Hydrogen transport and embrittlement in structural metals

The close relationship between hydrogen transport and embrittlement is indicated by evidence of hydrogen absorption preceding degradation of mechanical properties. Concentration of hydrogen at a crack or flaw by diffusion or by transport with moving dislocations is probably necessary also. Experimental studies show that hydrogen permeation is significantly influenced by surface conditions, particularly oxide films, and internal defects and impurities that trap diffusing hydrogen. The usual thermodynamic and diffusion relations, therefore, do not predict accurately the final distribution of hydrogen and the kinetics of the processes.Investigation of the effects of hydrogen on the mechanical properties of approximately fifty structural alloys at ambient temperature and pressures up to 69 MPa indicates that all alloys show evidence of susceptibility to hydrogen embrittlement. The degree of hydrogen embrittlement appears to be related to the amount of hydrogen absorbed and its distribution within the metal lattice. Surface condition, defect structure, hydrogen purity, and hydrogen pressure influence embrittlement. Of major importance is the transport of hydrogen with moving dislocations, a mechanism for concentration and redistribution of hydrogen that is operative at temperatures lower than for diffusion.