The transition from partial pressure to fugacity in the assessment of acid gas activity (concentration) for the design of qualification testing of metals to be used in sour service according to MR0175/ISO 15156 entails a number of important consequences. This transition came about in the wake of oil and gas production moving off-shore to ever higher pressures and temperatures. It was recognized that multiplying total pressure by the mol fraction of H2S in the “gas phase” could no longer reflect the physicochemical realities with respect to the reactions between H2S and the metal surfaces. As a consequence it was proposed that the activity of H2S in the gas phase should be replaced by the activity (concentration) of H2S in the aqueous phase. This change in paradigm had already been accepted in the ISO Standard but not implemented. Nevertheless it stands to reason that the dissolved H2S is the active corrosion vector rather than the H2S in the gas phase.An unintended consequence of this shift in thinking lies in the fact that a very large number of Heritage Metals have been qualified for partial pressure criteria as specified in MR0175/ISO-15156 by the use of the Crolet Diagram i.e. as function of pH vs. pH2S. In order to overcome this difficulty it is proposed to generate an array of look-up tables preferably in electronic form to translate the experimental conditions from pH2S to cH2S. This translation has to be made as a function of the test parameters (to the extent they may be known) as well as the field parameters. In parallel the pH2S axis in the Crolet diagram will need to be changed to a cH2S axis. In this manner it will be possible to assign to existing test data corresponding field conditions or vice versa specific field conditions can be used to select the appropriate metal from existing test data.An additional outcome of this methodology will be a quantitative assessment of the excess conservatism practiced in the past.

The current understanding of the corrosion mechanisms in H2S containing environments considers the direct electrochemical reduction of H2S as the main contribution of this species to the corrosion process. Such an argument has been developed based on the distinctive behavior of cathodic polarization curves in H2S containing solutions as compared to the behavior observed in the solutions of strong acids or those in presence of other weak acids such as carboxylic acids and carbonic acid. The direct reduction of H2S is generally associated with the observation of a “double wave” in a cathodic polarization curve. In the present study the mechanism of cathodic currents in H2S containing acidic solutions was studied theoretically through a comprehensive mathematical model. The model includes a mechanistic description of main processes including mass transfer chemical reactions and electrochemical reactions. A quantitative analysis based on this model showed that all the characteristic behaviors previously associated with the direct reduction of H2S including the “double wave” behavior can be explained based on the homogeneous chemical dissociation of H2S as a weak acid and hydrogen ion reduction as the sole cathodic reaction. This analysis suggests that H2S is not a significant electroactive species and its main contribution to the corrosion process is through its buffering ability as a weak acid similar to other weak acids such as carboxylic acids and carbonic acid. In order to validate these mechanistic observations the results from this model were compared to existing experimental data from the open literature. The model was found to be able to capture the main characteristic experimental behaviors with reasonable accuracy further supporting this mechanistic argument.