Corrosion Mechanism of Spheroidal Graphite Cast Iron in an Aerated Saline Wyoming Bentonite Slurry

The corrosion of GGG40 spheroidal graphite cast iron in a naturally aerated saline slurry of Wyoming bentonite was monitored at 30°C and 50°C over 3 month using polarization curves and electrochemical impedance spectroscopy (EIS). The bentonite slurry severely restricts the transport of oxygen toward the surface, reducing the oxygen reduction current to levels comparable to those of the concomitant, unhindered water reduction. This mass transport barrier introduced by the suspension also promotes local alkalization generated by the reduction reactions, thereby facilitating the precipitation of ferrous hydroxide. Furthermore, the interfacial bentonite reacts with ferrous ions to form iron silicates alongside Fe(II,III) oxides. Local cells formed at the graphite/ferrite interfaces induce the growth of circular pits that extend around the graphite nodules. The associated coupling of the limited oxygen reduction on the graphite nodules with iron dissolution introduces an Fe$^{2+}$/Fe$^{3+}$ redox dynamic, which is reflected in current fluctuations superimposed onto the anoxic corrosion system controlled by water reduction. Changes in the surface chemistry and morphology were elucidated via scanning electron microscopy-energy dispersive x-ray spectroscopy, transmission electron microscopy-energy dispersive x-ray spectroscopy, and localized x-ray photoelectron spectoscopy.