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Studies on white anodizing on aluminum alloy for space applications
Applied Surface Science 151 Ž1999. 280–286 www.elsevier.nlrlocaterapsusc
Studies on white anodizing on aluminum alloy for space applications C. Siva Kumar a , S.M. Mayanna a , K.N. Mahendra a , A.K. Sharma R. Uma Rani b a
b,)
,
Department of Post-Graduate Studies in Chemistry, Central College, Bangalore 560 001, India b Thermal Process Section, ISRO Satellite Centre, Bangalore 560 017, India Received 20 March 1999; accepted 31 May 1999
Abstract A process of white anodizing in an electrolyte system consisting of sulfuric acid, lactic acid, glycerol and sodium molybdate was studied for space applications. The influence of anodic film thickness and various operating parameters, viz., electrolyte formulation, operating temperature, applied current density, on the optical properties of the coating has been investigated to optimize the process. The coatings were characterized by atomic absorption spectroscopic analysis, thickness and microhardness evaluation. The space worthiness of the coating has been evaluated by humidity, thermal cycling, thermo-vacuum performance tests and measurement of optical properties. The anodic film developed herein provides solar absorptance value as low as 0.16, and infrared ŽIR. emittance of the order of 0.80. These results indicate that the process developed is suitable for thermal control applications in space environment. q 1999 Elsevier Science B.V. All rights reserved. Keywords: AA 2024; Anodize; Solar absorptance; Infrared emittance; Thermal control
1. Introduction A spacecraft in orbit undergoes extreme temperature cycling due to direct sun load on one side and deep cold space on the other. This causes large thermal gradients between the sunlit and shadowed sides of the vehicle. However, the various subsystems of the spacecraft can work at their fullest efficiency within the specified temperature limits. In the absence of an atmosphere, heat exchanged in the spacecraft is limited to radiation. The equilib)
Corresponding author. Tel.: q91-080-5083106; fax: q91080-5266552; Telex: 085-2325
rium temperature of any subsystem is controlled by the ratio of solar absorptance to infrared ŽIR. emittance of its surface. Chemical coatings applied to the spacecraft components play an important role in thermal control by providing suitable optical properties w1,2x. In the absence of any internal power dissipation, the steady state temperature of a spacecraft far from the Earth’s atmosphere may be expressed by Eq. Ž1. w3–5x. T s SA p ars A ´
1r4
Ž 1.
where T is the absolute temperature of the spacecraft, S is the solar constant, s is the Stefan–Boltz-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 2 9 0 - 1
C. SiÕa Kumar et al.r Applied Surface Science 151 (1999) 280–286
mann constant, A p is the projected surface area of the spacecraft perpendicular to the solar rays, A is the total surface area of the spacecraft, a is the solar absorptance of the surface of projected area and ´ is the IR emittance of the exposed surface to space. In this situation where S, A p , A and s are constants, the temperature of any given area of the spacecraft is directly controlled by the ar´ ratio. Here, the term absorptance refers to all solar radiation ŽX-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. It is customary to use Optical Solar Reflectors ŽOSRs. and white paint to maintain equilibrium temperature at certain external surfaces of the spacecraft where high heat dissipation equipment is located. Though the thermal-control white paints provide adequate optical properties Žsolar absorptance, a ; 0.20; IR emittance, ´ ; 0.85. and are used extensively in space missions, a high degradation in their absorptance value has been observed. The change in solar absorptance value is mainly associated with the degradation of their organic base material by exposure to ultraviolet radiation. During a 7-year geo-stationary orbit mission, the solar absorptance value of white paint, which is ; 0.20 at the beginning of life ŽBOL., is estimated to be as high as ; 0.60 at the end of life ŽEOL.. Other problems associated with white paints are their high mass loss, volatile condensable material percentage in space environment, limited shelf-life and poor adhesion. The minimum paint thickness required for optimum optical properties is 50–70 mm. This results in appreciable increase in overall weight of the spacecraft and makes these paints unsuitable on the surfaces where dimensional tolerance is critical. The OSRs Žsecond surface mirror, silverized quartzrteflon. on the other hand provide excellent optical properties and low degradation in solar absorptance ŽBOL: 0.08; EOL: 0.20.. However, the cost of OSRs is enormous and they require a meticulous process of bonding on spacecraft panels due to their delicate nature. Development of white anodic coating on AA 2024 is undertaken as an alternative to the application of white paint and OSRs. The copper content added to increase the strengthrdensity ratio of aluminum alloy produces a heavier and dull shade when
281
anodized w6x. This poses difficulties in optimizing the bath parameters to achieve desired optical properties. Reynolds Metals w7x has developed a white anodizing process for the National Aeronautics and Space Administration ŽNASA. of the USA, which provides 70–80% reflectivity. The main constituents of the anodizing bath are sulfuric acid, polyhydric alcohol, organic carboxylic acid and titanium ammonium lactate. In the present communication, a process of white anodizing with sulfuric acid, glycerol, lactic acid and sodium molybdate is studied.
