Wednesday, January 23, 2008

Metallic Impurities

Cations that may commonly be present in chromium plating baths include chromium (III), iron, copper, nickel, zinc and sodium. Chromium (III) usually result when baths are operated with too large a cathode area and too small an anode area or when organic matter is introduced. The chromium (III) content can be kept down by increasing the area of the lead anodes used relative to the cathode area, or, where this is not practical, by electrolyzing the solution for a time with a relatively large anode area and a small cathode area.
Iron, copper, nickel, zinc and other metallic impurities may be introduced into the bath in various ways; if permitted to accumulate, they cause an increase in the resistivity of the bath. None of the cations discussed has any noteworthy beneficial effect on the operating characteristics of the bath.

Chromium (III) particularly is detrimental, although contrary statements have appeared. The tradition that a small amount of chromium (III) is beneficial when added to a new bath may have grown up due to making such additions in the early days in the form of chromic sulfate or chromium hydroxide precipitated from chromic sulfate and containing some sulfate, thus affecting the catalyst content of the bath. A number of very painstaking investigations have failed to indicate any improvement in new baths with the addition of small amounts of chromium (III) and there is no need to electrolyze made up new bath for this purpose.

The next article is about how to maintain and to control the bath of chromium plating.

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