Formation of Cr(VI) in cobalt containing Cr(III)-based treatment solution

Zinc plated steel was formerly submitted to a Cr(VI)-based passivation treatment leading to enhanced corrosion resistance. However, Cr(VI) compounds were recognized as toxic and carcinogenic. Thereafter, hexavalent chromium was substituted by a safer, yet effective, trivalent chromium-based solution treatment. Due to health hazards, the formation of Cr(VI) species in Cr(III) based conversion coating must be avoided. The present work aims at elucidating the formation mechanism of hexavalent chromium in the Cr(III) conversion coatings particularly obtained from treating agents containing cobalt. It is shown that trivalent chromium compounds in the layer might be oxidized to hexavalent chromium by oxygen in the presence of water and under certain conditions. The presence of Cr(VI) was detected by means of spectrophotometry and the dissolved chromium amount was measured with the aid of ICP-OES. The morphology and structure of the films were observed with the aid of FIB-SEM. The total amount of water in the layers was measured employing Karl Fischer Titration. The results indicate that the morphology of the fluoride containing film with a high density of micropores increases the probability of water entrapment. This leads to the oxidation of Cr(III) to Cr(VI) by oxygen in the presence of water at elevated temperatures.