Effect of microstructure on passive film formation and breakdown in sputter-deposited AlTa alloy films

The role played by microstructural features in the passivation and breakdown processes which lead to pitting has been examined for the first time by the new techniques of dynamic imaging microellipsometry (DIM) and local electrochemical impedance spectroscopy (LEIS). Contrary to expectations, an examination of the passivation of sputter-deposited AlTa alloy films found that the film on the Al3Ta precipitates formed a thicker passive layer than that on the solid solution matrix surrounding the precipitates. The LEIS study of the microstructural effects under breakdown conditions found that the film on the Al3Ta precipitate had a higher capacitance than the dealloyed region surrounding it, indicating that the film on the precipitate has different properties from the dealloyed region in its vicinity. This difference in film thickness and properties, found by both DIM and LEIS, may be responsible for the breakdown which leads to pitting initiated in the region adjacent to the precipitates.