Large Area Photocurrent Behavior and Laser Spot Scanning of Passivated Stainless Steels

The semiconductor properties of passive films on different stainless steels were investigated by photoelectrochemical methods. Photocurrent-potential measurements together with data for the susceptibility of the materials to localized corrosion revealed that above a critical potential value (UCrlt) photocurrents as well as the pit initiation rate decrease with increasing potential. The occurrence of Uc~t could be explained by a Cr(III)/Cr(VI) oxidation in the passive film starting at this potential. By applying a laser spot scanning technique it was possible to resolve local variations in the photocurrent response of the passive films studied. Microscopic investigations revealed that sites wh ich exhibit an enlarged photocurrent coincide with inclusions present in the bulk material of the steels. In recent years the electronic properties of passive films on stainless teels have been the subject of several investi-gations. 1-7 In order to describe the films in terms of a semi-conductor approach most work has been carried out by capacitance (Mott-Schottky-type of analysis) and pho-recurrent measurements, which generally characterize the average behavior of samples with a large area. From pho