Chromium coatings decorative

Wesley, W. A., Pitting of Decorative Nickel-Chromium Coatings , Tech. Proc. Am. Electroplaters Soc., 50, 9 (1963)... 

Chromium coatings are applied on the surface of other metals for decorative purposes, to enhance resistance, and to lower the coefficient of friction. Radioactive chromium-51 is used as a tracer in the diagnosis of blood volume. 

Metallic coatings such zinc and its alloys are commonly used to protect steel against atmospheric corrosion. Copper and brass are often coated with metals for decorative purposes as well as for improved corrosion resistance. Examples include chromium coated water faucets or gold coated electric contacts in printed circuit boards. Metallic coatings also serve for wear protection in tribological applications. 

Microcracked chromium coatings are produced by first depositing a high stress nickel strike on a sulfur free nickel layer and then a decorative chromium deposit. The uniform crack network results from the interaction of the thin chromium layer and the high stress nickel deposit. The result is a mirror surface as well as a decorative chromium coating. 

As mentioned earlier, a thin overlay of chromium is applied on nickel coating to prevent the corrosion of nickel coatings. Chromium is generally applied in the form of electrodeposit. As chromium coating is resistant to corrosion and has a bright luster, it is used as a decorating coating. Chromium is more active than iron (E° = -0.76 V) and has a tendency to become passive. 

Nickel plating is widely used for a corrosion- and wear-resistant finish. Typical applications, with a thin top coat of electrodeposited chromium, are decorative trim for automotive and consumer products and office furniture. Nickel deposits are also used for nondecorative purposes for improved wear resistance, for example, on pistons, cylinder walls, ball studs, and so forth. 

Chromium electroplating is also used as decorative and hard coatings. Colored and tarnish-resistant chromium decorative coatings are produced over a base deposit of copper and/or nickel for applications such as those noted above for nickel. Hard chromium coatings are used for hydraulic pistons and cylinders, piston rings, aircraft engine parts, and plastic molds, where resistance to wear, heat abrasion, and/or corrosion are required. 

Such agents are of importance in the plating baths used for the deposition of decorative as well as corrosion-protective coatings, e.g. bright nickel plate which confers protection to nickel-chromium coatings. The addition agents in this instance are more specific, more stable and more controllable in their effects than the colloidal type of agent they are characterised by the presence of unsaturated bonds, e.g. dimethyl fumarate. 

Chromium phosphate coatings can be deposited with very low (0.05—0.15 g/m ) weights to give colodess coatings for appHcations such as a paint base or very heavy (2.0—5.0 g/m ) weights for decorative appHcations. 

A.STM B604, Std. Spec, for Decorative Electroplated Coatings of Copper) Nickel) Chromium on Plastics, American Society for Testing and Materials, Philadelphia, Pa., 1991. 

Decorative coatings of nickel plus chromium are cathodic to steel or zinc alloy substrates and with these protective systems deliberate use may be made of discontinuities in the chromium topcoat where corrosion of the underlying nickel will occur. If the number of these discontinuities in the chromium layer is greatly increased the current density at each individual corrosion site is reduced, penetration of corrosion through the thickness of the nickel layer is thus slowed down and the period of protection of the substrate metal is prolonged. 

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