Low Alloy X100 Pipeline Steel Corrosion and Passivation Behavior in Bicarbonate-Based Solutions of pH 6.7 to 8.9 with Groundwater Anions: An Electrochemical Study

This research investigates the fundamental corrosion and passivation processes occurring on API-X100 pipeline steels before, during, and after passive layer formation in bicarbonate-based simulated soil solutions of pH 6.7 to 8.9. Free corrosion potentials decrease exponentially with bicarbonate (and pH), owing to increased water and bicarbonate reduction in more alkaline conditions and the coupled iron oxidation reaction. Active corrosion rates at potentials slightly above open circuit potential increase with bicarbonate, until a concentration of 1.68 g L−1 sodium bicarbonate (plus dilute amounts of chlorides/sulfate) at which first signs of film formation appear. Thereon, increased bicarbonate concentration generally decreases current densities and resists ferrous-oxide product formations due to improved iron carbonate formation conditions precipitating more durable passive layers. Potentiodynamic polarization in the anodic regime reveals varying electrochemical processes involving interactions between hydroxide, carbonate, and oxide complexes with ferrous, with diverse pH and potential dependencies. The products of corrosion reactions were investigated by scanning electron microscopy, X-ray diffraction, and/or X-ray photoelectron spectroscopy.