Hard chromium plating from trivalent chromium solution

A new trivalent chromium process has been developed that allows a sustained deposition reaction at a high rate for more than 20 hours and provides a deposit with a thickness of 450 microns and hardnesses of up to 1200 Vickers. The effects of chromium concentration, a number of carboxylic acids, a group of buffers, temperature and solution pH on deposition rate, deposit thickness and the ability to plate continuously have been investigated. We propose that normal growth of the trivalent chromium deposit would gradually diminish to either an unacceptably low rate or a complete termination as a result of increasing pH, followed by the formation and precipitation of a series of Cr(III) hydroxides on the cathode surface. The key to the success of this plating process lies in the use of carboxylic acids as complexing agents to promote the chromium deposition reaction, and a group of buffers-including boric acid, aluminum salt and another carboxylic acid-to keep the solution pH sufficiently stable within a certain range, hereby preventing the precipitation of chromium hydroxides on the cathode surface, Ion exchange membranes must be used to separate the anode from the cathode compartment and to prevent oxidation of solution constituents at the anode. They also maintain the stability of the solution constituents and sustain a high rate of deposition over a prolonged period. The hardness of the deposit can be greatly increased after heat treatment in a suitable temperature range.