Current distribution and electrochemical environment in a cathodically protected crevice

Current and potential distributions inside the crevice of a simulated disbo nded coating with a holiday during cathodic protection (CP) of steel were m easured Experimentally in 0.001 M to 0.6 M sodium chloride (NaCl) solutions covering a range of conductivity from 0.012 Ohm(-1) m(-1) to 5.1 Ohm(-1) m (-1). The circular crevice was 95 mm in radius, 0.8 mm in thickness, and op en to the test solution through a holiday of 5 mm in radius on the center o f the coating, Microelectrodes mounted on the steel surface were used to de termine the local current densities inside the crevice. Change in the conce ntration 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 10 0-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 solution conductivity. In 0.6 M NaCl, the steady-state local current density on steel at the sealed end of the crevice was 200 times smaller than that at the holiday opening. In 0.00 1 M NaCl, the local current density at the sealed end became 1,000 times sm aller than that at the holiday opening. The dissolved oxygen within the cre vice was consumed largely by local corrosion cells when the crevice was for med initially. Any oxygen needed for the reduction reaction during CP had t o be transported into the crevice by diffusion through the holiday opening.