Effects of cathodic potential on the local electrochemical environment under a disbonded coating

A rectangular crevice assembly was used to investigate the effects of cathodic protection (CP) potential, bubbling CO₂ and surface condition on the crevice corrosion of X70 steel under a disbonded coating. The solution within the crevice becomes more alkaline due to the reduction of dissolved O₂. As a result, the potential of the steel reaches the protected potential range and thus the protection distance becomes longer when the applied CP potential is more negative. Potential drop (IR) mainly occurs in the vicinity of the opening. However, the introduction of CO₂ into the solution prevents the formation of an alkaline environment but gives rise to an environment with a nearly neutral pH and a uniform potential distribution in the crevice. In addition, it is found that the pre-corrosion product layer significantly decreases the polarization rate in the crevice.