Chromium coatings discontinuities

Under micro-discontinuous chromium coatings, copper undercoats improve corrosion resistance. On non-conductors, especially on plastic substrates, copper is often applied before nickel-chromium plating over the initial electroless copper or nickel deposit in order to improve ductility and adhesion, e.g. as tested by the standard thermal-cycling test methods ... 

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. 

The case is different again under micro-discontinuous (i.e. micro-cracked or micro-porous) chromium, on which a definite improvement in corrosion resistance can be achieved when copper is present under the nickel coating . 

G. Grube and co-workers hold that chromium goes primarily into soln. as bivalent ions at lower current densities, and that these are immediately oxidized to the tervalent condition, whilst at higher current densities sexivalent ions are formed. The anode potential in N-, 4Y-, 8N-, and 16Y-potassium hydroxide soln. has also been studied at different temp. for low current densities, the curve is discontinuous as in acid soln., and a similar explanation is adopted. In the alkaline soln., the anode becomes coated with a fine grey powder, which apparently consists... 

Chromium is cathodic to steel however, economic electrodeposited coatings of chromium are too thin and porous to give protection on their own and are used in conjunction with thick (25 pm) deposits of copper or nickel. Chromized coatings, on the other hand, are relatively thick and can confer protection by their barrier action, although corrosion of the steel substrate will be increased at any discontinuities in the coating. 

A satin finish nickel coating provides good corrosion resistance due to the discontinuity of the top coat of chromium. 

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