Examination of Oxides on Tin Surfaces by Cathodic Reduction

Abstract

Oxides formed on tin surfaces were examined by electrochemical reduction in galvanostatic and potentiodynamic experiments and also by electron diffraction. Reduction of oxide occurred at three distinct potentials (NHE scale): ?0·115 ?0·06 pH, ?0·150 ?0·06 pH and ?0·345 ?0·06 pH, although sometimes only one or two of them were observed. The most negative reduction step rarely appeared for electrodeposited tin coatings oxidised in air but was well developed for pure tin sheet and for hot-dipped tin coatings. Impurities in tin modified both the speed of oxidation and the form of the oxide reduction curves but their effect cannot account for the difference between electrodeposiied coatings and other tin surfaces. This difference is probably due to exposure of differing crystal faces. The two less negative reduction steps appeared for all forms of tin, but were not always clearly separated and they cover the possible reduction of both SnO and SnO₂. The most negative step is associated with SnO₂ only and its development is favoured by chemical or electrochemical oxidation, which may produce it for electrodeposited tin, or by high humidity of air. SnO₂ is also formed on all surfaces during long term storage at room temperature. Some thicker oxide films, formed during long storage or by anodic oxidation, may be partly undermined during reduction and the total reduction charge is not an accurate measure of their character.