OXIDE FILM FORMATION ON A MICROCRYSTALLINE AL-ALLOY IN SULFURIC-ACID

The differences in the electrochemical behavior of a rapidly solidifie d, two-phase (matrix and dispersoid) Al-based alloy containing Fe, V, and Si (FVS0812 alloy) and the bulk form of its matrix and dispersoid phases were investigated in sulfuric acid. FVS0812 exhibited generally higher electrochemical activity than the matrix material due to the p resence of the very active dispersoid phase in the alloy. Impedance st udies indicated that the properties of thin films formed on the FVS081 2 and matrix substrates during only 5 min of anodization are similar, whereas the film formed on the alloy during 2.5 h of anodization was s ubstantially less resistive and contained a thinner and/or damaged und erlying barrier oxide compared to a comparable film formed on the matr ix. Compared to the classical structure of porous Al oxide films with underlying barrier oxide, the oxide film on FVS0812, as seen by TEM, w as thinner, with an intermittent barrier oxide underlying a porous oxi de of contorted morphology. Evidence is seen for the loss of dispersoi ds from the oxide film, leaving voids throughout its structure, perhap s the reason for its lack, of physical adherence and its limiting thic kness.