2. Experimental The 50 mm = 60 mm = 0.5 mm samples of surface area 71 cm2 were processed for white anodizing with the following sequential operations: 1. solvent degreasing using tri-chloroethylene in an ultrasonic bath for 3–5 min; 2. alkaline cleaning at 30–408C for 3–4 min, in a solution containing 50 grl sodium hydroxide, 30 grl tri-sodium orthophosphate and 30 grl sodium meta silicate. Water rinsing and air-drying; 3. chemical polishing in a solution of 80% Žvrv. orthophosphoric acid, 3.5% Žvrv. nitric acid and 0.01% Žwt.. copper at ; 908C for 20–25 s followed by hot water rinsing Ž50 to 608C.; 4. de-smutting in a solution of 50% Žvrv. nitric acid at room temperature Ž23 " 28C. for 30 s followed by water rinsing; 5. anodizing in a solution formulated and operated as per the following parameters: Sulfuric acid ŽS.G. 1.84. Lactic acid Glycerol Sodium molybdate Current density Voltage Time Temperature Coating thickness Post treatment
200 mlrl 28 mlrl 16 mlrl 10 grl 1.3–1.4 Ardm2 Ž1.35. 7–10 V 22 min 23 " 28C ; 10.5 mm Water rinsing
6. sealing in hot distilled water Ž) 988C. for 15–20 min. The pH of the sealing water was maintained within 6.5–7.0, using ammonium acetate Ž1 grl. as a buffering agent.
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All solutions were prepared by using ARrLR grade chemicals and distilled water.
3. Results and discussion 3.1. Process optimization Process optimization was carried out by investigating the influence of coating thickness and various operating parameters, viz.: temperature, electrolyte formulation, and current density on the physico-optical properties of the coating. Wherever the anodic film thickness is not mentioned, the data are given for a constant thickness of 10.5 mm. These data were obtained by conducting a set of experiments to achieve a coating thickness close to the optimum value of 10.5 " 0.5 mm. The corresponding coating properties were then obtained at 10.5 mm thickness by graphical plotting. 3.1.1. Influence of chemical polishing Chemical polishing was performed in order to render a smooth surface prior to anodizing, which helps in improving the reflectance of the coating. Specular reflectance was further improved by the addition of copper w8,9x at 0.01% Žwt.. to the polishing bath. At optimum operating conditions, anodiz-
Fig. 1. Influence of chemical polishing time on solar absorptance of the coating. Conditions: coating thicknesss10.5 mm; current density s1.35 Ardm2 ; temperatures 238C.
Fig. 2. Influence of electrolyte temperature on the a r ´ ratio. Coating thicknesss10.5 mm; current density s1.35 Ardm2 .
ing after chemical polishing showed an improvement Ždrop. in the solar absorptance values by about 25% ŽFig. 1.. 3.1.2. Influence of electrolyte temperature Influence of the bath temperature on the coating properties was studied in the temperature range of 15–508C ŽFig. 2.. The optimum results were obtained at room temperature 23 " 28C. At lower temperatures, the coating formed with high solar absorptance value. At higher temperatures, the coating starts to dissolve in the electrolyte. Consequently, it is difficult to achieve the optimum coating thickness. The coatings so formed were very soft, highly porous and had low IR emittance. 3.1.3. Bath constituents: influence of sodium molybdate concentration In the present studies, sulfuric acid is used as a conducting media and lactic acid as a voltage suppressor. Glycerol is added to minimize the corrosive effect of electrolyte particularly on high copper containing alloys and sodium molybdate to increase the specular reflectivity of the coating. The high ionic conductivity and diffusion coefficient of molybdate anion w10x provides adequate chemical polishing effects on the sample surface while anodizing. This helps in improving the surface reflectivity of the anodic film.
