THESIS OF Martien Duvall Deffo Ayagou*

Hydrogen sulfide (H2S) is a toxic gas found both in nature (green algae fermentation) and the industrial environmenta. On contact with water, it forms an acid solution liable to cause steel corrosion via the oxidation of iron, with the formation of a deposit that acts as a protective barrier to varying degrees and the generation of hydrogen gas on the metal surface. H2S then facilitates the mass penetration of hydrogen into the steel, causing internal damage leading to fracture and, ultimately, structural failure.

Well documented in the oil sector, in which environments tend to be devoid of oxygen, this problem has not been studied in the biomass and geothermal sectors, with environments that can contain both H2S and air. It was therefore important to verify to what extent reactions between H2S and O2 affect steel corrosion and weakening by hydrogen.

The research carried out highlighted the main products of the reaction between H2S and O2 dissolved in aqueous medium, sulfates and sulfites, resulting in
acidification of this mediumb(1). In addition, the iron sulfide deposit that forms on the surface of the steel is less dense and less protective in the presence of O2. These modifications lead to a considerable increase in the speed of corrosion (figure). In addition, the concentration of hydrogen in the steel is much greater in the presence of air(2,3).

Figure Deffo
Variations in the speed of steel corrosion in aqueous media containing dissolved H2S, in the absence or presence of oxygen.

These results suggest that serious corrosion problems and increased weakening risks due to hydrogen are likely with the simultaneous presence of H2S
and O2.
For the new energy sector, where just such an environment may exist, these risks will have to be taken into account when selecting metals.

a - In natural gas or biogas, for example.
b - Fall of close to one unit of pH per month in the presence of O2.

*Thesis entitled "Impact of oxygen and H2S on the corrosion of pure iron and on hydrogen concentration"

(1) L. Gemello, V. Cappello, F. Augier, D. Marchisio, C. Plais, Chemical Engineering Research and Design, 2018, 136, 846-858.

(2) L. Gemello, C. Plais, F. Augier, D. Marchisio, Chemical Engineering Journal, 372, 2019, 590-604.

Scientific contact: Jean KITTEL