The Effects of Prior Pitting Damage on Repassivation Potential

Abstract

Abstract : There has been considerable controversy over the relationship between repassivation potential and pit size in passive metals such as aluminum and stainless steel. Understanding this behavior is very important when determining the appropriate level of cathodic protection for previously damaged systems. The repassivation-size relationship was explored in this experiment using a procedure known as the Scanning Reference Electrode Technique or SRET. Steel and aluminum samples were immersed in a 3.5 wt% NaCl solution simulating sea water. The samples were then polarized above the pitting potential and pitting activity was measured on a pit by pit basis with the SRET. The potential was then lowered in 0.1 V steps until all pits repassivated. No data was obtained for the aluminum due to its pitting characteristics, however, data was collected and analyzed on an individual pit basis for the 316 stainless steel. The pits analyzed varied in diameter from 0.31 mm to 1.63 mm, and showed repassivation potentials between 0.6 V and 0.5 V (vs. SCE). The data showed a decrease in repassivation potential of 0.093 V for an increase in diameter of 1 mm. This demonstrates that when cathodically protecting a damaged system, the level of protection must be held below the repassivation potential of the largest pit. Corrosion, Pitting, Repassivation potential, Prior damage, Stainless steel