Electrolytic post treatment of chromium substrates

Abstract

An improved process and aqueous electrolyte for imparting improved corrosion resistance to chromium substrates, and particularly nickel-plated substrates having a chromium plating thereover deposited from a trivalent chromium electrolyte. The post-treating solution contains solution soluble and compatible hexavalent chromium compounds including chromates and dichromates present in an amount sufficient to deposit a protective film on the chromium substrate and phosphate ions present in an amount of about 2.5 g/l up to the limit of solubility in the electrolyte. The electrolyte optionally, but preferably, further contains a buffering agent present in an amount to stabilize the pH of the solution within a range from about 2.5 up to about 10. Optionally, the electrolyte further contains at least one metal ion selected from the group consisting of nickel, cobalt and mixtures thereof in an amount up to about 2 g/l, conductivity salts to enhance the electrical conductivity of the solution and a compatible wetting agent. In accordance with the process aspects of the invention, the chromium plated substrate is immersed in the electrolyte and is cathodically electrified to effect the passage of current through the solution between an anode and the substrate at an average cathode current density below that at which metallic chromium is deposited on the substrate for a period of time sufficient to deposit a protective film of the desired thickness thereon.