The Mechanism of Corrosion Protection by Brush Plated (Selective Area Plated) Cadmium on Steel

Abstract

SUMMARYThe corrosion protection performance of selective area electroplated cadmium on mild steel has been assessed as a function of the depositing anode speed and current density, using salt spray, constant immersion, and electrochemical corrosion tests. The results indicated that the cadmium coatings provided very good corrosion protection performance without the standard Chromate conversion coating, providing the anode speed was maintained below 0.2 m s1. but not at very low speeds and extremes in the permitted deposition current density range. A good inverse correlation was found between values of corrosion current obtained from the electrochemical tests, and constant immersion corrosion rates for the cadmium coatings, with high currents corresponding to low dissolution rates. This effect is thought to be associated with the porosity of the coatings. A mechanism of corrosion protection by the cadmium based on the formation of a bi-polar precipitate of corrosion product consisting of cadmium chloride hydroxide and cadmium carbonate has been proposed.