Local additional potential model for effect of strain rate on SCC of pipeline steel in an acidic soil solution

Stress corrosion cracking (SCC) behavior of X70 pipeline steel in an acidic soil solution was investigated by slow strain rate test, surface characterization, potentiodynamic polarization curve measurement and electrochemical hydrogen permeation technique. A local additional potential model (LAPM) was developed to illustrate the critical role of strain rate in SCC of the steel. According to LAPM, both density and mobility of local active spots on the steel surface, i.e., dislocation emergence point, increase linearly with strain rate. Generation of such active spots introduces an additional negative potential locally, affecting the electrochemical reaction and, consequently, the susceptibility of the steel to SCC. It is found that a maximum of the SCC susceptibility occurs at strain rate of 10-6 s-1, which is associated with an enhanced hydrogen evolution due to the local additional potential (LAP) effect. When strain rate is sufficiently high to exceed 10-6 s-1, the mobility of the dislocation emergence points is so fast that the reactive species in solution cannot combine with them for cathodic reaction, resulting in a decrease of the SCC susceptibility. Similarly, a maximum of hydrogen permeation current observed at the strain rate of 10-6 s-1 is also attributed to the effect of strain rate on the density and mobility of dislocations in the steel. Diffusion of hydrogen atoms in a strained steel is through both body diffusion and dislocation diffusion, with the latter enhanced by an increasing strain rate. When strain rate is so high that the dislocation mobility is sufficiently fast, hydrogen atoms become incapable of catching up with the dislocations. As a result, the hydrogen diffusion is dominated by the body diffusion mode.