Corrosion of carbon steel and the passivating properties of corrosion films formed under high-PT geothermal conditions

Corrosion is a major obstacle to a safe implementation of geotechnical applications. Using a novel approach that includes vertical scanning interferometry (VSI) and electrochemical impedance spectroscopy (EIS) we discuss time-dependent carbon steel corrosion and film formation at geothermally relevant temperatures (80-160 degrees C) in CO2-saturated mildly acidic Na-Cl brine. Iron dissolution kinetics follows a logarithmic rate at 80 and 160 degrees C and a linear rate at 120 degrees C. At 80 degrees C, high initial corrosion rates (first 24 h) generate H-2 at a minimum rate of 12 mu mol h(-1) cm(-2) and lead to the formation of a continuous similar to 100 mu m thick porous corrosion film. It exhibits a duplex structure with a crystalline outer FeCO3 layer and an inner layer composed of a skeletal network of Fe3C impregnated with FeCO3. Being an electrical conductor we hypothesize the Fe3C to strongly enhance corrosion rates by providing additional cathodic sites. Pseudo-passivity due to an anodic film-forming reaction (presumably Fe-oxide) was observed at 120 and 160 degrees C, soon followed by the initiation of pitting at 120 degrees C. Steady-state corrosion rates at 160 degrees C are at least one order of magnitude lower than for 120 degrees C. Our experimental approach demonstrated potential for general applicability in studying corrosion-related phenomena. (C) 2019 Elsevier B.V. All rights reserved.