Crevice corrosion behavior of X70 steel in HCO−3 solution under cathodic polarization

The crevice corrosion behavior of X70 steel was investigated with a wedge-shaped crevice assembly under −1000 mV (SCE) cathodic polarization in the solutions with various HCO−3 concentrations. The potential, current, pH and the oxygen content within the crevice were measured with or without outside coupled specimen. The results indicated that the polarization potential of X70 steel in the crevice dropped with the increase of time under the cathodic polarization. There was a remarkable influence of HCO−3 concentration on the potential of X70 steel in the crevice. When HCO−3 concentration was up to 0.125%, the surface of the metal was covered with the corrosion products that resulted in the polarization extent of X70 steel decreased. The pH value in the crevice rose and it dropped gradually from the crevice mouth to the bottom under the cathodic polarization. With the increasing of HCO−3 concentration, the hydrolyzation reaction of metal in the crevice bottom aggravated. Most of the dissolved oxygen in the crevice was consumed by the cathodic current. The maximum cathodic current on the metal surface was at the crevice mouth and it was much more than that at the crevice bottom.