TEM Characterization of Corrosion Initiation in Passive Films

This research program is intended to develop new insight into the mechanisms of localized corrosion initiation in passive metals through unique approaches based on examining corrosion initiation on needle-shaped samples similar to field emitter tips. Samples with this geometry allow localization of corrosion initiation to the tip itself and therefore pit initiation can be confined to a small lateral area. Local electric fields can also be controlled, thus permitting damage to be confined in the passive over layer, and characterized before pit initiation. High purity Al wire was used for the initial studies. Wire shaped specimens were electropolished in a 20 % perchloric in methyl alcohol solution at -30°C to obtain the fine needle geometry. The resultant tip radii ranged from 40 to 150 nm with no further sample prep needed and without the presence of artifacts.A custom-designed TEM holder for a Philips CM30 was fabricated, which allowed the needles to be characterized by TEM both prior to and after experiments.