Stress-corrosion cracking in plastic solids including the role of hydrogen

Abstract

Small changes in surface environments can change the energy needed to create a surface shear step. Increases in this energy tend to shift a delicate balance between glide and cleavage initiation at a crack tip. By inhibiting plastic deformation this causes an increased tendency for cleavage. Thus a material that is ductile in a vacuum can become quite brittle in the presence of certain surface-active environments, particularly atomic hydrogen. The quantitative criterion for deciding whether flow or cleavage will prevail is the ratio of the appropriate glide plane's surface energy to the cleavage plane's surface energy. A survey of the strengths of the interactions between hydrogen and metals has been made. Throughout the periodic table strong diatomic interactions occur. At solid surfaces the interactions remain strong although they are somewhat weaker than for diatoms. In solid hydrides they tend to be much weaker or non-existent. Thus the strength of the interaction depends on the metal-metal distance. It is shown that for a typical transition metal the interaction with hydrogen is strong enough to readily cause embrittlement.