A MICROELLIPSOMETRIC STUDY OF THE PASSIVE FILM FORMATION ON AL-TA ALLOYS .2. THE ROLE OF AL3TA PRECIPITATES IN BREAKDOWN

Al3Ta precipitates act as sites for pit initiation and propagation in Al-Ta alloys. Dynamic imaging microellipsometry was used to measure th e change in oxide film thickness and the refractive indexes of the fil ms that form on the precipitates and matrix to examine the role of the precipitates in breakdown. Film formation was measured on an Al-1.5 a /o Ta alloy, containing Al3Ta precipitates approximately 50 mum in dia meter at applied potentials of 0.0, 1.0, 2.0, and 5.0 V SCE in a pH 7. 2 borate buffer solution. At 0.0 V SCE the change in passive film thic kness on the precipitate was greater than that on the Al matrix. The c hanges in film thicknesses at 1.0 V SCE were approximately equal and a t higher potentials (2.0 and 5.0 V SCE) the matrix film thickened more than the film on the precipitate. SEM observations demonstrate that t he precipitate-matrix interface is highly susceptible to localized att ack when the passive film on the Al3Ta is thicker than on the matrix ( at 0.0 V SCE). We propose that pit initiation occurs at both the inter face of the Al3Ta precipitation and in the dealloyed region around its periphery.