Electrodeposited multilayer coating for titanium

Abstract

A process for the electrodeposition of a multilayered coating on an article formed of titanium or titanium alloy comprises the steps of: (i) cleaning the article by conventional means, such as by descaling, dry blasing, soaking, and rinsing, so as to remove oxides and other surface contaminants therefrom and expose the article's titanium substrate; (ii) immmersing the clean article in an agitated acid bath at room temperature to activate the surface thereof; (iii) immersing the thus activated article in an agitated aqueous nickel bath preferably comprising nickel chloride at room temperature and cathodically applying a relatively thin non-porous nickel base coat directly on the exposed titanium substrate to improve subsequent zinc adhesion thereto; and (iv) immersing the nickel-plated article in an agitated aqueous zinc bath preferably comprising zinc oxide, sodium cyanide, sodium hydroxide and sodium carbonate and electrodepositing a zinc layer of desired thickness over the nickel base coat. The thus coated titanium article is thereafter preferably baked to provide hydrogen relief therefor. Where a bright surface finish is desired, the article is thereafter dipped for a short period in a brightening solution preferably comprising sodium cyanide. Alternatively, an extra measure of corrosion resistance is obtained, where desired, by supplementary chromate treatment of the article subsequent to the hydrogen relief-baking thereof.