Experimental evidence of hydrogen evolution from local anodic corrosion sites and its consequences for corrosion cracking mechanisms

Abstract

Abstract Based on observation of gas bubbles evolving from crevices on slow strain rate test pieces during anodic corrosion the effect of applied potentials in chloride aqueous solutions on amount and composition of the gas together with fracture strains from the slow strain rate test of a 17-4 PH steel was investigated. As a result, increasing applied potentials provide a reduction of fracture strains together with an initial increase in volume of released hydrogen gas. The fracture surfaces exhibit increasing brittle appearance, as found in hydrogen cracking failures. The results confirm that during local anodic corrosion, hydrogen is produced that provokes hydrogen induced cracking following anodic dissolution. Respective local pH measurements under varying applied electric potentials, show the acidification of the pit-electrolyte. Additional model calculations elucidate the stepwise local anodic corrosion and its acidification process together with the hydrogen supported cracking. The model shows that the relative amount of hydrogen cracking will depend on the provided material data as well as on solution properties such as pH, chloride level, temperature and oxygen content. As an all over result, together with the model calculations, the experimental evidence of hydrogen evolution during anodic local corrosion confirms the contribution of hydrogen cracking to anodic stress corrosion cracking.