- Email: [email protected]
Blackening of aluminium-zinc (galvalume) coatings for solar energy utilization
Energy Convers. Mgmt Vol. 26, No. 3/4, pp. 317-320, 1986 Printed in Great Britain. All rights reserved
0196-8904/86 $3.00+ 0.00 Copyright © 1986 Pergamon Journals Ltd
BLACKENING OF ALUMINIUM-ZINC (GALVALUME) COATINGS FOR SOLAR ENERGY UTILIZATION S. J O H N , V. BALASUBRAMANIAN, N. V. S H A N M U G A M , M. SELVAM, K. N. SRINIVASAN and B. A. SHENOI,I, Central Electrochemical Research Institute, Karaikudi 623006, Tamil Nadu, India (Received 30 October 1985)
Abslraet--The authors have reported a method for immersion blackening of Galvalume coatings for use as a selective surface for solar collectors. Such coatings have a solar absorptance (a) of 0.90-0.92 and thermal emittance (E) of 0.25-0.40. The coating has moderate corrosion resistance. In order to improve this, a post-treatment is necessary. The post-treated coatings in dichromate solution offer good corrosion resistance. Thermal cycling tests show that the coatings are stable to 220°C. Tape tests show that the coating is strongly adherent. Selective coatings Black coatings Solar energy Nickel-black
Galvalume
INTRODUCTION Energy is vital to all our endeavours and indeed to the maintenance of life itself. With increases in the cost of fossil fuels, mankind is facing an acute energy crisis. Furthermore, due to the war between oilproducing countries, the cost of oil is continuously rising due to reduced production. Earlier calculations forecast that the presently available oil can last only for 20-30 years. In such a critical situation, the entire world is focusing its attention on unconventional and renewable sources of energy. Among them, solar energy now holds much promise. Efficient conversion of solar radiation into heat requires a selective absorber surface which has high absorptance (ct) across the incoming solar radiation (0.2-2.5/~m) with low thermal emittance (E) in the i.r. region ( > 2.5/~m). Such selective coatings play a major role in improving the efficiency of flat-plate collectors. Among the various selective coatings that have been studied, high-performance coatings based on black chrome [1-7] and black nickel [8-10] increase the efficiency of the collectors. As the cost of production of these coatings is high, attempts are being made to develop cheap and durable coatings. Among the various conversion coatings [11-16], blackening of aluminium [14] has moderate corrosion resistance. Hot dipped aluminium-zinc coatings on steel can replace aluminium. Such a coating on steel, called Galvalume,t is somewhat similar to galvanized iron in that it consists of mild steel carrying a hot dipped protective coating. The coating, however, consists of aluminium-zinc rather than zinc alone. Since Galvalume coating on steel is a newcomer to the market, with 5-10 times salt fog resistance and 3--4 times ,l,Deceased. 1"Trade Mark Bethlehem Steel Corporation, U.S.A.
Flate-plate collectors
Metal colouring
atmospheric corrosion resistance than galvanized iron and has better corrosion resistance than aluminium, the authors have reported a method for immersion blackening of Galvalume coatings for use as a selective surface for solar collectors. Such a coating has a solar absorptance (ct) of 0.90-0.92 and thermal emittance (E) of 0.25--0.40. Cathro [15] has reported a method for producing immersion blackening of Galvalume coatings in a solution containing nickel, zinc and thiocyanate ions with an intermediate step of zincating on Galvalume coatings. The black coating contains nickel, zinc and sulfur and has only moderate corrosion resistance. Blackening of Galvalume in an electrolyte containing nickel and molybdenum salts has been reported earlier by Selvam et al. [17]. The present authors have made an attempt to blacken the Galvalume coatings, avoiding the intermediate zincating step, by immersion in a solution containing only nickel salt and thiocyanate. The characteristics of the black coating have been studied with regard to suitability for solar thermal energy conversion.
