Abstract In order to control and predict the influence of hydrogen sulfide on carbon steel corrosion, it is of a paramount importance to understand its corrosion mechanism. What is the mechanism and kinetics of formation and growth of a FeS layer? Highlight as a very important issue because of the different electrochemical roles that these corrosion products can accomplish. One of these roles is to assist the hydrogen evolution reaction, where the iron sulfide corrosion product will act as a cathode. However, another role could be its direct action as a cathodic fuel. In this paper, it was found that Pyrite and Pyrrhotite act as cathodic fuels when they are galvanically coupled to steel. Furthermore, the nucleation and growth of FeS in pH 8.40 buffered solution are investigated. It is possible to identify three nucleation species with different nucleation rates, acting at the same time. These species were identified as Mackinawite, Troilite and Cubic iron sulfide. The proportion of these species changed with the nucleation potential. These findings indicate that at the most fundamental level, the FeS layer consist on a blend of different iron sulfide polymorphs rather than a pure iron sulfide form.