Current Distribution and Electrochemical Environment in a Cathodically Protected Crevice

Abstract

Abstract Current and potential distributions inside the crevice of a simulated disbonded coating with a holiday during cathodic protection (CP) of steel were measured experimentally in 0.001 M to 0.6 M sodium chloride (NaCl) solutions covering a range of conductivity from 0.012 Ω−1 m−1 to 5.1 Ω−1 m−1. The circular crevice was 95 mm in radius, 0.8 mm in thickness, and open to the test solution through a holiday of 5 mm in radius on the center of the coating. Microelectrodes mounted on the steel surface were used to determine the local current densities inside the crevice. Change in the concentration of dissolved oxygen inside the crevice was monitored with a micro-oxygen electrode. Current distribution was extremely nonuniform. Depending upon NaCl concentrations, the local current density on steel exhibited a 100-fold decrease from that at the holiday opening within 20 mm to 40 mm into the crevice. Current and potential distribution curves became less uniform with a decrease in NaCl concentration and solut...