C. SiÕa Kumar et al.r Applied Surface Science 151 (1999) 280–286
283
The concentration of sodium molybdate varied from 2.5 grl to 20 grl in successive steps and the changes in the optical properties were investigated. Fig. 3 shows the initial gradual improvement of the optical properties, with increasing molybdate concentration, initially. The optimum value Žlowest ar´ ratio. was obtained at a concentration of 10 grl. Further addition of molybdate results in an increase in the ar´ ratio of the coating. The anodic film obtained at optimum operating conditions showed incorporation of molybdenum Ž32 ppmrgm of coating. when analyzed through atomic absorption spectroscopy. 3.1.4. Influence of the anodic film thickness Emittance is a surface phenomenon. During anodizing, the aluminum surface is converted into alumina. The emittance of the sample surface increases with the growth of anodic film thickness. The effect of anodic film thickness on solar absorptance Ž a ., IR emittance Ž ´ . and the ar´ ratio is shown in Figs. 4–6. In the initial stages, a sharp increase in the emittance value of the surface was observed with the growth of the anodic film; however, it becomes slow at latter stages. The solar absorptance of the film also increases with film thickness, but with a reverse pattern, to that observed for IR emittance. A slow increase was observed with initial increase in
Fig. 3. Effect of salt concentration on the a r ´ ratio. Coating thicknesss10.5 mm; current density s1.35 Ardm2 ; temperature s 238C.
Fig. 4. Variation of solar absorptance with the coating thickness. Current density s1.35 Ardm2 ; temperatures 238C.
coating thickness and a sharp increase at the latter stage. It is very important therefore, to limit the anodic film thickness for optimum optical properties. The optimum coating thickness of anodic film was found to be 10.5 mm. No change in optical properties of the anodic film was observed after pore-sealing operation. 3.1.5. Influence of current density The current density determines the rate of film growth and nature of deposits. To investigate the
Fig. 5. Variation of IR emittance with the coating thickness. Current density s1.35 Ardm2 ; temperatures 238C.
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Fig. 6. Variation of the a r ´ ratio with the coating thickness. Current density s1.35 Ardm2 ; temperatures 238C.
influence of applied current density on the optical behavior of the coating, experiments were conducted at various constant current densities varying from 0.5 to 2.15 Ardm2 . The electrolyte temperature was maintained at 23 " 28C throughout the experiment. At lower applied current density Ž- 1.0 Ardm2 ., the rate of film growth was too low, and the coating obtained showed poor absorptance. At higher current density Ž) 1.6 Ardm2 ., anodic coatings obtained had a chalky appearance with an increase in absorptance value. The influence of applied current density on the ar´ ratio of anodic film is presented in Fig. 7. It is apparent from this figure that the lowest ratio is obtained at an applied current density range of 1.3–1.4 Ardm2 Ž7–10 V..
All anodized specimens were visually examined for any defects. The coatings were perfectly uniform, and no defects such as discontinuity, patches, or powdery deposition were observed. After visual inspection, the anodized specimens were subjected to evaluation tests and measurements. Adhesion was evaluated by a Scotch tape peel-off test. Masking tape of 25-mm width type 3M500 Žpressure 200 grcm2 . was applied over the film by passing a 2-kg rubber roller over the tape twice. The tape was then quickly removed in a direction normal to the surface, and the test specimens were examined visually for any coating removal. No detachment of the film from the substrate was observed. This test was also conducted after humidity, thermal cycling and thermovacuum performance tests. The test results showed excellent adhesion of the coating to the substrate. The thickness of the coating was measured using an ISOSCOPE MP 2BEB, Helmut Fischer ŽGermany., coating thickness tester. A coating thickness of 10.5 mm is recommended for optimum optical properties Žlowest ar´ ratio.. The microhardness of the white anodized coupons were measured with a Shimadzu Microhardness tester type M ŽKyoto, Japan. using a Vickers indent. Vickers hardness numbers were obtained by averaging seven measurements of each specimen with a load of
3.2. Testing and eÕaluation To evaluate the performance of the anodic film in pre-launch and post-launch environments, test coupons anodized at optimum conditions were subjected to humidity, thermal cycling and thermovacuum performance tests. Humidity test was conducted to examine the resistance of the anodic film to corrosive pre-launch and post-launch conditions. The thermal cycling and thermovacuum performance tests were designed to evaluate the effect of on-orbit cycling temperature of the spacecraft on the physico-optical properties of anodic film.
Fig. 7. Effect of current density on the a r ´ ratio. Coating thicknesss10.5 mm; temperatures 238C.
