Monitoring of Corrosion Rate of Carbon Steel under Wet/Dry Cycle Conditions and Its Corrosion Mechanism
Atsushi Nishikata, Takehiko Takahashi, Bao-Rong Hou, Tooru Tsuru
AC impedance technique has been applied to the monitoring of corrosion rates of a carbon steel in wet/dry cycle condition. The wet/dry cycles were conducted by exposure to alternate conditions of 1h-immersion in 3% NaCl solution and 5h-drying at 25°C and 60% RH. The corrosion rate of the carbon steel is greatly accelerated by the wet/dry cycles. Because the active FeOOH species, which are produced by the oxidation of Fe3O4 in air during drying, act as very strong oxidants to the corrosion of carbon steel in wet condition.
Shortly before the surface dried out, a large increase in the corrosion rate and a small shift of the corrosion potential to the positive values were observed. This can be explained by an acceleration of the rates of oxygen transport through the thin electrolyte layers. When the carbon steel is immersed again in the solution after the surface almost dried out, it exhibited a high corrosion rate and a noble potential, followed by a gradual decrease in the corrosion rate and a shift of the corrosion potential in negative direction. This decline of corrosion rate is caused by a decrease of the amount of active FeOOH species. Based on the results obtained by the AC impedance and the corrosion potential measurements, the corrosion mechanisms of the carbon steel in wet/dry cycle conditions has been discussed.