EXPERIMENTAL
Galvalume panels (manufactured by Bethlehem Steel Corporation, U.S.A.) of size 100 x 100mm were degreased with a solvent such as trichloroethylene to remove organic compounds, if any were present on the surface, cleaned in an alkaline solution containing 50g/l sodium hydroxide, washed, desmudged in 10% v/v nitric acid, washed and then immersed in the blackening solution containing nickel sulfate (100g/l), ammonium thiocyanate (10 g/l) and ammonium sulfate (30 g/l), at pH 4.5-5.0 and temperature 60-80°C. Immersion time was 15--60s. All the chemicals used were of laboratory 317
JOHN et al.: COATINGS FOR SOLAR ENERGY USE
318
reagent grade. Solar absorptance (ct) and thermal emittance (E) of the black coatings were measured using alphatometer and emissometer (manufactured by Devices and Services Co,, U.S.A.). The adhesion of the coating was tested by the tape test, in which an adhesive tape was pressed evenly on the black coating and then pulled off suddenly with a swift rapid motion. If deposit particles did not come off on the tape, then the coating was considered good and adherent. This test on the blackened panels shows that the coating is strongly adherent.
Thermal cycling test The samples were placed in an electric oven and the temperature was raised from ambient temperature to 220°C within 30 min and maintained for the next 8 h. The oven was switched off and the samples were allowed to cool overnight. This was repeated for seven consecutive days. The object here was to test the coating under conditions of overheating due to failure in the circulation of heat-extracting fluid through fiat-plate collectors. After this test, the samples were scanned under an optical microscope at a magnification of 100 × to detect any possible corrosion or visible damage to the coated surface. ~t and c values were measured before and after this test to note the changes in optical properties.
Post-treatment Since the black deposit is thin and porous in nature, upon exposure to atmosphere, white rust is formed on the coating within 2 days. Hence, it has been decided to give a suitable post-treatment which will minimise the degradation of the coating. The coating immediately after blackening, exhibits gas evolution upon being dipped in water, and the black coating decolourised to a whitish shade. Hence, to test the effectiveness of the post-treatment, the panels were immersed in boiling distilled water for 30 min. If there was any gas evolution and the coating decolourised, then the post-treatment was not good. If there was no gas evolution and the black coating was retained, then the post-treating solution was considered as good. RESULTS AND DISCUSSION
concentration of 100 g/1 was used for our studies. At this level, the coatings produced have optical properties of ct = 0.90 and E = 0.30.
Influence of ammonium thiocyanate concentration Ammonium thiocyanate in the solution produces better coatings than sodium and potassium thiocyanate. The concentration of ammonium thiocyanate was varied between 0 and 20 g/1 by keeping the concentration of nickel sulfate IO0 g/l and that of ammonium sulfate 30 g/l, temperature 70'~C and immersion time 20 s. Without ammonium thiocyanate, only nickel is deposited on the surface of the panel. Addition of thiocyanate ions as ammonium thiocyanate produce black coatings. At lower concentrations ( < 5 g/l), the coating is grey in colour. At higher concentrations ( > 15 g/l), a powdery nonadherent deposit was produced. Better black coatings are obtained within the concentration range of 5-15 g/1. The optimum concentration was chosen as 10 g/l. At this concentration, the black coatings have optical properties of ~t = 0.90 and ~ = 0.30.
Influence of ammonium sulfate concentration The addition of ammonium sulfate to the solution has been found necessary in the production of good coatings. Without ammonium sulfate, the deposit is whitish, non-adherent and smutty. The function of ammonium sulfate is to prevent too great a rise in pH by buffering action and to prevent the formation of basic salts of metal ion. The amount of ammonium sulfate was varied between 10 and 50 g/l. After conducting a few experiments, an optimum concentration of 30 g/l was fixed to produce quality black coatings.
Influence of solution pH The solution pH is an important parameter for the production of better coatings by immersion or by electrodeposition. The pH of the solution was varied between 3 and 6 and was adjusted electrometrically using dilute sulfuric acid and ammonia for an electrolyte containing 100g/l nickel sulfate, 10g/! ammonium thiocyanate and 30g/l ammonium sulfate. Uniform adherent black coating was produced at pH va'lues between 3.5 and 4.5.
