Selective solar surfaces

Claim 1

A process for the production of a solar collector for operation at temperatures in the range 100° to 300° C. or more comprises immersing a stainless steel plate in a solution of chromic acid and sulphuric acid, with or without other constituents, at a temperature of about 65° C. or more, to give a colored heavily hydrated chromium-rich spinel oxide surface film of thickness of from about 0.02 to about 0.5 μm, said surface film comprising a tortuous network of interlinking pathways of effective length of about twice the film thickness, washing and drying the surface and subjecting it to DC electrolysis in a bath selected from the group comprising a nickel plating bath and a palladium plating bath, said electrolysis continuing from a time sufficient to deposit metal into the pores and thereby effect a color change of the oxide film but insufficient for metal to be deposited on the surface of the film.