C. SiÕa Kumar et al.r Applied Surface Science 151 (1999) 280–286
15 g for 15 s. The microhardness of the coating was found in the range of 180 to 190 HV0.015 . The humidity test is designed to check the effect of humidity and high temperature, which in turn shows the resistance of the coatings to corrosive atmosphere. The test was conducted in a thermostatically controlled humidity chamber. The relative humidity in the chamber was maintained at 95 " 0.5% at 508C. After the test, specimens were visually examined, and their optical properties Žsolar absorptance and IR emittance. were measured. No changes in optical properties and no physical degradation Ždiscoloration, corrosion spots, patches or peeling. of the coating were observed. The thermal cycling test is designed to evaluate the effect of cycling temperatures, which are likely to be encountered throughout the life span of spacecraft, on the physico-optical properties of the coating. The test was conducted in a thermostatically controlled chamber. A total of 1500 cycles was applied. A cycle consists of lowering the temperature to y458C, a dwell of 5 min, and raising the temperature to 858C with a dwell of 5 min. After the thermal cycling, the test specimens were inspected visually, and their optical properties were measured. No degradation was observed. The thermovacuum performance test is designed to examine the effect of cycling temperature on the coating in the space environment Ži.e., in vacuum.. The test was conducted in a thermostatically controlled vacuum chamber. The test consists of lowering the temperature to y458C with a dwell of 2 h, and increasing the temperature to 858C with a dwell of 2 h. Ten cycles of hot and cold soak were applied, and a vacuum level below 10y5 Torr was maintained inside the chamber during the test. No sign of any degradation on the coating was noticed after the test. The optical properties, specifically solar reflectance and IR emittance of the coating, were measured using a solar reflectometer version 50, Model SSR-ER and an emissometer model RD-1, respectively. Both these instruments provide an average value of solar absorptance and IR emittance digitally over the entire solar or IR region. To evaluate the environmental stability of the anodic coating, the measurement of optical properties were carried out before and after each of the environment tests Žhumidity, thermal cycling and thermovacuum tests..
285
No change in optical properties was observed. This clearly indicates excellent stability of the coating to the space environment.
4. Conclusion Ž1. A white anodizing process on AA 2024 alloy was obtained in an electrolyte system containing: 200 mlrl sulfuric acid, 16 mlrl glycerol, 28 mlrl lactic acid and 10 grl sodium molybdate. The operating temperature was 23 " 28C at a current density of 1.3–1.4 Ardm2 for 22 min. The process provides low solar absorptance and high thermal emittance values, which are extremely suitable for thermal control applications. Ž2. A drop in the solar absorptance value of the white anodic film by ; 25% was achieved by chemical polishing pre-treatment at 908C for 20–25 s in a solution containing 80% orthophosphoric acid Žvrv., 3.5% nitric acid Žvrv. and 0.01% copper Žwt... Ž3. The process described herein provides highly reproducible results. A coating thickness Žmost important parameter for optical properties. of 10.5 " 0.5 mm is obtained under optimum operating conditions. Ž4. The white anodic coatings are highly stable. The humidity, thermal cycling and thermovacuum performance tests have no adverse effect on the physico-optical properties of the coating. This shows high stability of the coating in adverse space conditions.
Acknowledgements The author gratefully acknowledges the financial support of ISRO Satellite Centre, Bangalore for this project. The authors are thankful to A.V. Patki, H. Narayanamurthy and H. Bhojraj of ISRO Satellite Centre for encouragement, guidance and providing necessary testing and evaluation facilities.
References w1x A.K. Sharma, Trans. SAEST 30 Ž1995. 1. w2x M. Kelleher Therese, in: Symposium on Anodizing Aluminum, Aluminum Federation, Birmingham, 1967, 71.
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w3x A.K. Sharma, H. Bhojraj, V.K. Kaila, H. Narayanamurthy, J. Aerospace 102 Ž1993. 865, Section 1. w4x A.K. Sharma, H. Narayanamurthy, H. Bhojraj, J.Md. Mohideen, J. Spacecr. Technol. 2 Ž2. Ž1992. 35. w5x B.N. Agarwal, Design of Geo-synchronous Spacecraft, Prentice-Hall, Englewood Cliffs, NJ, 1986, p. 281. w6x A. Charles Grubbs, in: Metal Finishing Guidebook 1998, Vol. 26 Ž1., p. 480. w7x NASA Washington, 1969. 54P.PB-184005; S. Wernick, R.
Pinner, Surface Treatment and Finishing of Al and its Alloys, Vol. 2, Robert Draper, NY, 1972, p. 545. w8x J.B. Mohler, Finishing of Aluminum, Van Nostrand-Reinhold, New York, 1963, p. 51. w9x E.A. Culpan, D.J. Arrowsmith, Transactions of Institute of Metal Finishing 51 Ž1973. 17. w10x R.L. David, CRC Handbook of Chemistry and Physics, 76th edn., CRC Press, Boca Raton, FL, USA, 1995–1996, pp. 5–90.
