Charge transfer mechanism of hydrogen-induced intergranular embrittlement of iron

Impurity-induced reduction of intergranular cohesion - a major factor limiting the usable strength level of ultrahigh-strength steels - is particularly severe when aggravated by mobile hydrogen through environmental interaction, as in the case of hydrogen stress corrosion cracking. As an aid in establishing an understanding on the electronic level, the influence of hydrogen on the cohesion of an iron grain boundary was determined using the full-potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation. Through precise calculations on both grain boundary and free surface environments, we found that hydrogen is a strong embrittler. Analysis of the results in terms of structural relaxation, bonding character, and magnetic interactions shows that the hydrogen-iron chemical bond is stronger on the free surface and a charge-transfer mechanism is found to play a dominant role for the hydrogen-induced reduction of cohesion across the iron grain boundary. These results provide a quantitative explanation from first principles for the technologically important phenomenon of hydrogen-induced intergranular embrittlement.