Abstract
The structure of hydrated chromium oxide on Tin Free Steel (TFS) formed in a low concentration CrO3 bath containing a small amount of fluoride, silicofluoride or sulfate was investigated by XPS. Lacquer adhesion before and after immersing in a hot citric acid solution was also examined. The lacquer adhesion of TFS was discussed based on the structure of hydrated chromium oxide. The results obtained were as follows: 1) The hydrated chromium oxide films on TFS basically consisted of polymerized chromium (III) hydroxide, in which a part of OH ions was replaced by fluoride ions or sulfate ions. In the case of silicofluoride bath, silicon was not incorporated into the film. 2) When the films formed in the fluoride baths were heated at a lacquer curing temperature, both the oxolation of hydrated chromium oxide and the removal of incorporated fluorine occurred. On the other hand, in the case of the film formed in the sulfate bath, incorporated sulfate ions were kept in the film after heating, and the oxolation of hydrated chromium oxide occurred slightly. 3) After immersing in a hot citric acid solution, TFS formed in the fluoride baths showed much better lacquer adhesion than TFS formed in the sulfate bath. This was explained by considering larger amount of hydroxyl ions in the films formed in the fluoride baths, when assuming that the adhesion is supported by hydrogen bonding between the hydroxyl ions and the lacquer. 4) After immersion in a hot citric acid solution TFS heated before lacquer coating showed lower adhesive force than TFS without heating because of the slight oxolation of hydroxyl bridges in films on TFS.
