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EIS study of iron corrosion in aqueous solutions at various concentrations of dissolved H2S. Impact of oxygen contamination

Picture for EIS study of iron corrosion in aqueous solutions at various concentrations of dissolved H2S. Impact of oxygen contamination
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Mildly acidic water containing dissolved H2S presents a strong risk in the cracking of mild steels. The main driving force is associated with the ability of H2S to promote hydrogen entry into steel. Lots of studies on H2S cracking mechanisms have been performed for oil and gas applications since it is a huge concern for this industrial sector. Standard test methods have been developed and published as NACE technical methods (e.g. NACE TM0284 and NACE TM0177). Though it is recognized that oxygen pollution should be avoided during H2S cracking tests there is still a lack of experimental data to illustrate the potential impacts of a small oxygen pollution. In standard glass vessels used for the evaluation of steel resistance to H2S cracking appropriate test procedures should allow for the controlling of O2 levels below 50 ppb. However higher values can be reached in the case of poor laboratory practices.In the following study continuous O2 injection at a level corresponding to 500 ppb is applied together with H2S bubbling in the test solutions. This paper will focus on the interactions between oxygen and H2S on electrochemical behavior of unalloyed steel. A test duration of the same order of standard SSC tests is applied. Surface reactions in H2S saturated water with or without oxygen pollution are studied through electrochemical impedance spectroscopy. EIS diagrams showed a strong influence of the highly porous and conductive iron sulfide scale that forms rapidly in H2S environments. This FeS film had to be taken into account in the impedance model in the form of a porous electrode element placed in parallel to the more conventional circuit corresponding to anodic dissolution with adsorbed intermediates already used in the literature for H2S corrosion.The evolution of corrosion rates obtained from impedance analysis was compared to two other independent methods: i/ weight loss measurements and ii/ hydrogen permeation. Without O2 pollution a permeation efficiency of 100% was obtained as expected. Permeation current density was thus found to match precisely with the corrosion current density determined by impedance analysis at different times. Weight-loss measurements also confirmed the validity of impedance analysis. On the other hand when a continuous O2 pollution was added in the system significantly higher corrosion rates were observed associated with test solution acidification. At the same time permeation efficiency was decreased by up to one order of magnitude.

Product Number: 51319-13041-SG
Author: Jean Kittel
Publication Date: 2019
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$20.00
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