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Iron sulfide corrosion product layers commonly form on mild steel surfaces corroding in aqueous H2S environments. These porous layers present a barrier which may reduce the corrosion rate, however, their semi-conductive nature leads to an acceleration of corrosion via galvanic coupling, by increasing the cathodic surface area. The electrocatalytic properties of different iron sulfides, which are important in this process, were unknown. The current study looks at the cathodic reaction rates on the surfaces of geological pyrite, geological pyrrhotite, and mild steel in HCl, CO2 and H2S aqueous solutions at different pH. Results show that in solutions where H+ reduction dominates, pyrite has similar electroactivity as X65 steel, while pyrrhotite exhibits approximately one order of magnitude smaller current densities. An extra wave observed in the cathodic sweeps on pyrrhotite was due to conversion of pyrrhotite to troilite. In aqueous CO2 solutions, similar results were obtained, while in H2S aqueous environments, both pyrite and pyrrhotite showed similar electroactivity that was slightly lesser than that of X65 steel. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.1381712jes] All rights reserved.