OXIDE FILM FORMATION AT A MICROCRYSTALLINE AL-ALLOY IN ROOM-TEMPERATURE NEUTRAL BORATE SOLUTION

The differences in the electrochemical behavior of a rapidly solidifie d, two-phase (matrix and dispersoid) microcrystalline Al-based alloy c ontaining Fe, V, and Si (FVS0812 alloy), the bulk forms of its phases, and polycrystalline Al were investigated in neutral berate buffer sol ution. FVS0812 shows higher activity than the matrix and pure Al, whil e the dispersoid is the most active, due to oxygen evolution and disso lution, even in this benign solution. Barrier oxide films formed at FV S0812 appear to be generally uniform and similar in thickness to those formed at the matrix material. At low anodizing voltage the dispersoi d phase appears to form an oxide film, although some loss of Fe and V is inferred. However, at higher voltages, evidence for the incomplete oxidation of the dispersoid phase and its incorporation and/or subsequ ent loss from some sites in the oxide film is seen, leading to a more flawed film of less uniform thickness. The higher capacitance of the o xide film at FVS0812 compared to that of films formed under identical conditions at Al and the matrix is consistent with the penetration of solution, via flaws, into the oxide film and the development of region s of thinner oxide film at dispersoid containing sites.