ARTICLE HAVING A LAYER OF A NICKEL-PHOSPHOROUS ALLOY AND COATED WITH A PROTECTIVE LAYER

摘要

An article which covered with a nickel-phosphorus alloy and which is covered on its outer surface with a protective layer which comprises CrPO4, Cr(OH)3 and may also comprise Cr2(SO4)3 and in which no nickel is present. The layer provides an excellent corrosion protection. In a particular embodiment the layer is black, scratch-resistant in normal use, cannot be wiped off and is not sensitive to finger prints and is extremely suitable for decorative applications. The layers can be obtained electrolytically in a bath which comprises chromic acid and optionally reserved current in a given frequency range, a given range of current densities and a given range of ratios of cathodic to anodic pulse durations. In order to obtain the black layers the cathodic time duration is only a fraction of black layers the anodic time duration.