Studies on white anodizing on aluminum alloy for space applications C. Siva Kumar a , S.M. Mayanna a , K.N. Mahendra a , A.K. Sharma R. Uma Rani b a
b,)
,
Department of Post-Graduate Studies in Chemistry, Central College, Bangalore 560 001, India b Thermal Process Section, ISRO Satellite Centre, Bangalore 560 017, India Received 20 March 1999; accepted 31 May 1999
Abstract A process of white anodizing in an electrolyte system consisting of sulfuric acid, lactic acid, glycerol and sodium molybdate was studied for space applications. The influence of anodic film thickness and various operating parameters, viz., electrolyte formulation, operating temperature, applied current density, on the optical properties of the coating has been investigated to optimize the process. The coatings were characterized by atomic absorption spectroscopic analysis, thickness and microhardness evaluation. The space worthiness of the coating has been evaluated by humidity, thermal cycling, thermo-vacuum performance tests and measurement of optical properties. The anodic film developed herein provides solar absorptance value as low as 0.16, and infrared ŽIR. emittance of the order of 0.80. These results indicate that the process developed is suitable for thermal control applications in space environment. q 1999 Elsevier Science B.V. All rights reserved. Keywords: AA 2024; Anodize; Solar absorptance; Infrared emittance; Thermal control
1. Introduction A spacecraft in orbit undergoes extreme temperature cycling due to direct sun load on one side and deep cold space on the other. This causes large thermal gradients between the sunlit and shadowed sides of the vehicle. However, the various subsystems of the spacecraft can work at their fullest efficiency within the specified temperature limits. In the absence of an atmosphere, heat exchanged in the spacecraft is limited to radiation. The equilib)
Corresponding author. Tel.: q91-080-5083106; fax: q91080-5266552; Telex: 085-2325
rium temperature of any subsystem is controlled by the ratio of solar absorptance to infrared ŽIR. emittance of its surface. Chemical coatings applied to the spacecraft components play an important role in thermal control by providing suitable optical properties w1,2x. In the absence of any internal power dissipation, the steady state temperature of a spacecraft far from the Earth’s atmosphere may be expressed by Eq. Ž1. w3–5x. T s SA p ars A ´
1r4
Ž 1.
where T is the absolute temperature of the spacecraft, S is the solar constant, s is the Stefan–Boltz-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 2 9 0 - 1
C. SiÕa Kumar et al.r Applied Surface Science 151 (1999) 280–286
mann constant, A p is the projected surface area of the spacecraft perpendicular to the solar rays, A is the total surface area of the spacecraft, a is the solar absorptance of the surface of projected area and ´ is the IR emittance of the exposed surface to space. In this situation where S, A p , A and s are constants, the temperature of any given area of the spacecraft is directly controlled by the ar´ ratio. Here, the term absorptance refers to all solar radiation ŽX-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. It is customary to use Optical Solar Reflectors ŽOSRs. and white paint to maintain equilibrium temperature at certain external surfaces of the spacecraft where high heat dissipation equipment is located. Though the thermal-control white paints provide adequate optical properties Žsolar absorptance, a ; 0.20; IR emittance, ´ ; 0.85. and are used extensively in space missions, a high degradation in their absorptance value has been observed. The change in solar absorptance value is mainly associated with the degradation of their organic base material by exposure to ultraviolet radiation. During a 7-year geo-stationary orbit mission, the solar absorptance value of white paint, which is ; 0.20 at the beginning of life ŽBOL., is estimated to be as high as ; 0.60 at the end of life ŽEOL.. Other problems associated with white paints are their high mass loss, volatile condensable material percentage in space environment, limited shelf-life and poor adhesion. The minimum paint thickness required for optimum optical properties is 50–70 mm. This results in appreciable increase in overall weight of the spacecraft and makes these paints unsuitable on the surfaces where dimensional tolerance is critical. The OSRs Žsecond surface mirror, silverized quartzrteflon. on the other hand provide excellent optical properties and low degradation in solar absorptance ŽBOL: 0.08; EOL: 0.20.. However, the cost of OSRs is enormous and they require a meticulous process of bonding on spacecraft panels due to their delicate nature. Development of white anodic coating on AA 2024 is undertaken as an alternative to the application of white paint and OSRs. The copper content added to increase the strengthrdensity ratio of aluminum alloy produces a heavier and dull shade when
281
anodized w6x. This poses difficulties in optimizing the bath parameters to achieve desired optical properties. Reynolds Metals w7x has developed a white anodizing process for the National Aeronautics and Space Administration ŽNASA. of the USA, which provides 70–80% reflectivity. The main constituents of the anodizing bath are sulfuric acid, polyhydric alcohol, organic carboxylic acid and titanium ammonium lactate. In the present communication, a process of white anodizing with sulfuric acid, glycerol, lactic acid and sodium molybdate is studied.
