Stephen N. Smith, Exxon Mobile Production
Recent developments in the understanding of FeS chemistry are having an impact upon the definition of the role that the various forms of FeS have upon the continued corrosion of steel in environments that contain H2S.
- Stoichiometry of mackinawite has been reevaluated. The new findings indicate that mackinawite has a 1:1 iron to sulfur stoichiometry as opposed to the previous belief that mackinawite is an iron excess compound. This could be significant since any corrosion model that results in the formation of mackinawite would have needed to address the non-stoichiometric ratio.
- Some recent corrosion research has included the concept of FeS supersaturation in the proposed mechanism building upon the supersaturation that is observed in CO2 - bicarbonate corrosion mechanisms. However, the kinetics of FeS precipitation have long been known to be far faster than carbonate precipitation, so the observed excess ferrous ion concentrations were difficult to understand. However, a number of iron-sulfur complexes, ion pairs and hydrated species have been identified that possibly contribute to the "excess" iron concentrations in solution. The continuum of mackinawite that ranges from single FeS molecules to clusters of 4 to 150 molecules through nanoparticles with more than 150 molecules may also help to explain the extra iron that has been reported to be in solution.
- This improved understanding of the complicated Fe-S chemical system raises questions about the conditions that exist of fixed inventory corrosion tests procedures with low H2S concentrations. Not only is the H2S adsorbed by the test equipment surfaces and is consumed to form the corrosion product, but it also is consumed to reach equilibrium with all of the complexes, nanoparticles, etc. Under some conditions, the tests may actually not be tests of corrosion in a low H2S environment, but may end up as CO2 corrosion tests of specimens with partial coatings of FeS.
- The existence of a large number of Fe-S corrosion product species has long been known as has the fact that some FeS species are precursors for the development of other species. What has not always been clear has been the mechanism of the transition from one morphology to another. Research into the mechanism of transition from mackinawite to pyrite is not one of transformation but is based upon the dissolution of the mackinawite to provide a reactant with sulfur from the solution to reprecipitate as pyrite. This requires not only the presence of the mackinawite, but also a source of elemental sulfur and a mechanism to transport the sulfur to the region near the mackinawite and pyrite crystal surfaces.
