Corrosion Tests of Tin-Cadmium Coatings on Steel

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Abstract
Coatings containing tin and cadmium, produced either as a layered electrodeposit of the two metals or by electrodepositing alloys from sulphate or fluosilicate baths, have been subjected to corrosion tests in comparison with other coatings. Coatings of tin-cadmium alloy or of cadmium deposited over an equal thickness of tin have been found to have outstanding durability in laboratory salt-spray tests and, in all environments, are as well able as cadmium coatings to prevent rusting at pores when freshly exposed to corrosion. The alloy coatings have an advantage over cadmium in that they are less affected by the organic vapours emitted by some insulating materials. It has also been shown, by other investigators, that the alloy coatings have useful anti-galling properties. Protection of steel exposed to the weather by the sea has lasted longer for some alloy coatings than it has for cadmium coatings of the same thickness, but it is doubtful whether this superiority can be obtained consistently. Compared with cadmium coatings, alloy coatings were no better in continuous or intermittent immersion in the sea and were inferior in exposure to inland urban atmospheres. A tin undercoating 0·05mil‡ thick improved the performance of zinc coatings on flat plates in a marine atmosphere, but such an undercoating had no appreciable effect on the life of cadmium coatings in any type of exposure or of zinc coatings exposed inland.

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