A RADIOANALYTICAL STUDY OF THE CHROMATE CONVERSION COATING FORMED ON ALUMINIUM

Phosphate/chromate and accelerated chromate coatings were produced on commercially available aluminium. The Cr, P and Fe components of the conversion coatings were determined by radioactive tracer technique. The tracer technique was combined with ion-exchange and film-sectioning methods to determine the chromium (III)/total chromium ratio in the accelerated chromate coating. It was found that during the acidic dissolution of the conversion coating the chromium (III)/total chromium ratio may suffer changes. The filmforming components in the metal/bath interface are supersaturated and deposited onto the aluminium. The identified components of the accelerated chromate coatings are Cr/OH/3, Cr/OH/CrO4 and Cr4[Fe/CN/6]3 and the chromium (III)/total chromium ratio was found to be cca. 2/3.