Energy Convers. Mgmt Vol. 26, No. 3/4, pp. 317-320, 1986 0196-8904/86 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1986 Pergamon Journals Ltd

BLACKENING OF ALUMINIUM-ZINC (GALVALUME) COATINGS FOR SOLAR ENERGY UTILIZATION

S. JOHN, V. BALASUBRAMANIAN, N. V. SHANMUGAM, 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 Galvalume Flate-plate collectors Metal colouring Solar energy Nickel-black

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 oil- producing 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 con- version of solar radiation into heat requires a selec- tive 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 Gal- valume 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.

atmospheric corrosion resistance than galvanized iron and has better corrosion resistance than alumi- nium, the authors have reported a method for im- mersion 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 im- mersion 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 trichloro- ethylene to remove organic compounds, if any were present on the surface, cleaned in an alkaline solution containing 50g/l sodium hydroxide, washed, de- smudged 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

318 JOHN et al.: COATINGS FOR SOLAR ENERGY USE

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

Influence of nickel sulfate concentration

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 concentra- tions (< 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 ob- tained is grey in colour and has optical properties of

= 0.85 and E = 0.48. The coatings with better opti- cal properties are obtained at nickel sulfate concen- trations between 50 and 150 g/l. Hence, an optimum

concentration of 100 g/1 was used for our studies. At this level, the coatings produced have optical proper- ties 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 thio- cyanate. The concentration of ammonium thio- cyanate 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 im- mersion time 20 s. Without ammonium thiocyanate, only nickel is deposited on the surface of the panel. Addition of thiocyanate ions as ammonium thio- cyanate produce black coatings. At lower concen- trations ( < 5 g/l), the coating is grey in colour. At higher concentrations (> 15 g/l), a powdery non- adherent 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 con- ducting a few experiments, an optimum concen- tration 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 electro- lyte containing 100g/l nickel sulfate, 10g/! ammo- nium thiocyanate and 30g/l ammonium sulfate. Uniform adherent black coating was produced at pH va'lues between 3.5 and 4.5.

Influence of solution temperature

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 im- mersion time is reduced for producing black coatings with better optical properties. At elevated tem- peratures (>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 319

Table 1. Influence of immersion time, in the black- ening 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

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 .32) and after (~t =0.91; ~ =0.31) thermal cycling. This test shows that there was not much change in optical properties.

be obtained at temperatures between 70 and 80°C. Hence, an opt imum temperature was chosen as 70°C. At this temperature, the coating obtained by im- mersing 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 am- monium thiocyanate and 30 g/l ammonium sulfate at 70°C. The immersion time was varied between 10 and 40 s. Blackening at shorter durat ion (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 im- mersion time. Hence, an immersion time of 20 s was considered as the opt imum 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 post- treated samples were immersed in boiled distilled water for 30 min and no decolorat ion 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 durat ion of dipping time at 50°C. It is evident that an im- mersion time of 30 s is opt imum 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 post- treating 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

CONCLUSION

A method for producing a black coating on Gal- valume by an immersion technique has been studied for solar thermal energy conversion. Based on the above investigation, the following composit ion and operating conditions are recommended: nickel sulfate (100g/l), ammonium thiocyanate (10g/l), ammo- nium 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 post- treatment, white rust is formed on the coating in a shorter durat ion of exposure to the atmosphere. Optical properties of the coating produced under the opt imum 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 suit- able 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.

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