Iron sulfide corrosion product layers have recently become the center of attention in H₂S corrosion. The iron sulfides role on mild steel corrosion exposed to H₂S containing environments has been reported to be related to their semi-conductive properties. The two most stable phases, pyrite and pyrrhotite have similar resistivity and a more positive open circuit potential (OCP) when compared to mild steel OCP. In an earlier study, the authors reported a comparable electroactivity to what was measured on an X65 mild steel surface. Given conditions for establishing a galvanic cell are present in a conductive electrolyte, a corrosion product layer containing sulfide phases could enhance the rate of cathodic reactions by providing a larger cathodic surface area. Also, the mixed potential established between these layers with the mild steel is often more positive than the OCP for the uncoupled steel. This results in an intensified iron dissolution rate. In conditions where the steel is locally exposed to the corrosive media, this leads to localized corrosion.

The evidence for a galvanic effect related to iron sulfides coupling to mild steel has been mostly circumstantial. The galvanic corrosion that was attributed to such conditions was a conclusion that was reached through deductive reasoning, without direct proof. In the current study, zero resistance ammeter (ZRA) measurements were utilized to measure the magnitude of the galvanic current between coupled X65-pyrite and X65-pyrrhotite electrodes. Pyrite and pyrrhotite electrodes were prepared using geological specimens. The experiments were conducted in a deaerated conductive aqueous solutions at different pH at room temperature with iron sulfide surface area being seven times higher than the X65 steel surface area.

The experimental results for the same cathode to anode ratio showed that a higher galvanic current was measured at the X65-pyrrhotite couple versus X65-pyrite surfaces. It is noteworthy that upon exposure to an aqueous solutions containing H⁺, pyrrhotite undergoes a surface reduction to troilite and produces H₂S. In a more acidic environments the produced H₂S does not have a substantial effect on the corrosion currents. However, in aqueous solutions with limited available H⁺, the produced H₂S plays a significant role in enhancing the corrosion currents.

It was shown in a prior publication that at pH 5.0 the currents at the pyrrhotite surface was higher than what was measured at the pyrite or X65 surfaces. This resulted in a much higher galvanic current at the X65-pyrrhotite surface.