Abstract
The surface layer produced on tinplate by chromate treatments was examined by X-ray fluorescence measurement of residual chromium after alkaline extraction, acid extraction and anodic oxidation and by chemical estimation of dissolved chromium. Two main chromium-containing constituents are formed, one probably Cr0, dissolved by sulphuric acid or anodic oxidation and one soluble in concentrated alkali. Calibration of the charge passed in anodic oxidation in terms of total Cr has held good for most commercial tinplates because they have varied little in composition of the surface film but it breaks down for experimental samples or new developments. Values for the presumed Cr0 constituent good enough for routine control may be obtained from coulometric oxidation of new or alkali-extracted surfaces, provided that an appropriate calibration is used. For research and estimation of total surface Cr, chemical analysis of alkali and acid extracts is needed.
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