10365 Algorithm of the Protectiveness of Corrosion Layers: 2 - Protectiveness Mechanisms and H2S Corrosion Prediction

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Product Number: 51300-10365-SG

ISBN: 10365 2010 CP

Author: Jean-Louis Crolet and Michel R. Bonis

Publication Date: 2010

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The predictions of the so called CO2 or sweet corrosion and the so called H2S or sour corrosion are questions raised and still pending for over 60 years. A reliable prediction actually requires one to understand when high or low corrosion rates are encountered in E&P operations (field data), how they can occur (comprehension of the various mechanisms ruling the corrosion layer protectiveness), and finally why they do occur (link between data and mechanisms). So far, however, and despite long lasting researches, these three aspects had always been studied separately.

After a short reminder of the chronology of each theme, the protectiveness concept is revisited in depth, and without any equation… In this way, major differences can be emphasized between a unique acid gas or a mix of acid gases, and then between two acid gases with insoluble iron salts, and a single gas plus a second acid with a soluble iron salt. The latter may be the well known free acetic acid (HAc), but also a potential contribution of the direct diffusion of H+ ions from the bulk medium through the deposit. At the metal surface, this is locally equivalent to a strong acid, i.e. HCl in brines or HClO4 in condensed water. Thanks to the user friendliness of a recent calculation tool on water chemistry, which now includes corrosion products, the leading parameters of protectiveness were systematically quantified, or at least their base value in the bulk medium: pH, potential corrosivity (PC), solubility limit of iron as an added corrosion product (Fes) and pH at saturation. This results in a genuine algorithm of protectiveness, i.e. allowing us to understand why what was observed in the field did occur. Consequently, one will at last be certain that this cannot but reoccur in the same conditions as those identified.

Keywords: CO2 corrosion, gas wells, oil wells, carbon steel, corrosion layers, protectiveness