PITTING IN ALUMINUM THIN-FILMS - SUPERSATURATION AND EFFECTS OF DICHROMATE IONS

The growth of pits in 209 nm thick Al films in chloride solutions with and without dichromate ions was examined using image analysis of the growing pits to determine pit current density. In pure chloride soluti ons, the pit current density decreased at high potentials after reachi ng a maximum value, and then was almost independent of applied potenti al. A hysteresis in the pit current density-potential behavior was obs erved during downward stepping of the potential from high values. This is a result of a combination of supersaturation of the pit electrolyt e followed by salt film formation, and changes in mass transport from hydrogen bubbles that increase convection and lift the remnant passive film away from the dissolving surface. In solutions containing dichro mate ions, the corrosion and repassivation potentials shifted in the n oble direction, and rather large metastable pits formed at the open ci rcuit. A large concentration of dichromate ions was needed to inhibit pit growth. In dichromate solutions, subsequent pit growth at higher p otentials often initiated at the edge of the open-circuit pits. The ra te of pit growth was lower for these pits because the remnant passive film layer was not Easily lifted up at these sites, and thus created a barrier for mass transport away from the dissolving pit Edge.