Influence of nickel sulfate concentration
Influence of solution temperature
Nickel sulfate concentration was varied between 25 and 200g/l by keeping the concentration of ammonium thiocyanate 10 g/l and that of ammonium sulfate 30 g/l, at a solution temperature of 70°C and immersion time of 20 s. At lower concentrations ( < g/l), the coating is brown in colour and has optical properties of ~ = 0.80 and E = 0.42. At higher concentrations ( > 150g/1), the coating obtained is grey in colour and has optical properties of = 0.85 and E = 0.48. The coatings with better optical properties are obtained at nickel sulfate concentrations between 50 and 150 g/l. Hence, an optimum
At temperatures < 50°C, the reaction is very slow, and the coating is brownish black in colour and also requires longer immersion time ( > 120 s). When the solution temperature is increased, the required immersion time is reduced for producing black coatings with better optical properties. At elevated temperatures (>90°C), copious gas evolution is seen from the substrate, and also the immersion time is reduced very much (i.e. < 5 s). At this level, the control is made very difficult and the coating so produced is rough and dark black, having optical properties of ct = 0.95 and E = 0.52. Better results can
JOHN et al.: COATINGS FOR SOLAR ENERGY USE Table 1. Influence of immersion time, in the blackening solution, on optical properties (~t, E) Optical properties Immersion time Absorptance Emittance (s) (:c) (~) 15 0.87 0.28 20 0.90 0.30 25 0.91 0.36
319
and it was found that there was no corrosion or visible damage to the coated surface. Absorptance (a) and emittance (E) values were measured before (a =0.90; E = 0 . 3 2 ) and after (~t =0.91; ~ =0.31) thermal cycling. This test shows that there was not much change in optical properties. CONCLUSION
be obtained at temperatures between 70 and 80°C. Hence, an optimum temperature was chosen as 70°C. At this temperature, the coating obtained by immersing the substrate for 20 s had optical properties of ~t = 0.90 and E = 0.30. Influence o f immersion time Pretreated Galvalume panels were immersed in the solution containing 100 g/1 nickel sulfate, 10 g/l amm o n i u m thiocyanate and 30 g/l a m m o n i u m sulfate at 70°C. The immersion time was varied between 10 and 40 s. Blackening at shorter duration (10 s) produces grey-coloured coatings and at longer durations ( > 30 s) rough and dark black coatings having optical properties of a = 0.92 and E = 0:45. Table 1 shows the change of value of optical properties with immersion time. Hence, an immersion time of 20 s was considered as the optimum value. Immersion in the above solution for 20s produced a coating of a = 0.90 and E = 0.30. Influence o f post-treatment For post-treatment, a number of solutions based on chromic acid and dichromate have been tried, among which the following solution exhibits good corrosion resistance: sodium dichromate, 5 g/l, at 40-50°C, with an immersion time of 30 s. The posttreated samples were immersed in boiled distilled water for 30 min and no decoloration of the coating was observed. Immersion time in the above solution is very important as it affects the optical properties, because of the formation of a yellowish film. Table 2 shows the optical properties obtained for different duration of dipping time at 50°C. It is evident that an immersion time of 30 s is optimum for good corrosion resistance without affecting the optical properties. Thermal cycling test After conducting thermal cycling tests at 220°C for 7 consecutive days, the coating was scanned under an optical microscope at a magnification of 100 x , Table 2. Influence of dipping time in the posttreating solution on optical properties (~t, E) Optical properties Immersion time Absorptance Emittance (s) (~) (~) 0 0.90 0.30 15 0.90 0.31 30 0.90 0.32 45 0.91 0.34
A method for producing a black coating on Galvalume by an immersion technique has been studied for solar thermal energy conversion. Based on the above investigation, the following composition and operating conditions are recommended: nickel sulfate (100g/l), a m m o n i u m thiocyanate (10g/l), ammonium sulfate (30 g/l), at pH 3.5-4.5 and temperature 70°C, with an immersion time of 20 s. The coating is dipped in 5 g/l sodium dichromate at 50°C for 30 s to enhance the corrosion resistance. Without posttreatment, white rust is formed on the coating in a shorter duration of exposure to the atmosphere. Optical properties of the coating produced under the optimum conditions are found to be ~t = 0.90 and E = 0.30. Further work is necessary to improve the optical properties, i.e. to increase absorptance to 0.98 and to decrease emittance to 0.1. This is possible by modifying the pretreatment conditions, adding suitable additives and by changing operating conditions. Acknowledgement--The authors wish to express their sincere thanks to the Director, Central Electrochemical Research Institute, Karaikudi-6, for kind permission to publish this paper. REFERENCES
