No abstract

No abstract

The effects of additions of fluorozirconic acid to sulfuric acid on the anodizing behavior of aluminum have been investigated under a constant voltage at temperatures of 0 and 20 • C. The fluoroacid increased the rate of film growth, with a dependence on the fluoroacid concentration, the electrolyte temperature and the anodizing time. Compositional analyses showed that fluorine species were present in the films. However, zirconium species were absent. The fluoroacid generally enhanced film dissolution, although this effect was less important at low fluoroacid concentration, low electrolyte temperature and short anodizing times. Chromium (VI)-free conversion treatments that include fluorotitanic and fluorozirconic acids in the bath formulations have been developed for aluminum alloys.1-6 The resultant coatings comprise chromium (III) (if present in the bath), titanium and zirconium fluorides, oxides and hydroxides, which constitute the main part of the coating thickness, and a comparatively thin layer of aluminum oxyfluoride next to the substrate. In comparison with the findings from the relatively numerous studies of conversion coatings produced using fluoroacids, very little information is available on the use of fluoroacids in anodizing of aluminum and aluminum alloys. One study has been made of anodizing aluminum and aluminum alloys at a constant current density in fluoroboric acid solutions at temperatures ranging from 0 to 30• C. 7 Porous films were formed, with a pore size and population density that depended on the concentration and temperature of the fluoroboric acid solution and the time of anodizing. However, the information provided on the morphology and composition of the films was very limited and the role of the fluoroacid in the growth of the film was not considered. More recent studies examined the influence of ammonium hexafluorosilicate 8 and ammonium fluoride 8,9 additions to oxalic acid on the formation of porous anodic films on aluminum. The effect of hexafluorosilicate ions was more pronounced than that of fluoride ions, suggesting that Si