Immersion blackening of galvalume coatings for photothermal conversion of solar energy

A method for producing black coatings on 'tiahralume' by immersion technique has been studied for solarthermal energy utilization. An electrolytecontaining 100gll nickel sulphate,25 gll ammonium molybdate, 30 gll ammonium sulphate operating at 70°C at a pH of 3.5-4.5 for an immersion time of 20 seconds produces black coatings with optical properties of K= 0.90 and L = 0.30. The coating is post treated in 5 gll sodium dichromate at 60°C for 30 seconds to enhance the corrosion resistance, as otherwise the black coating gets discoloured to brownish shade upon exposure to atmosphere. The influence of bath composition, pH, temperature and immersion time on the optical.properties (