1. G. E. McDonald, Solar Energy 17, 119 (1975). 2. L. Sivasamy, S. Gowri and B. A. Shenoi, Met. Fin. 72, 48 (1974). 3. K. J. Cathro, Met. Fin. 76, 57 (1978). 4. A.C. Benning, AES Coatings for Solar Collectors Syrup. American Electroplaters Society, Atlanta, G. A. (1976). 5. M. Selvam, N. V. Shanmugam, K. N. Srinivasan, S. John and B. A. Shenoi Proc. Natl. Solar Energy Convention, p. 246. Allied Publishers, New Delhi (1980). 6. M. Selvam, K. N. Srinivasan, N. V. Shanmugam, S. John and B. A. Shenoi, Met. Fin. 80, 107 (1982). 7. M. Selvam, K. N. Srinivasan, N. V. Shanmugam, S. John and B. A. Shenoi, Met. Rin. 81, 37 (1983). 8. R. E. Peterson and J. H. Lin, Improvement of black nickel coatings. NASA, CR 149928 (1976). 9. E. A. Serfass, R. F. Muraca and W. R. Mayer, Proc. Am. Electropleters Soc. An. Conf. 39, 101 (1952). 10. K. N. Srinivasan, N. V. Shanmugam, M. Selvam, S. John and B. A. Shenoi, Energy Convers. Mgmt 24, 255 (1984). I 1. H. Tabor, J. Harries, H. Wenberger and B. Doron, UN Conf. on New Sources of Energy, Paper E, 35/546 (1961). 12. P. K. Gogra and K. L. Chopra, Solar Energy 23, 405 (1976). 13. J. B. Hajdu and T. E. Sullivan, A note on conversion coatings as selective surface. Enthone, Inc. Product Catalogue (1978). 14. N. V. Shanmugam, K. N. Srinivasan, S. John, M. Selvam and B. A. Shenoi, Proc. Natl. Solar Energy Convention, 1981, SESI, India. SS:05, 7.021 (1982).
320
JOHN et al.: COATINGS FOR SOLAR ENERGY USE
15. K. J. Cathro, Solar Energy Mat. 5, 317 (1981). 16. S. John, N. V. Shanmugam, K, N. Srinivasan, M. Selvam and B. A. Shenoi, Surface Technol. 20, 331 (1983).
17. M. Selvam, S. John, V. Balasubramanian, N. V. Shanmugam, K. N. Srinivasan and B. A. Shenoi, Bull. electrochem. (India) 2, 1, (1986).
0196-8904/86 $3.00+ 0.00 Copyright © 1986 Pergamon Journals Ltd
BLACKENING OF ALUMINIUM-ZINC (GALVALUME) COATINGS FOR SOLAR ENERGY UTILIZATION S. J O H N , V. BALASUBRAMANIAN, N. V. S H A N M U G A M , M. SELVAM, K. N. SRINIVASAN and B. A. SHENOI,I, Central Electrochemical Research Institute, Karaikudi 623006, Tamil Nadu, India (Received 30 October 1985)
Abslraet--The authors have reported a method for immersion blackening of Galvalume coatings for use as a selective surface for solar collectors. Such coatings have a solar absorptance (a) of 0.90-0.92 and thermal emittance (E) of 0.25-0.40. The coating has moderate corrosion resistance. In order to improve this, a post-treatment is necessary. The post-treated coatings in dichromate solution offer good corrosion resistance. Thermal cycling tests show that the coatings are stable to 220°C. Tape tests show that the coating is strongly adherent. Selective coatings Black coatings Solar energy Nickel-black
Galvalume
INTRODUCTION Energy is vital to all our endeavours and indeed to the maintenance of life itself. With increases in the cost of fossil fuels, mankind is facing an acute energy crisis. Furthermore, due to the war between oilproducing countries, the cost of oil is continuously rising due to reduced production. Earlier calculations forecast that the presently available oil can last only for 20-30 years. In such a critical situation, the entire world is focusing its attention on unconventional and renewable sources of energy. Among them, solar energy now holds much promise. Efficient conversion of solar radiation into heat requires a selective absorber surface which has high absorptance (ct) across the incoming solar radiation (0.2-2.5/~m) with low thermal emittance (E) in the i.r. region ( > 2.5/~m). Such selective coatings play a major role in improving the efficiency of flat-plate collectors. Among the various selective coatings that have been studied, high-performance coatings based on black chrome [1-7] and black nickel [8-10] increase the efficiency of the collectors. As the cost of production of these coatings is high, attempts are being made to develop cheap and durable coatings. Among the various conversion coatings [11-16], blackening of aluminium [14] has moderate corrosion resistance. Hot dipped aluminium-zinc coatings on steel can replace aluminium. Such a coating on steel, called Galvalume,t is somewhat similar to galvanized iron in that it consists of mild steel carrying a hot dipped protective coating. The coating, however, consists of aluminium-zinc rather than zinc alone. Since Galvalume coating on steel is a newcomer to the market, with 5-10 times salt fog resistance and 3--4 times ,l,Deceased. 1"Trade Mark Bethlehem Steel Corporation, U.S.A.
