Reduction Reactions on Iron Sulfides in Aqueous Acidic Solutions

Abstract

Iron sulfide corrosion product layers commonly form on mild steel surfaces corroding in aqueous H 2 S 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, CO 2 and H 2 S 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 CO 2 solutions, similar results were obtained, while in H 2 S aqueous environments, both pyrite and pyrrhotite showed similar electroactivity that was slightly lesser than that of X65 steel. Iron sulfides can be found in various forms, the most common being: mackinawite (FeS) that has a tetragonal crystalline structure, cubic ferrous sulfide (FeS), troilite (FeS) with a hexagonal structure, polymorphous pyrrhotite (Fe 1-x S), hexagonal smythite (Fe 3+x S 4 ), cubic greigite (Fe 3 S 4 ), cubic pyrite (FeS 2 ) and orthorhombic marcasite (FeS 2 ).1 They contain iron in different oxidation states (Fe 2+ and Fe 3+ ), with a broad range of non-stoichiometric compositions and have distinct physicochemical and electrical properties. Iron sulfides are classified as semiconductors 1 with their electrochemistry being an important area of investigation across different fields of application, including clean energy related research, 2-6 corrosion of steel in H 2 S containing environments, 7-12 geochemical studies, 12-15 etc. For example pyrite, and pyrrhotite are two most abundant iron sulfides found in the Earth's crust 1,[16][17][18] and are at the same time among the most common corrosion products of steel corrosion due to H 2 S.Pyrite is the most thermodynamically stable stoichiometric iron sulfide, 1 having the lowest solubility in water as compared to other iron sulfides.19 Pyrite is a semiconductor with a resistivity in the range of 10 −5 _ 10 1 m 20-22 and is found as both n-type and ptype.23 Pyrrhotite (Fe (1-x) S (x = 0-0.2)) 17,24-27 is an iron deficient iron sulfide, with a crystalline structure changing from monoclinic to hexagonal depending on its iron deficiency. For example, hexagonal stoichiometric pyrrhotite is known as troilite. Both pyrrhotite and troilite are p-type semiconductors with a resistivity [20][21][22] in the range of 10 −6 _ 10The electrochemistry of iron sulfides investigated to date, has been mostly focused on anodic reactions, such as phase transformation and dissolution [28][29][30][31][32] which are predominantly important in weathering of iron sulfide...

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