The acceleration mechanism of stress on anodic dissolution of bare metal surface

Abstract

The difference in energy between the atoms in dislocations and those in a dislocation-free lattice is analyzed. The analysis suggests that the electromotive force of atoms in dislocations is 2.1 V higher than that of the atoms in dislocation-free lattices, and the anodic dissolution rate of atoms in dislocations is 1017 times that of the atoms in dislocation-free lattices. A model of the anodic dissolution of bare metal surface has been proposed. The theoretical equation proposed according to the model is i = i0 exp [flnF(q~a - ~e)/RT] �9 exp [flnFV/RT]. p. qJ  10 -15 The relationship between the increase of current density of anodic dissolution of bare metal surface and the increase of dislocation density has been calculated by this equation. The theoretical calculation shows that the current density rises slowly with dislocation density when the dislocation density is below 101~ . However, above 101~ , it increases greatly. The effect of the stress-strain on the current density of anodic dissolution of a bare single-crystal surface of 321 stainless steel in 3.5 pct MgC12 solution has been measured by a rapid scratching and recording device designed by the authors. The experimental results show that the effect of stress-strain on the current density of anodic dissolution of a bare singlecrystal surface is small in the range of elastic deformation; however, it is marked in the range of plastic deformation. The experimental results are consistent with the theoretical calculation. Several research workers 11'2m have shown that the electrode potential decreases and the anodic current rises as the metals are stressed or strained. Despic et al. [3] studied the effect of strain on anodic dissolution kinetics of several metals. It has been found in the case of iron in acid solutions that the overall effect of elastic strain is small and the effect of yielding on the rate of the anodic dissolution of metals is much more pronounced than that of the elastic deformation. Hoar and co-workers I4,5] and Hines tTI found that anodic dissolution is highly stimulated by the mechanically induced movement of the surface metal atoms. The influence of macroscopic plastic strain upon the propensity to stress corrosion cracking has been considered by a number of researchers. ~13-2~ Some authors considered that the increasing rate of corrosion or anodic