During the past decade or so, there has been an increase in the use of a rule-of-thumb that
there is a dividing line between CO2 and H2S corrosion where the CO2/H2S gas partial
pressure is 500. Most users of this ratio are familiar with neither the origins of the ratio, the
assumptions that it implies nor of its practical limits. This work investigated both the history
and the basis for the ratio. In doing so, it was determined that although there is a sound
scientific basis for the ratio, the error band produced by variations in thermodynamic data
inputs is so large as to make the ratio’s use impractical as an engineering guideline.
Key words: CO2/H2S Ratio, H2S Corrosion, Mackinawite, Siderite