A major problem in the oil and gas industry is corrosion in sour (or H2S-containing) environments. Low
concentrations of H2S act to inhibit corrosion, because of the formation of a protective iron sulfide film/scale
on steel surfaces; large concentrations can lead to pitting, severe localized corrosion, and catastrophic failure.
Therefore, the solubility and hydrothermal stability of iron sulfides is particularly relevant to corrosion. To
understand the mechanisms that are associated with the hydrothermal formation of iron sulfides, it is important
to reliably estimate equilibrium constants for all the reactions involved, including those for the dissolution
and dissociation of H2S. In this study, equations for determining the equilibrium constants that are related to
the solubility of mackinawite are evaluated, and the most appropriate expressions are described. Also, the
most relevant expressions for the solubility of H2S and for the first dissociation constant of H2S are reviewed.
It is recommended that the second dissociation constant be avoided in calculations, because of the large
uncertainty involved in previously reported values