Surface Modification Effects on Hydrogen Permeation in High-Temperature, High-Pressure, Hydrogen-Hydrogen Sulfide Environments^✫

Surface modifications on chromium-molybdenum (Cr-Mo) steel were tested for hydrogen (H) permeation and corrosion protection in a hydrogen-hydrogen sulfide (H₂-H₂S) environment. Virgin thermal spray coating showed no resistance to H permeation. Virgin metal coating, aluminum (Al) hot-dipped plating, Al diffusion coating, and Cr diffusion coating worked as barriers to H permeation, apparently by means of a superficial oxide or passivation film. Virgin ceramic coating by chemical vapor deposition (CVD) also worked as a barrier to H permeation. After testing of specimens as coated, coating materials were sulfidized to simulate the surface after use in a corrosive environment containing H₂S and then were tested for H permeation. Thermal spray coatings with a layer of aluminum oxide (Al₂O₃)-50% nickel (Ni)-50% Cr acquired resistance to H permeation after sulfidization. Resistance to H permeation of metal coatings such as the Al hot-dipped plating, Al diffusion coating, and Cr diffusion coating decreased after sulfidization, but remained at a satisfactory level. Ceramic coatings by CVD lost resistance to H permeation with increasing times of sulfidization. Thermal spray and metal coatings retained corrosion resistance against sulfidization. The corrosion resistance of ceramic coatings by CVD gradually deteriorated with time of sulfidization. Of the surface modifications studied, thermal spray coatings were considered feasible for practical applications.