Effect of Cathodic Potential on Hydrogen Content in a Pipeline Steel Exposed to NS4 Near-Neutral pH Soil Solution Available to Purchase

The effect of cathodic potential on hydrogen generation and permeation behavior of a X-65 pipeline steel exposed to near-neutral pH NS4 soil environment was studied. The steel-hydrogen interaction was evaluated in terms of the total hydrogen content in the steel and the hydrogen permeation flux through the steel coupon. It was found that hydrogen content in the steel charged in near-neutral pH solution depended significantly on a combined effect of cathodic potential and the formation of calcium carbonate (CaCO₃) film on the specimen surface that builds up with increasing charging time. It was confirmed by hydrogen permeation testing that the calcium carbonate film reduces the adsorption of atomic hydrogen to the steel surface. In the solution with diluted Ca²⁺, $CO32−$⁠, and $HCO3−$⁠, the hydrogen content in the steel was less sensitive to cathodic potential, and was lower than that produced in the solution with concentrated Ca²⁺, $CO32−$⁠, and $HCO3−$ under the same cathodic condition. The mechanisms and the implication related to the observations were discussed.