment and Finishing of Al and its Alloys, Vol. II, Robert Draper Ltd., New York, 1972, pp. 545. NASA Washington, 1969, 54P.PB-184005. [11] J.B. Mohler, Finishing of Aluminum, Reinhold, New York, 1963, pp. 51. [12] V.F. Henley, Anodic Oxidation of Al and its Alloys, Pergamon Press, Oxford, 1982, p. 98. [13] T. Pavlovic, A. Ignatiev, Sol. Energy Mater. 16 (1987) 319. [14] B.A. Scott, Trans, Inst. Met. Fin., 39 (1) (1962) 29. [15] W.E. Cooke, Plating 49 (11) (1962) 1157.