The effect of diffusion-mobile and combined hydrogen on hydrogen brittleness of steel

Abstract

Diagnostics of the emergencies attributed to the pipeline destruction resulted from hydrogen charging and corrosion is one of the important problems of the oil and gas industry. Corrosion and hydrogen charging of the metal lead to formation of holes in the pipe wall at a certain critical ratio of the metal strength and thickness resulted from the impact of the internal pressure of the gas transported through the pipeline. Despite extensive research, many issues related to this problem require further consideration. This concerns, in particular, the issues regarding kinetics of the brittle fracture of metal of the pipeline walls and the role of diffusion-mobile (active) and combined (bound) hydrogen in this process. Electrochemical hydrogenation (saturation) of cylindrical steel samples revealed that the zone of penetration of electrolytic hydrogen match the region of the cleavage crack origin, i.e., the area of the subsurface hydrogen distribution can be considered a local area of micro-cleavage in the peel tests thus making possible the study of kinetics and concentration dependences of steel brittle fracture under the impact of both diffusion-mobile and combined hydrogen, i.e., under conditions of reversible and irreversible hydrogen brittleness. The dependences of the peel resistance (micro-cleavage) on the concentration of diffusion-mobile (active) and bound hydrogen in the micro-cleavage zone are obtained as a result of kinetic studies. Electron-fractographic study of fractures of 18KhGMF steel samples showed that fracture morphology changes in accordance with the stages of brittleness development. The initial ductile fracture is followed by a typically brittle peeling at reversible brittleness. Then, with the development of irreversible fragility, a mixed character of destruction is observed. The obtained three-stage dependence of the peeling strength, taking into account different hydrogen states, proves the different physical nature of the processes at different stages of hydrogen brittleness and connects the known reversible and irreversible hydrogen brittleness with the effect of diffusion-mobile and bound hydrogen, respectively.