Study of current/potential distribution in a cathodically protected crevice

This work is concerned with experimental and theoretical study of current and potential distribution inside a cathodically protected crevice in order to investigate the effectiveness of cathodic protection against crevice corrosion. Crevice corrosion occurs on underground steel pipelines whose exterior coating develops pinholes and cracks through the aging process. In low conductivity soils, cathodic protection has not been able to mitigate corrosion beneath the disbonded coatings. The cathodic protection criteria available in the literature suggest that the ability of cathodic protection to be effective against crevice corrosion depends on the current and potential distribution inside crevice.; Experiments were carried out to measure current and potential distribution, and chemical changes inside a simulated crevice. The crevice was formed between a metal surface and a Plexiglas insulating surface with a holiday opening at the center of insulating surface. Since mass transfer plays an important role in determining current and potential distribution inside crevice, the following ideal mass transfer systems were investigated: (1) reduction of ferricyanide ions and (2) reduction of dissolved oxygen on gold surface inside crevice. Experiments were also performed for the cathodic protection of steel in aerated sodium chloride solutions. The effect of solution conductivity, applied holiday potential and the crevice geometry was investigated.; The results of ferricyanide and oxygen reduction on the gold surface inside crevice indicate that there exists a 'transition time' corresponding to the complete depletion of a limiting reactive species inside crevice. Within the transition time, the current distribution curves showed a maxima at the interior locations of the crevice. At the steady state, only a small amount of current penetrated inside the crevice. Depending on the solution conductivity, current density at the end of crevice was two to three order of magnitudes smaller than the current density at the center of holiday opening.; Experiments for cathodic protection of steel were conducted over a wide range of solution conductivities from 0.012 to 5.1 {dollar}rm ohmsp{lcub}-1{rcub}msp{lcub}-1{rcub}{dollar} and the applied holiday potentials from {dollar}-{dollar}0.8 to {dollar}-{dollar}1.2 V vs. Ag/AgCl. In these experiments, a slight to mild corrosion of steel was observed. The cathodic protection criteria of the metal-to-solution potential being more negative than {dollar}-{dollar}0.78 V vs. Ag/AgCl was met in all the experimental runs. There was no transition time phenomena observed in the cathodic protection experiments. The local current density always decreased with increasing distance inside crevice. The current distribution inside crevice was very non-uniform with a sharp drop in current density near the holiday opening. The current and potential distribution became less uniform with decrease in the solution conductivity.; A two dimensional steady state mathematical model was developed to investigate the transport processes inside crevice. The model is capable of predicting (1) concentration profiles of various chemical species present in the crevice solution; (2) local potential; and (3) local cathodic current density inside crevice for a given applied holiday potential and bulk concentrations. The model predicted a very short depth of current penetration of 4-5 times the crevice thickness when the total crevice depth was 120 times the crevice thickness. The depth of current penetration decreased with decrease in the radius of holiday opening, {dollar}rm rsb{lcub}H{rcub}.{dollar} The crevice depth had no influence on current distribution for the crevice depth to thickness ratio greater than 10 when {dollar}rm rsb{lcub}H{rcub}/delta{dollar} was 6.