2. Experimental The 50 mm = 60 mm = 0.5 mm samples of surface area 71 cm2 were processed for white anodizing with the following sequential operations: 1. solvent degreasing using tri-chloroethylene in an ultrasonic bath for 3–5 min; 2. alkaline cleaning at 30–408C for 3–4 min, in a solution containing 50 grl sodium hydroxide, 30 grl tri-sodium orthophosphate and 30 grl sodium meta silicate. Water rinsing and air-drying; 3. chemical polishing in a solution of 80% Žvrv. orthophosphoric acid, 3.5% Žvrv. nitric acid and 0.01% Žwt.. copper at ; 908C for 20–25 s followed by hot water rinsing Ž50 to 608C.; 4. de-smutting in a solution of 50% Žvrv. nitric acid at room temperature Ž23 " 28C. for 30 s followed by water rinsing; 5. anodizing in a solution formulated and operated as per the following parameters: Sulfuric acid ŽS.G. 1.84. Lactic acid Glycerol Sodium molybdate Current density Voltage Time Temperature Coating thickness Post treatment
200 mlrl 28 mlrl 16 mlrl 10 grl 1.3–1.4 Ardm2 Ž1.35. 7–10 V 22 min 23 " 28C ; 10.5 mm Water rinsing
6. sealing in hot distilled water Ž) 988C. for 15–20 min. The pH of the sealing water was maintained within 6.5–7.0, using ammonium acetate Ž1 grl. as a buffering agent.
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All solutions were prepared by using ARrLR grade chemicals and distilled water.
3. Results and discussion 3.1. Process optimization Process optimization was carried out by investigating the influence of coating thickness and various operating parameters, viz.: temperature, electrolyte formulation, and current density on the physico-optical properties of the coating. Wherever the anodic film thickness is not mentioned, the data are given for a constant thickness of 10.5 mm. These data were obtained by conducting a set of experiments to achieve a coating thickness close to the optimum value of 10.5 " 0.5 mm. The corresponding coating properties were then obtained at 10.5 mm thickness by graphical plotting. 3.1.1. Influence of chemical polishing Chemical polishing was performed in order to render a smooth surface prior to anodizing, which helps in improving the reflectance of the coating. Specular reflectance was further improved by the addition of copper w8,9x at 0.01% Žwt.. to the polishing bath. At optimum operating conditions, anodiz-
Fig. 1. Influence of chemical polishing time on solar absorptance of the coating. Conditions: coating thicknesss10.5 mm; current density s1.35 Ardm2 ; temperatures 238C.
Fig. 2. Influence of electrolyte temperature on the a r ´ ratio. Coating thicknesss10.5 mm; current density s1.35 Ardm2 .
ing after chemical polishing showed an improvement Ždrop. in the solar absorptance values by about 25% ŽFig. 1.. 3.1.2. Influence of electrolyte temperature Influence of the bath temperature on the coating properties was studied in the temperature range of 15–508C ŽFig. 2.. The optimum results were obtained at room temperature 23 " 28C. At lower temperatures, the coating formed with high solar absorptance value. At higher temperatures, the coating starts to dissolve in the electrolyte. Consequently, it is difficult to achieve the optimum coating thickness. The coatings so formed were very soft, highly porous and had low IR emittance. 3.1.3. Bath constituents: influence of sodium molybdate concentration In the present studies, sulfuric acid is used as a conducting media and lactic acid as a voltage suppressor. Glycerol is added to minimize the corrosive effect of electrolyte particularly on high copper containing alloys and sodium molybdate to increase the specular reflectivity of the coating. The high ionic conductivity and diffusion coefficient of molybdate anion w10x provides adequate chemical polishing effects on the sample surface while anodizing. This helps in improving the surface reflectivity of the anodic film.
