An article which is covered with a nickel-phosphorus alloy is covered on its outer surface with a protective layer which comprises CrPO.sub.4, Cr(OH).sub.3 and may also comprise Cr.sub.2 (SO.sub.4).sub.3 and in which no nickel is present. The layer provides 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 phosphoric acid and sulfuric acid using reversed 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 the anodic time duration.