Electrochemical sorption of H into Fe and mild-steel: kinetic conditions for enhancement or inhibition by adsorbed HS-

Abstract

Sulfide species, HS^-, S^2- and H₂S, are often the most common and important poisons that arise in practical situations of embrittlement of ferruginous metals by sorbed H. The absorption and permeation of H can be either enhanced or inhibited under conditions of cathodic polarization in the presence of HS^-. The effect of chemisorption of S on H transfer into and through Fe and mild-steel in aqueous NaOH solution has been studied comparatively in relation to poisoning by species derived from AsO₂^- using a kinetic equation-fitting procedure applied to H permeation currents in relation to simultaneously measured polarization behaviour on the H₂-evolution side of a bielectrode membrane. A kinetic-fitting model is applied to all the constituent partial reactions involved at the electrode, including processes involving chemisorbed poison (P) species. The important relation of surface coverage by H to the H concentration in a sub-surface state is examined in relation to the kinetics of steps in the cathodic H₂ evolution reaction (HER) and the step of interfacial transfer of H into the metal host. The rates of absorption and desorption of H, and its diffusion from the sub-surface layer into the bulk metal, are important in determining the effects of the poisons on the sub-surface concentration of H and therefore the H permeation current.