The Anodic Behavior of Iron in Hydrogen Sulfide Solutions

The anodic behavior of iron in 0.032 mol·L⁻¹ NaCl/0.003 mol·L⁻¹ NaHCO₃ and 1 mol·L⁻¹ Na₂SO₄/0.003 mol·L⁻¹ NaHCO₃ solutions containing 0.05 mol·L⁻¹ H₂S at ambient temperature was studied using cyclic voltammetry and the potentiostatic technique. In both solutions, a nonprotective film of mackinawite (Fe_(1+x)S) was formed on the iron, but the film growth kinetics differ for the two solutions. The film growth in chloride solutions does not appear to follow a conventional type of model for multilayer films whereas the formation of the film in sulfate solutions can be represented in terms of a pore-resistance model. At more anodic potentials, the mackinawite is oxidized to a higher sulfide, possibly pyrite (FeS₂), as suggested from a comparison of the anodic and cathodic peak potentials with the equilibrium potential. The formation of the higher sulfide(s) in the chloride solution appears to follow a similar film growth mechanism to that for mackinawite, whereas it is not clear whether this is the case for the sulfate solution.