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 d eposit with a thickness of 450 microns and hardnesses of up to 1200 Vickers . The effects of chromium concentration, a number of carboxylic acids, a gr oup of buffers, temperature and solution pH on deposition rate, deposit thi ckness and the ability to plate continuously have been investigated. We pro pose that normal growth of the trivalent chromium deposit would gradually d iminish to either an unacceptably low rate or a complete termination as a r esult of increasing pH, followed by the formation and precipitation of a se ries of Cr(III) hydroxides on the cathode surface. The key to the success o f this plating process lies in the use of carboxylic acids as complexing ag ents to promote the chromium deposition reaction, and a group of buffers-in cluding boric acid, aluminum salt and another carboxylic acid-to keep the s olution pH sufficiently stable within a certain range, hereby preventing th e 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 m aintain the stability of the solution constituents and sustain a high rate of deposition over a prolonged period. The hardness of the deposit can be g reatly increased after heat treatment in a suitable temperature range.