Effect of Cr, Mo and Ni on Hydrogen Embrittlement of Martensitic Steels

Abstract

The susceptibility to hydrogen embrittlement in martensitic steels containing Cr, Ni and Mo under stress and hydrogen diffusion have been investigated by electrochemical methods. As the content of alloying elements was increased, the apparent diffusion coefficient of hydrogen (D) and the threshold hydrogen permeation rate (J_th) for hydrogen embrittlement were decreased. The threshold hydrogen content (C_th), which was calculated by the formula; C_th=J_th×L/D (L; thickness), was almost constant. This means that the steel with smaller D has the higher susceptibility to hydrogen embrittlement because the hydrogen content in steel (C) becomes higher.
On the other hand, the C in NACE solution (0.5% CH₃COOH+5% NaCl, saturated with 0.1MPa H₂S, 25°C) was increased due to the decrease in D with the increase in the content of Cr although the hydrogen permeability (J×L) was decreased, and the susceptibility to Sulfide Stress Cracking (SSC) was increased as the result. However, Mo showed the remarkable effect of decreasing the J×L and the consequent improvement in the resistance to SSC according to its content.