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

Abstract

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.