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Chloride pitting corrosion of API X-80 and X-100 high strength low alloy pipeline steels in bicarbonate solutions Alwaranbi, Maslat S

Abstract

The corrosion polarization behaviors of API X-80 (550 Grade) and API X-100 (690 Grade) pipeline steels in bicarbonate/chloride media were investigated by means of electrochemical techniques. In bicarbonate free chloride containing solutions, both steels revealed practically similar polarization behaviors exhibiting active/passive transitions with a varying number of distinct oxidation peaks. The bicarbonate concentration increase in the solutions was found to expand the passive potential region and result in more noble breakdown potentials. In contrast, the addition of small amounts of chloride ions evidently influenced the shape of the polarization curves where the corrosion rates were increased, the passive potential regions were narrowed in width, and the pitting potentials were shifted in the negative direction. The corrosion characteristics under the same prevailing experimental conditions of both steels indicated that API X-100 steel grade had lower corrosion rates in all tested bicarbonate solutions with and without the presence of chloride ions and better pitting corrosion resistance for the range of chloride concentrations investigated. This may have been attributed to the higher alloying content of molybdenum, nickel and copper in API X-100 steel over API X-80 grade and accordingly in the variance of the final microstructure formed for both steels which in turn could have made a change in the thickness and the protectiveness of the passive layers formed on the surfaces of the steels. Pitting morphology features such as the average diameter and density of active pits were examined by the Scanning Electron Microscopy (SEM). SEM observations showed that solutions of low bicarbonate concentrations (0.01 and 0.05M) resulted in a comparatively bigger size of pits, while solutions of higher bicarbonate concentrations (0.1 and 0.5M) showed no pits. This major difference may be due to the more stable passive films produced on the steel surfaces having higher bicarbonate concentrations. On the other hand, pit morphology as examined by the SEM technique for the bicarbonate solutions consisting of different concentrations of chloride ions indicated that the density and the average diameter of the stable corrosion pits increased with increasing the chloride concentrations in the test solutions.

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