Please see attached this figure from a journal paper I am currently reviewing. I made slight changes to it to maintain the anonymity.
The paper addresses the significance of H2S with the corrosion reactions of a mild steel in CO2-saturated brines. Increasing the H2S concentration increases the total cathodic currents as it gets reduced to H2 and HS-.
Now, how would this kind of reduction interfere with those of H2CO3, H+, and H2O? Which are the main reactions that drive the cathodic reduction in the CO2-saturated solutions?
One reason I am posting this is that the authors believe that H2S might have slowed down the kinetics of H2O reduction, is that possible?
Any inputs would be highly appreciated.
In these types of environments with both H2S and a steel surface, there is a direct reaction where the H2S reacts on the surface of the steel to form an FeS layer and atomic hydrogen. The FeS layer is electrically semi-conductive, so further reactions are possible, but the kinetics will be different.
As for the CO2 cathodic reaction, H2CO3 does not react directly with the metal surface, so with only CO2, the cathodic reactions are the reduction of H+ and water.
As for your comment that the H2S is reduced to H+ and HS-, this is not the case. The reaction is simply one of H2S (or HS- depending on pH) reacting directly on the surface of the steel to form atomic hydrogen and FeS. I should also add that the kinetics of this reaction are very fast, much faster than H2CO3 dissociation and far faster than the precipitation of FeCO3 in the absence of H2S.
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