Quantitative Study of the Hydrogen Entry Behavior of Low Alloy Steels for Various Sour Environments

Hydrogen entry behavior was investigated with different H2S partial pressures over pH 5.0 and was quantified for various sour environments using American Petroleum Institute grade X65 line pipe and line pipe plate. Hydrogen permeability dramatically decreased for H2S partial pressures of 0.1 MPa exceeding pH 5.5 and 0.01 MPa exceeding pH 6.2. This is caused by the formation of a stable iron sulfide film. On the other hand, hydrogen permeability proportionally decreased with increasing pH for H2S partial pressure of 0.001 MPa up to pH 6.0. The critical pH at which iron sulfide becomes quite stable was predicted from the equation of the relation among pH, H2S partial pressure, and iron ion activity based on potential vs. pH in Fe–S–H2O. Hydrogen concentration into steel invading from various sour environments was proposed and quantified.