Influence of electrochemical hydrogenation parameters on microstructures prone to hydrogen-induced cracking

Abstract

When pipeline steels are exposed to hydrogen-rich environments during service, this can lead to a degradation in mechanical properties and premature failure. These steels are often susceptible to hydrogen embrittlement and hydrogen-induced cracking (HIC) and contain very sensitive microstructures for HIC formation. This work aims to find a suitable hydrogenation methodology that allows to charge pipeline steel specimens without inducing any hydrogen related damage during the charging process. Therefore, different electrochemical hydrogen charging methodologies are evaluated. The susceptibility to HIC and blisters of two pipeline steels (API 5L grades X70 and X56), is first investigated in a sulfuric acid electrolyte. Both grades are prone to HIC in the mid-thickness section of the pipeline wall. For X56, significant blistering occurs as well, and hydrogen-induced damage could be linked to elongated MnS inclusions. For the X70 steel, hard bands are found to be vulnerable to HIC. The electrochemical charging conditions are modified for the X56 steel, where the extent of the damage is greatest due to its prone microstructure, as revealed by scanning electron microscopy. Lowering the sulfuric acid concentration and applied charging current density does lead to some reduction of blistering. However, changing to a different, sodium hydroxide-based electrolyte is more effective in mitigating the extent of the hydrogen-induced damage and the time at which it appears.