Electromechanical breakdown mechanism of passive film in alternating current-related corrosion of carbon steel under cathodic protection condition

Buried carbon steel pipe are provided with corrosion prevention systems, namely an insulating coating and a cathodic protection system which reduces (or halts) corrosion rate as a result of soil corrosiveness. The presence of alternating current (AC) interference may cause serious corrosion damages on metallic structures corresponding to coating defects, even if the -0.850 VCSE protection criterion is matched. Nowadays, the AC corrosion mechanism, as well as the protection criteria in the presence of AC, is still controversial and debated. In this paper, a two-step AC corrosion mechanism of carbon steel under cathodic protection condition is proposed: (1) AC causes the electromechanical breakdown of the passive film formed on carbon steel under overprotection condition, and (2) after passive film breakdown, high-pH chemical corrosion takes place. In order to investigate AC effects on passive condition (Step 1), experimental tests have been performed on stainless steels in neutral solution, as a result of its passive behavior: AC has harmful effects on passive condition, reducing anodic overpotential, and causing film breakdown over a critical AC interference level. The electromechanical oxide breakdown mechanism resulting from high alternating electric fields (of the order of 106 V/cm) within the passive film is proposed. Electrostriction stresses are a possible explanation. After film breakdown, corrosion can occur if the pH inside the crack is close to 14; this strong alkalization can be reached in overprotection condition (high cathodic current density) and in the presence of AC.