Cadmium coatings compared with

Compared with other methods, vacuum evaporation produces coatings that have a most satisfactory corrosion resistance, e.g. 0.005 mm of evaporated Cd gives a degree of protection to steel similar to that afforded by 0.01 mm of electrodeposited Cd. Cadmium coatings on ferrous and other substrates can meet authoritative specifications concerning corrosion... 

One-year test results on zinc and cadmium are in Table 2.22. The results of long-term atmospheric corrosion tests on zinc and cadmium coatings (Fig. 2.35) are compared with those of accelerated corrosion tests (Fig. 2.36) on the same coatings in three cabinets warm and humid, SO2 gas, and sea mist (Strekalov and Berukshtis, 1965). 

In a laboratory study by Schlekat et al. [15], it was demonstrated that coating silica particles with an exopolymer prepared from an estuarine bacterium enhanced the sorption of cadmium on to the particles. The composition of the exopolymer was glucose, galactose and glucuronic acid in the ratio 5 2 1. These investigations also compared the effects of salinity, pH and different concentrations of cadmium. Increasing salinity resulted in less cadmium associated with the particles, presumably due to competition with the chloride ion. The pH had a dramatic effect, resulting in only ca. 10% absorbed at pH 5 to more than 95% at pH 9. 

Carter also quotes Le Que on the unsuitability of salt spray tests for sacriflcial coatings and comments on the failure of the latter investigator to place cadmium and zinc in the same order as in outdoor exposure. He makes the further point that the comparative performance of zinc and cadmium varies with the composition of the salt solution used for spraying. For example, the superiority of cadmium is lost when natural seawater (containing, e.g., magnesium salts also) is substituted for a pure sodium chloride solution. It is suggested that variations in relative performance may be explained by the nature of the corrosion products formed and the extent to which condensation occurs on the surface of the specimens. 

---