Sulphide Staining inside Tinplate Cans and its Prevention

Abstract

Production of sulphides of iron or of tin inside tinplate cans filled with sulphur-containing foodstuffs may cause stains which, although harmless, are a source of complaint from consumers. The formation of iron sulphide is widelythought to be a conversion by H₂S or HS-ions of a preliminary Iron corrosion product. Evidence now obtained suggeststhat tm sulphide is formed by a corresponding process and that Its direct formation from metallic tin or tin oxide is unlikely. Formation of tm sulphide may be prevented by screening the tin from the corrosive agent with lacquers or with the better passivation films formed by cathodic treatment in dichromate solutions on tinplate production lines. It is shown that the effectiveness of the protection that is given by passivation films depends on the amount of metallic chromium that they contain. Films which contain only oxides of tin and/or of chromium can give a useful degree of protection only when they are relatively thick.