Effect of Microstructure on Passive Film Formation and Breakdown on Al Alloys
Abstract
Dynamic imaging microellipsometry (DIM) has been utilized to examine the passive film growth on a dynamically compacted Al-1% Ta alloy. SEM observation revealed Al3Ta precipitates up to 20 microns in diameter. These precipitates were identified ellipsometrically demonstrating in-situ spatial resolution of at least 20 microns. In addition it was observed that a thinner passive film formed over the precipitates than over the matrix material. This may result in an incompatibility between the two films that results in local strains or flaws, such as dislocations, that promote (Cl-) ingress and degrade the protective nature of the passive film. Ellipsometric analysis of the passivation of Al-8at%Ta and Al-11at%W has demonstrated that the passive films formed on the alloys are thinner than on pure aluminum. To a first approximation the film thickness on both alloys is half that of pure aluminum at a given applied potential. It has also been observed that the index of the passive film on the Al-W alloy increases at an applied overpotential of approximately 1 volt. The scanning electrochemical impedance spectroscopy (EIS) technique developed during the first two years has been improved and quantitatively demonstrated. This was accomplished by examining two electrochemical systems. The first was a homogeneous 99.999% aluminum electrode. To demonstrate that this technique could be used to accurately locate and measure the impedance properties of surface heterogeneities, a second system containing a model defect was examined.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 27, 1992
- Accession Number
- ADA245863
Entities
People
- C. C. Streinz
- J. Kruger
- J. W. Wagner
- P. J. Moran
- R. S. Lillard
Organizations
- Johns Hopkins University