Contribution to the Understanding of H2S Corrosion

Abstract H2S corrosion is controlled by processes occurring across the corrosion product (FeS) layer and by chemical effects which affect the properties of this layer. Laboratory data are reviewed which clearly demonstrate the parabolic nature of the corrosion kinetics. Along with the corrosion rates the film growth rates were experimentally determined. It was postulated that that the permeability was in essence the parabolic rate constant and could be calculated from the differential weight loss corrosion and the associated film weight. The permeability could then be examined as a function of pH. The difficulty in relating the corrosion rate to the permeability of FeS is the lack of knowledge regarding the actual rate controlling thickness of the film. The semiconducting properties of the film were formulated based on several mechanistic hypotheses. It was found that two of them fit the experimental evidence but no distinction could be made between the two. The corrosion rate was found to vary with flow rate to the 1/6th power. It had been possible to formulate this dependence on the basis that the corrosion rate was controlled by masstransfer both in solution as well as in the scale. Diagnostic equations had been developed for the action of amine type corrosion inhibitors. It appears that the more basic the amine, the more effective the inhibitor, which seems to have been experimentally confirmed, however, inhibitor effectiveness also depends on the pH of the solution and as expected the size and hydrophobicity of the inhibitor molecule.