Electrochemical behavior of X70 and X80 pipeline steels in a simulated soil environment with and without the presence of chlorides

The present work investigates the effect of microstructure, pH as well as the role of aggressive chloride ions on the corrosion behavior of American Petroleum Institute (API) X70 and X80 steel grades. The corrosion behavior was studied using single-run dynamic and cyclic polarization in different solutions followed by microstructural analysis of the corroded samples to determine the mode and extent of corrosion damage. The solutions consist of a near-neutral aqueous sodium chloride solution, mildly alkaline carbonate-bicarbonate solution (pH-8.8), and highly alkaline carbonate-bicarbonate solution (pH-12) with and without the presence of chlorides. Ferrite phase in both the steel was found to dissolve preferentially in the different solutions leading to pitting as a result of microgalvanic coupling with the cementite and/or bainite phase. Interestingly, the corrosion resistance of the X70 grade was found to be greater than X80 at near-neutral as well as low alkaline carbonate-bicarbonate solutions, whereas the X80 performed better in higher pH solutions. Such contrasting corrosion behavior is attributed to the compactness, thickness, and electronic resistance of the passive oxide/hydroxide films formed during polarization in both the steel.