Crack and blister initiation and growth in purified iron due to hydrogen loading

Purified iron was loaded electrochemically with hydrogen in the presence of a hydrogen promoter, leading to the formation of cracks inside of the bulk and blisters on the surface. The mechanism for the crack initiation was investigated using SEM cross-section images and by investigating the fracture surface of a ruptured sample, where preexisting cracks were exposed for observation. Cracks were found to originate at inclusions. It was observed that blisters grow with time, leading to the conclusion that the underlying growth process is discontinuous. The surface morphology of the blisters consists of steps and in the underlying microstructure investigated by TEM shear bands were found. Hydrogen gas pressures in the range of half of the yield strength of iron were determined directly after hydrogen loading using density measurements. Therefore, the hydrogen gas pressure in the cracks was concluded to be the driving force for crack advance. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.