Stagewise behavior of the electrochemical dissolution of a loaded metal under the conditions of its local interaction with corrosive media

We show that the process of local electrochemical dissolution of iron of loaded low-carbon 20 steel (σ0.2=270 MPa) in a corrosive medium (3% aqueous NaCl solution, pH 6.9) is multistage. The characteristics of the process depend on the form of the surface interacting with the corrosive medium. The investigated cases are arranged in the following order according to the rate of dissolution of the metal: a smooth surface, a stress concentrator, and a stress concentrator with a short crack-like defect. We propose and detect the corresponding electrochemical reactions and their sequence (stagewise behavior) realized in the course of the process. The electrochemical parameters of local anodic dissolution of the metal at tips of corrosion cracks of various depths and openings are determined. On the basis of these results, we suggest an autocatalytic mechanism of dissolution of the metal in which Fe2+ ions are autocatalytic particles. We demonstrate agreement between the results calculated by using classical electrochemical equations for the kinetics of electrode processes and experimental data obtained in corrosive media with various concentrations of Fe2+ ions in the rangeCFe2+=1.0·10-5-2.0·10-1. This means that the indicated mechanism can be realized at the tip of a corrosion crack.