A REVIEW ON CORROSION OF LOW CARBON STEELS IN AQUEOUS HYDROGEN SULFIDE

Sour corrosion has been a problem for the oil exploration and extraction industry for many decades. It can manifest itself in different ways and one of which is the formation of a scale on the inner walls of pipelines. Although the consequences of corrosion are well known, the combination of factors that determine the composition and physicochemical characteristics of the scale remains unclear. This one is initially composed of mackinawite, which is relatively stable in the absence of oxygen. At high partial pressures of hydrogen sulfide and/or temperatures, other more stable sulfides such as troilite and pyrrhotite are formed. Thus, the protectiveness of the scale is enhanced by the formation of these new compounds. An undesirable entry of air in pipelines during drilling and oil recovery operations changes scale composition and therefore polysulfides, oxides and oxyhydroxides can also be found as corrosion products. The degree of oxygen contamination determines the relative proportions of these compounds, so low concentrations favor the formation of polysulfides, while high concentrations favor the formation of oxides. In this context, this paper discusses the effects of partial pressure of hydrogen sulfide, dissolved oxygen and water chemistry on scale formation. Particular attention is paid to the nature and stability of the aqueous species formed during the corrosion process, as well as the thermodynamic and kinetic aspects lead to scale formation. The main objective is to enhance the understanding of the formation and precipitation conditions of mackinawite and pyrrhotite and their effects on corrosion processes involving low carbon steels