Flate-plate collectors
Metal colouring
atmospheric corrosion resistance than galvanized iron and has better corrosion resistance than aluminium, the authors have reported a method for immersion blackening of Galvalume coatings for use as a selective surface for solar collectors. Such a coating has a solar absorptance (ct) of 0.90-0.92 and thermal emittance (E) of 0.25--0.40. Cathro [15] has reported a method for producing immersion blackening of Galvalume coatings in a solution containing nickel, zinc and thiocyanate ions with an intermediate step of zincating on Galvalume coatings. The black coating contains nickel, zinc and sulfur and has only moderate corrosion resistance. Blackening of Galvalume in an electrolyte containing nickel and molybdenum salts has been reported earlier by Selvam et al. [17]. The present authors have made an attempt to blacken the Galvalume coatings, avoiding the intermediate zincating step, by immersion in a solution containing only nickel salt and thiocyanate. The characteristics of the black coating have been studied with regard to suitability for solar thermal energy conversion.
EXPERIMENTAL
Galvalume panels (manufactured by Bethlehem Steel Corporation, U.S.A.) of size 100 x 100mm were degreased with a solvent such as trichloroethylene to remove organic compounds, if any were present on the surface, cleaned in an alkaline solution containing 50g/l sodium hydroxide, washed, desmudged in 10% v/v nitric acid, washed and then immersed in the blackening solution containing nickel sulfate (100g/l), ammonium thiocyanate (10 g/l) and ammonium sulfate (30 g/l), at pH 4.5-5.0 and temperature 60-80°C. Immersion time was 15--60s. All the chemicals used were of laboratory 317
JOHN et al.: COATINGS FOR SOLAR ENERGY USE
318
reagent grade. Solar absorptance (ct) and thermal emittance (E) of the black coatings were measured using alphatometer and emissometer (manufactured by Devices and Services Co,, U.S.A.). The adhesion of the coating was tested by the tape test, in which an adhesive tape was pressed evenly on the black coating and then pulled off suddenly with a swift rapid motion. If deposit particles did not come off on the tape, then the coating was considered good and adherent. This test on the blackened panels shows that the coating is strongly adherent.
Thermal cycling test The samples were placed in an electric oven and the temperature was raised from ambient temperature to 220°C within 30 min and maintained for the next 8 h. The oven was switched off and the samples were allowed to cool overnight. This was repeated for seven consecutive days. The object here was to test the coating under conditions of overheating due to failure in the circulation of heat-extracting fluid through fiat-plate collectors. After this test, the samples were scanned under an optical microscope at a magnification of 100 × to detect any possible corrosion or visible damage to the coated surface. ~t and c values were measured before and after this test to note the changes in optical properties.
Post-treatment Since the black deposit is thin and porous in nature, upon exposure to atmosphere, white rust is formed on the coating within 2 days. Hence, it has been decided to give a suitable post-treatment which will minimise the degradation of the coating. The coating immediately after blackening, exhibits gas evolution upon being dipped in water, and the black coating decolourised to a whitish shade. Hence, to test the effectiveness of the post-treatment, the panels were immersed in boiling distilled water for 30 min. If there was any gas evolution and the coating decolourised, then the post-treatment was not good. If there was no gas evolution and the black coating was retained, then the post-treating solution was considered as good. RESULTS AND DISCUSSION
concentration of 100 g/1 was used for our studies. At this level, the coatings produced have optical properties of ct = 0.90 and E = 0.30.
