Method of making an article having a layer of a nickel-phosphorus alloy and coated with a protective layer
First Claim
1. A method of producing a protective layer consisting of a nickel-free conversion layer of a hydrated basic chromium phosphate which comprises the components CrPO4 and Cr(OH3), wherein Cr2 (SO4)3 may be present and in which the weight ratio Cr:
- P;
S=1;
(0.2-1.5);
(0-0.5) on an article coated with a layer consisting of a nickel-phosphorus alloy, by treating the coated article in an aqueous solution containing chromate ions characterized in that the coated article is connected as an electrode in aqueous solution of chromic acid at ambient temperature, the electrode being subjected to a periodically reversed current with a frequency between 0.1 and 50 Hz with a current density between 0.2 and 1 A/dm2 and a ratio of cathodic to anodic pulse durations between 0.05 and 20.
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Abstract
An article which is covered with a nickel-phosphorus alloy 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 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.
9 Citations
3 Claims
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1. A method of producing a protective layer consisting of a nickel-free conversion layer of a hydrated basic chromium phosphate which comprises the components CrPO4 and Cr(OH3), wherein Cr2 (SO4)3 may be present and in which the weight ratio Cr:
- P;
S=1;
(0.2-1.5);
(0-0.5) on an article coated with a layer consisting of a nickel-phosphorus alloy, by treating the coated article in an aqueous solution containing chromate ions characterized in that the coated article is connected as an electrode in aqueous solution of chromic acid at ambient temperature, the electrode being subjected to a periodically reversed current with a frequency between 0.1 and 50 Hz with a current density between 0.2 and 1 A/dm2 and a ratio of cathodic to anodic pulse durations between 0.05 and 20. - View Dependent Claims (2, 3)
- P;
Specification