Effect of trivalent iron and trivalent chromium ions on chromium plating

Chromium electrodeposition is nowadays considered as one of the most imporomt electrodeposition processes. Chrome deposits are used both for decorative and for functional applications. A thin, hlue-tinged chrome deposit is normally applied on nickel undercoats in the decorative nickel-chromium finish. Such nickel-chromium decorative finishes are applied on a number of decorative articles like cycle components, automobile components, household articles, office furnitures, etc. In industry a thick chromium deposit is used as hard chromium and is used to coat on a numher of components which are resistant to wear and tear. The high hardness of chromium resists wear and tear and improves the service life of the components. Such an industrially important chromium is normally deposited from conventional chrome hath consisting of 250 g/l chromic acid and 2.5 g/l sulphuric acid and operated at 323 K and at a current density of 15.5 and 46.5 A/dm2• Chromium is deposited from hexavalent chromium using insoluble anodes. However, during the normal working of the bath, there is a gradual increase in Cr3+ concentration in the hath due to the incomplete oxidation of Cr3 + and Cr6+ at the anode. Buildup of Iron(III) in chrome bath occurs in a number of ways. They are a) carry over from previous operations due to poor washing of the components d) dissolution of substrate when jobs are allowed to be in chrome bath without current, c) due to anudic etching from the plating bath itself and d) etching of fallen articles remaining in the hath. Thus the entry of Cr3+ and Fe3+ are inevitahle in the chromium bath. In order to study tbe effect of iron and trivalent chromium in the production of chrome deposit the study was undertaken and the results on the nature of the deposit, current efficiency, rate of build up, hardness and porosity are presented in this paper