AC Corrosion Mechanism: Effects on Overpotentials of Anodic and Cathodic Processes

Despite the increased number of publications during the last years, AC induced corrosion continues to be a controversial subject and many aspects need to be clarified, especially the mechanism and the relationship between AC density and corrosion rate.
Different theories have been proposed about the mechanism by which AC produces and enhances corrosion, although no one clearly explains the phenomenon.
In this paper the attention is focused on the effects that AC interference may have on the overpotentials of the anodic and cathodic electrochemical processes. The polarisation curves obtained in different solutions on metallic materials (carbon steel, galvanised steel, zinc and cooper) in the presence of AC interference (AC density in the range 30 to 1000 A/m²) are presented. The values of corrosion rates obtained by Tafel extrapolation are compared with previous weight loss tests. The results clearly show that AC has an effect on the overpotentials, but that the magnitude of such effect is not sufficient to explain the increment of the corrosion rates measured with weight loss tests.