The Effect of the Partial Pressure of H₂S and CO₂ on the Permeation of Hydrogen in Carbon Steel by Using Pressure Buildup Techniques

There are concerns in the industry about using an electrochemical technique for actual hydrogen permeation measurements where charging current is not a field condition. The objective of this work is to use pressure buildup techniques to study the influence of H₂S and CO₂ partial pressure on the relationship between hydrogen permeation and corrosion rate measured by different techniques. Sulfide films formed on carbon steel in a solution containing 5 wt% NaCl and 0.5 wt% acidic acid at various H₂S and CO₂ partial pressures were characterized, and the effect of the film on hydrogen permeation was also investigated. Field conditions were included in this study for comparison purposes. The relationship was modeled at the steady state of both hydrogen flux and corrosion rate. The results confirmed by use of two hydrogen flux measurement techniques (eudiometer and high-pressure buildup probe) and two corrosion measurement methods (weight loss coupons and coupled multiarray electrode system), that there is no direct correlation between hydrogen flux and corrosion rate. Therefore, the hydrogen permeation rate in H₂S and CO₂ environments was found to be more controlled by partial pressure of H₂S than corrosion rate. The amount of descent in hydrogen flux, after reaching maximum of hydrogen permeation rate and before reaching a steady state, depends on the morphology and structure of corrosion films which are mainly controlled by concentration of H₂S.