Comparative Assessment of Zinc and Cadmium Electroplates by the Weight of Retained Corrosion Products and the Total Weight

The correlation between the corrosion weight losses, the weight of retained corrosion products, and the total weight–change of steel specimen, which has a plain or superficially converted zinc or cadmium electroplate 24 m thick, is studied. The specimens measuring 50 × 70 × 2 and 100 × 150 × 2 mm were exposed for 1 to 5 years in the open and semienclosed atmosphere in coastal and continental test stations and for up to 20 years in the enclosed atmosphere on coasts. The retained product weight and the total specimen weight gain (or loss) vary linearly with the corrosion metal weight loss (K= a+ bX) both in low- and high-corrosive atmospheres. Cadmium coatings retain a lower amount of products than zinc ones, while superficially converted ones retain lower amount than plain ones. The largest amount of products is retained in enclosed atmosphere. In semienclosed and open atmospheres, the amounts are smaller by factors of approximately 1.5 and 2–4, respectively. Knowing the coefficients of a corresponding linear equation and the weight of retained products of zinc or cadmium corrosion on specimens (in a certain time), one can calculate the corrosion rate of a coating in each atmosphere. For a predicted corrosion of 50 g/m², the estimation error will not exceed ±20–30%.