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

Mundhenk, Niklas, Knauss, Kevin G., Bandaru, Siva R.S., Wonneberger, Robert, Devine, Thomas M.
The Science of the total environment 2019 v.677 pp. 307-314
carbon, corrosion, dielectric spectroscopy, hydrogen, interferometry, iron oxides, sodium chloride, steel, temperature
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 °C) in CO2-saturated mildly acidic NaCl brine. Iron dissolution kinetics follows a logarithmic rate at 80 and 160 °C and a linear rate at 120 °C. At 80 °C, high initial corrosion rates (first 24 h) generate H2 at a minimum rate of 12 μmol h−1 cm−2 and lead to the formation of a continuous ~100 μm 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 °C, soon followed by the initiation of pitting at 120 °C. Steady-state corrosion rates at 160 °C are at least one order of magnitude lower than for 120 °C. Our experimental approach demonstrated potential for general applicability in studying corrosion-related phenomena.