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283
The concentration of sodium molybdate varied from 2.5 grl to 20 grl in successive steps and the changes in the optical properties were investigated. Fig. 3 shows the initial gradual improvement of the optical properties, with increasing molybdate concentration, initially. The optimum value Žlowest ar´ ratio. was obtained at a concentration of 10 grl. Further addition of molybdate results in an increase in the ar´ ratio of the coating. The anodic film obtained at optimum operating conditions showed incorporation of molybdenum Ž32 ppmrgm of coating. when analyzed through atomic absorption spectroscopy. 3.1.4. Influence of the anodic film thickness Emittance is a surface phenomenon. During anodizing, the aluminum surface is converted into alumina. The emittance of the sample surface increases with the growth of anodic film thickness. The effect of anodic film thickness on solar absorptance Ž a ., IR emittance Ž ´ . and the ar´ ratio is shown in Figs. 4–6. In the initial stages, a sharp increase in the emittance value of the surface was observed with the growth of the anodic film; however, it becomes slow at latter stages. The solar absorptance of the film also increases with film thickness, but with a reverse pattern, to that observed for IR emittance. A slow increase was observed with initial increase in
Fig. 3. Effect of salt concentration on the a r ´ ratio. Coating thicknesss10.5 mm; current density s1.35 Ardm2 ; temperature s 238C.
Fig. 4. Variation of solar absorptance with the coating thickness. Current density s1.35 Ardm2 ; temperatures 238C.
coating thickness and a sharp increase at the latter stage. It is very important therefore, to limit the anodic film thickness for optimum optical properties. The optimum coating thickness of anodic film was found to be 10.5 mm. No change in optical properties of the anodic film was observed after pore-sealing operation. 3.1.5. Influence of current density The current density determines the rate of film growth and nature of deposits. To investigate the
Fig. 5. Variation of IR emittance with the coating thickness. Current density s1.35 Ardm2 ; temperatures 238C.
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Fig. 6. Variation of the a r ´ ratio with the coating thickness. Current density s1.35 Ardm2 ; temperatures 238C.
influence of applied current density on the optical behavior of the coating, experiments were conducted at various constant current densities varying from 0.5 to 2.15 Ardm2 . The electrolyte temperature was maintained at 23 " 28C throughout the experiment. At lower applied current density Ž- 1.0 Ardm2 ., the rate of film growth was too low, and the coating obtained showed poor absorptance. At higher current density Ž) 1.6 Ardm2 ., anodic coatings obtained had a chalky appearance with an increase in absorptance value. The influence of applied current density on the ar´ ratio of anodic film is presented in Fig. 7. It is apparent from this figure that the lowest ratio is obtained at an applied current density range of 1.3–1.4 Ardm2 Ž7–10 V..
All anodized specimens were visually examined for any defects. The coatings were perfectly uniform, and no defects such as discontinuity, patches, or powdery deposition were observed. After visual inspection, the anodized specimens were subjected to evaluation tests and measurements. Adhesion was evaluated by a Scotch tape peel-off test. Masking tape of 25-mm width type 3M500 Žpressure 200 grcm2 . was applied over the film by passing a 2-kg rubber roller over the tape twice. The tape was then quickly removed in a direction normal to the surface, and the test specimens were examined visually for any coating removal. No detachment of the film from the substrate was observed. This test was also conducted after humidity, thermal cycling and thermovacuum performance tests. The test results showed excellent adhesion of the coating to the substrate. The thickness of the coating was measured using an ISOSCOPE MP 2BEB, Helmut Fischer ŽGermany., coating thickness tester. A coating thickness of 10.5 mm is recommended for optimum optical properties Žlowest ar´ ratio.. The microhardness of the white anodized coupons were measured with a Shimadzu Microhardness tester type M ŽKyoto, Japan. using a Vickers indent. Vickers hardness numbers were obtained by averaging seven measurements of each specimen with a load of
3.2. Testing and eÕaluation To evaluate the performance of the anodic film in pre-launch and post-launch environments, test coupons anodized at optimum conditions were subjected to humidity, thermal cycling and thermovacuum performance tests. Humidity test was conducted to examine the resistance of the anodic film to corrosive pre-launch and post-launch conditions. The thermal cycling and thermovacuum performance tests were designed to evaluate the effect of on-orbit cycling temperature of the spacecraft on the physico-optical properties of anodic film.
Fig. 7. Effect of current density on the a r ´ ratio. Coating thicknesss10.5 mm; temperatures 238C.
