Effect of Electrolyte Composition on Near-Neutral pH Stress Corrosion Crack Propagation

The paper describes the influence of electrolyte composition and potential on crack growth rate in pipeline steel X70. The growth rate was determined using electrical resistance test of samples with pre-grown fatigue crack.  Citrate buffer (pH 5.5) and mixture of NS-4 and borate buffer were used as background solutions under static and cyclic loading correspondingly. Various substances were added to the solutions to change anodic dissolution (AD) and hydrogen absorption (HA) of metal. The tests both under static and cyclic loading demonstrated that SCC propagation corresponded to visual information and metallographic analysis of the walls of gas pipelines. The effect of electrolyte composition on crack growth rate depends on AD and HA processes. Thus, sulfide and phosphate ions, which stimulate the both processes, accelerate crack propagation.  AD and HA inhibitors decrease the crack growth rate. The influence of AD promoters  may be diverse. The data obtained testify that the SCC leading mechanism of pipe steel X70 under corrosion potential is the anodic dissolution of the metal. The influence of HA promoters on the crack propagation could be connected with the activating effect of atomic hydrogen on the anodic dissolution of iron.