Internal hydrogen cracks, produced in iron-3 wt% silicon by cathodic hydrogen charging, have been examined using high resolution scanning electron microscopy and scanning tunnelling microscopy. Parallel striations with a spacing of around 0.3 μm were observed on the fracture surface. The crack front was not parallel to the striations, but was jogged and inclined at a small angle to them. Between the striations, the fracture surface had a stepped crystallographic appearance on a scale of less than 50 nm. The pressure-driven model for internal hydrogen cracking is re-examined and it is found that previous observations are consistent with hydrogen-assisted stable crack growth. An anti-shielding dislocation mechanism for hydrogen-assisted stable crack growth is proposed, suggested by the high resolution fractography.
