Passivity breakdown of carbon steel in hot potassium carbonate solutions
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This research studied passivity breakdown of carbon steel in conditions typical of hot potassium carbonate (HPC) acid gas processing plants. The pitting potential, evaluated from polarisation curves, indicated resistance to pitting increased with increasing equivalent carbonate concentration. The pitting potential was much more positive than the free corrosion potential for uninhibited concentrated carbonate solutions with pH values of 8.5 to 9.5. Therefore pitting is not an issue in these solutions. However, the carbon steel was susceptible to pitting corrosion in the dilute 1.0% bicarbonate solution of pH 8.0 for chloride concentrations in excess of 0.5 wt%. The critical chloride concentration was considerably above the maximum concentration of 0.1 wt% recorded in a typical HPC plant. Consequently chlorides should not usually be of concern to plant integrity.
KeywordsChloride Concentration Passive Film Passive Layer Thiosulphate Free Corrosion Potential
The support of Santos for this work and permission to publish this article is gratefully acknowledged. Thank you to P. Wilson, of Corrosion Risk Management Pty Ltd, for his work on developing the pH model, Porbaix Diagram and his technical advice.
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