C. SiÕa Kumar et al.r Applied Surface Science 151 (1999) 280–286
15 g for 15 s. The microhardness of the coating was found in the range of 180 to 190 HV0.015 . The humidity test is designed to check the effect of humidity and high temperature, which in turn shows the resistance of the coatings to corrosive atmosphere. The test was conducted in a thermostatically controlled humidity chamber. The relative humidity in the chamber was maintained at 95 " 0.5% at 508C. After the test, specimens were visually examined, and their optical properties Žsolar absorptance and IR emittance. were measured. No changes in optical properties and no physical degradation Ždiscoloration, corrosion spots, patches or peeling. of the coating were observed. The thermal cycling test is designed to evaluate the effect of cycling temperatures, which are likely to be encountered throughout the life span of spacecraft, on the physico-optical properties of the coating. The test was conducted in a thermostatically controlled chamber. A total of 1500 cycles was applied. A cycle consists of lowering the temperature to y458C, a dwell of 5 min, and raising the temperature to 858C with a dwell of 5 min. After the thermal cycling, the test specimens were inspected visually, and their optical properties were measured. No degradation was observed. The thermovacuum performance test is designed to examine the effect of cycling temperature on the coating in the space environment Ži.e., in vacuum.. The test was conducted in a thermostatically controlled vacuum chamber. The test consists of lowering the temperature to y458C with a dwell of 2 h, and increasing the temperature to 858C with a dwell of 2 h. Ten cycles of hot and cold soak were applied, and a vacuum level below 10y5 Torr was maintained inside the chamber during the test. No sign of any degradation on the coating was noticed after the test. The optical properties, specifically solar reflectance and IR emittance of the coating, were measured using a solar reflectometer version 50, Model SSR-ER and an emissometer model RD-1, respectively. Both these instruments provide an average value of solar absorptance and IR emittance digitally over the entire solar or IR region. To evaluate the environmental stability of the anodic coating, the measurement of optical properties were carried out before and after each of the environment tests Žhumidity, thermal cycling and thermovacuum tests..
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No change in optical properties was observed. This clearly indicates excellent stability of the coating to the space environment.
4. Conclusion Ž1. A white anodizing process on AA 2024 alloy was obtained in an electrolyte system containing: 200 mlrl sulfuric acid, 16 mlrl glycerol, 28 mlrl lactic acid and 10 grl sodium molybdate. The operating temperature was 23 " 28C at a current density of 1.3–1.4 Ardm2 for 22 min. The process provides low solar absorptance and high thermal emittance values, which are extremely suitable for thermal control applications. Ž2. A drop in the solar absorptance value of the white anodic film by ; 25% was achieved by chemical polishing pre-treatment at 908C for 20–25 s in a solution containing 80% orthophosphoric acid Žvrv., 3.5% nitric acid Žvrv. and 0.01% copper Žwt... Ž3. The process described herein provides highly reproducible results. A coating thickness Žmost important parameter for optical properties. of 10.5 " 0.5 mm is obtained under optimum operating conditions. Ž4. The white anodic coatings are highly stable. The humidity, thermal cycling and thermovacuum performance tests have no adverse effect on the physico-optical properties of the coating. This shows high stability of the coating in adverse space conditions.
Acknowledgements The author gratefully acknowledges the financial support of ISRO Satellite Centre, Bangalore for this project. The authors are thankful to A.V. Patki, H. Narayanamurthy and H. Bhojraj of ISRO Satellite Centre for encouragement, guidance and providing necessary testing and evaluation facilities.
References w1x A.K. Sharma, Trans. SAEST 30 Ž1995. 1. w2x M. Kelleher Therese, in: Symposium on Anodizing Aluminum, Aluminum Federation, Birmingham, 1967, 71.
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w3x A.K. Sharma, H. Bhojraj, V.K. Kaila, H. Narayanamurthy, J. Aerospace 102 Ž1993. 865, Section 1. w4x A.K. Sharma, H. Narayanamurthy, H. Bhojraj, J.Md. Mohideen, J. Spacecr. Technol. 2 Ž2. Ž1992. 35. w5x B.N. Agarwal, Design of Geo-synchronous Spacecraft, Prentice-Hall, Englewood Cliffs, NJ, 1986, p. 281. w6x A. Charles Grubbs, in: Metal Finishing Guidebook 1998, Vol. 26 Ž1., p. 480. w7x NASA Washington, 1969. 54P.PB-184005; S. Wernick, R.
Pinner, Surface Treatment and Finishing of Al and its Alloys, Vol. 2, Robert Draper, NY, 1972, p. 545. w8x J.B. Mohler, Finishing of Aluminum, Van Nostrand-Reinhold, New York, 1963, p. 51. w9x E.A. Culpan, D.J. Arrowsmith, Transactions of Institute of Metal Finishing 51 Ž1973. 17. w10x R.L. David, CRC Handbook of Chemistry and Physics, 76th edn., CRC Press, Boca Raton, FL, USA, 1995–1996, pp. 5–90.