Influence of ammonium thiocyanate concentration Ammonium thiocyanate in the solution produces better coatings than sodium and potassium thiocyanate. The concentration of ammonium thiocyanate was varied between 0 and 20 g/1 by keeping the concentration of nickel sulfate IO0 g/l and that of ammonium sulfate 30 g/l, temperature 70'~C and immersion time 20 s. Without ammonium thiocyanate, only nickel is deposited on the surface of the panel. Addition of thiocyanate ions as ammonium thiocyanate produce black coatings. At lower concentrations ( < 5 g/l), the coating is grey in colour. At higher concentrations ( > 15 g/l), a powdery nonadherent deposit was produced. Better black coatings are obtained within the concentration range of 5-15 g/1. The optimum concentration was chosen as 10 g/l. At this concentration, the black coatings have optical properties of ~t = 0.90 and ~ = 0.30.
Influence of ammonium sulfate concentration The addition of ammonium sulfate to the solution has been found necessary in the production of good coatings. Without ammonium sulfate, the deposit is whitish, non-adherent and smutty. The function of ammonium sulfate is to prevent too great a rise in pH by buffering action and to prevent the formation of basic salts of metal ion. The amount of ammonium sulfate was varied between 10 and 50 g/l. After conducting a few experiments, an optimum concentration of 30 g/l was fixed to produce quality black coatings.
Influence of solution pH The solution pH is an important parameter for the production of better coatings by immersion or by electrodeposition. The pH of the solution was varied between 3 and 6 and was adjusted electrometrically using dilute sulfuric acid and ammonia for an electrolyte containing 100g/l nickel sulfate, 10g/! ammonium thiocyanate and 30g/l ammonium sulfate. Uniform adherent black coating was produced at pH va'lues between 3.5 and 4.5.
Influence of nickel sulfate concentration
Influence of solution temperature
Nickel sulfate concentration was varied between 25 and 200g/l by keeping the concentration of ammonium thiocyanate 10 g/l and that of ammonium sulfate 30 g/l, at a solution temperature of 70°C and immersion time of 20 s. At lower concentrations ( < g/l), the coating is brown in colour and has optical properties of ~ = 0.80 and E = 0.42. At higher concentrations ( > 150g/1), the coating obtained is grey in colour and has optical properties of = 0.85 and E = 0.48. The coatings with better optical properties are obtained at nickel sulfate concentrations between 50 and 150 g/l. Hence, an optimum
At temperatures < 50°C, the reaction is very slow, and the coating is brownish black in colour and also requires longer immersion time ( > 120 s). When the solution temperature is increased, the required immersion time is reduced for producing black coatings with better optical properties. At elevated temperatures (>90°C), copious gas evolution is seen from the substrate, and also the immersion time is reduced very much (i.e. < 5 s). At this level, the control is made very difficult and the coating so produced is rough and dark black, having optical properties of ct = 0.95 and E = 0.52. Better results can
JOHN et al.: COATINGS FOR SOLAR ENERGY USE Table 1. Influence of immersion time, in the blackening solution, on optical properties (~t, E) Optical properties Immersion time Absorptance Emittance (s) (:c) (~) 15 0.87 0.28 20 0.90 0.30 25 0.91 0.36
319
and it was found that there was no corrosion or visible damage to the coated surface. Absorptance (a) and emittance (E) values were measured before (a =0.90; E = 0 . 3 2 ) and after (~t =0.91; ~ =0.31) thermal cycling. This test shows that there was not much change in optical properties. CONCLUSION
