Iron Sulfides Electroactivity in Environments in the Presence of Strong Acid, Carbonic Acid and Aqueous Hydrogen Sulfide

Abstract

Iron sulfides, particularly the two most stable polymorphs: pyrite and pyrrhotite, have captured the attention of scholars due to their semi-conductive properties. Their semi-conductive nature allows for the corrosive species to be reduced at their surface while electrons are transported from the steel surface via their network. Thus, as a corrosion product layer, pyrite or pyrrhotite do not offer sufficient protection to the steel underneath in the form of a mass transfer barrier for corrosive species. In addition, these layers could accelerate the corrosion rate via an enhancement of reduction of corrosive species rate on their surface. The electrochemistry of iron sulfide layers as it is related to the field of corrosion is unclear and the reported data in the literature has not reached a conclusion. Most of the available literature about electrochemistry of iron sulfides reports on their anodic, rather than cathodic behavior. However, it is noteworthy that in the mechanism related to a mild steel corrosion under such layers, the mild steel is the anode, and the iron sulfide layers are the cathode. A much higher cathodic surface area emerges due to the porous nature of these layers, which results in a higher corrosion currents of the mild steel. Therefore, in order to understand their main role on mild steel corrosion, it is important to study the rate of reduction of corrosive species such as: hydrogen (H+), carbonic acid (H2CO3) and hydrogen sulfide (H2S) on their surface to investigate their electroactivity. This would extend the current understanding of galvanic corrosion of a mild steel under iron sulfide layers. In the current study, pyrite and pyrrhotite electrodes made from geological specimens were polarized negatively and the results were compared with the data obtained from a mild steel electrode. The experiments were conducted in deaerated aqueous solutions with and without H2S at different pH at a room temperature. The experimental results revealed the significance of electroactivity of pyrite and pyrrhotite related to H+, H2CO3 and H2S reduction rates. A comparison of their polarization data with a bare mild steel data showed a similar level of electroactivity. The measured data were compared with the calculated data from an electrochemical model developed and validated for a mild steel electrode exposed to aqueous environments with and without H2S. The experimental results from the pyrrhotite electrode showed an extra wave on the polarization curves with a distinct limiting current, it remains unclear what this wave represents. The nature of this wave is still under investigation. Figure 1