Tin-Nickel Alloy Coatings—Estimation of Thickness and Porosity

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Abstract
The dissolution of tin-nickel alloy coatings in concentrated phosphoric acid at 180°C. or by anodic treatment at 300 amps, per sq. ft. in 10% hydrochloric acid has been shown to provide a means of determining the thickness of coatings on copper, or brass and, with care, on copper undercoats over steel. Neither these agents nor any others were found able to remove deposits applied direct to steel without serious solution of the basis metal hut a magnetic instrument may be used for such coatings. Mesle's chord method tended to give low values for coating thickness. For the estimation of porosity of coatings on steel, the ferricyanide test gave good results only if the samples were first cleaned cathodically, salt-spray tests did not reproduce normal behaviour out of doors and the hot-water test failed to give sufficiently conspicuous spots. Exposure of samples over a dilute solution of sulphur dioxide for 24 hours, followed by a brief exposure to ammonia, gave satisfactory results.