be obtained at temperatures between 70 and 80°C. Hence, an optimum temperature was chosen as 70°C. At this temperature, the coating obtained by immersing the substrate for 20 s had optical properties of ~t = 0.90 and E = 0.30. Influence o f immersion time Pretreated Galvalume panels were immersed in the solution containing 100 g/1 nickel sulfate, 10 g/l amm o n i u m thiocyanate and 30 g/l a m m o n i u m sulfate at 70°C. The immersion time was varied between 10 and 40 s. Blackening at shorter duration (10 s) produces grey-coloured coatings and at longer durations ( > 30 s) rough and dark black coatings having optical properties of a = 0.92 and E = 0:45. Table 1 shows the change of value of optical properties with immersion time. Hence, an immersion time of 20 s was considered as the optimum value. Immersion in the above solution for 20s produced a coating of a = 0.90 and E = 0.30. Influence o f post-treatment For post-treatment, a number of solutions based on chromic acid and dichromate have been tried, among which the following solution exhibits good corrosion resistance: sodium dichromate, 5 g/l, at 40-50°C, with an immersion time of 30 s. The posttreated samples were immersed in boiled distilled water for 30 min and no decoloration of the coating was observed. Immersion time in the above solution is very important as it affects the optical properties, because of the formation of a yellowish film. Table 2 shows the optical properties obtained for different duration of dipping time at 50°C. It is evident that an immersion time of 30 s is optimum for good corrosion resistance without affecting the optical properties. Thermal cycling test After conducting thermal cycling tests at 220°C for 7 consecutive days, the coating was scanned under an optical microscope at a magnification of 100 x , Table 2. Influence of dipping time in the posttreating solution on optical properties (~t, E) Optical properties Immersion time Absorptance Emittance (s) (~) (~) 0 0.90 0.30 15 0.90 0.31 30 0.90 0.32 45 0.91 0.34
A method for producing a black coating on Galvalume by an immersion technique has been studied for solar thermal energy conversion. Based on the above investigation, the following composition and operating conditions are recommended: nickel sulfate (100g/l), a m m o n i u m thiocyanate (10g/l), ammonium sulfate (30 g/l), at pH 3.5-4.5 and temperature 70°C, with an immersion time of 20 s. The coating is dipped in 5 g/l sodium dichromate at 50°C for 30 s to enhance the corrosion resistance. Without posttreatment, white rust is formed on the coating in a shorter duration of exposure to the atmosphere. Optical properties of the coating produced under the optimum conditions are found to be ~t = 0.90 and E = 0.30. Further work is necessary to improve the optical properties, i.e. to increase absorptance to 0.98 and to decrease emittance to 0.1. This is possible by modifying the pretreatment conditions, adding suitable additives and by changing operating conditions. Acknowledgement--The authors wish to express their sincere thanks to the Director, Central Electrochemical Research Institute, Karaikudi-6, for kind permission to publish this paper. REFERENCES
1. G. E. McDonald, Solar Energy 17, 119 (1975). 2. L. Sivasamy, S. Gowri and B. A. Shenoi, Met. Fin. 72, 48 (1974). 3. K. J. Cathro, Met. Fin. 76, 57 (1978). 4. A.C. Benning, AES Coatings for Solar Collectors Syrup. American Electroplaters Society, Atlanta, G. A. (1976). 5. M. Selvam, N. V. Shanmugam, K. N. Srinivasan, S. John and B. A. Shenoi Proc. Natl. Solar Energy Convention, p. 246. Allied Publishers, New Delhi (1980). 6. M. Selvam, K. N. Srinivasan, N. V. Shanmugam, S. John and B. A. Shenoi, Met. Fin. 80, 107 (1982). 7. M. Selvam, K. N. Srinivasan, N. V. Shanmugam, S. John and B. A. Shenoi, Met. Rin. 81, 37 (1983). 8. R. E. Peterson and J. H. Lin, Improvement of black nickel coatings. NASA, CR 149928 (1976). 9. E. A. Serfass, R. F. Muraca and W. R. Mayer, Proc. Am. Electropleters Soc. An. Conf. 39, 101 (1952). 10. K. N. Srinivasan, N. V. Shanmugam, M. Selvam, S. John and B. A. Shenoi, Energy Convers. Mgmt 24, 255 (1984). I 1. H. Tabor, J. Harries, H. Wenberger and B. Doron, UN Conf. on New Sources of Energy, Paper E, 35/546 (1961). 12. P. K. Gogra and K. L. Chopra, Solar Energy 23, 405 (1976). 13. J. B. Hajdu and T. E. Sullivan, A note on conversion coatings as selective surface. Enthone, Inc. Product Catalogue (1978). 14. N. V. Shanmugam, K. N. Srinivasan, S. John, M. Selvam and B. A. Shenoi, Proc. Natl. Solar Energy Convention, 1981, SESI, India. SS:05, 7.021 (1982).
320
JOHN et al.: COATINGS FOR SOLAR ENERGY USE
15. K. J. Cathro, Solar Energy Mat. 5, 317 (1981). 16. S. John, N. V. Shanmugam, K, N. Srinivasan, M. Selvam and B. A. Shenoi, Surface Technol. 20, 331 (1983).
17. M. Selvam, S. John, V. Balasubramanian, N. V. Shanmugam, K. N. Srinivasan and B. A. Shenoi, Bull. electrochem. (India) 2, 1, (1986).