Carbon or low-alloy steel corrosion, by aqueous media containing CO2, hampers the development of numerous technologiesa for the energy transition. Among the electrochemical processes involved, the reaction between metal cations and anions present in the electrolyte produces the precipitation of compounds that are deposited on the surface of the steel and modify corrosion kinetics through processes that are not yet fully understood.
A collaborative project conducted with ANDRA (French National Agency for Radioactive Waste Management) demonstrated experimentally, at a temperature of 80°C and under 0.54 bars of CO2, that increasing pH beyond 6.6 led to a reduction in corrosion kinetics, due to the formation of a film qualified as “pseudo-passive” (final corrosion rate below 0.05 mm/year)(1).
A study combining different techniquesb demonstrated that the effect of this protective porous film on corrosion rate was primarily due to its surface-coating action on the metal, thereby reducing active dissolution zones. SEM-EBSDc analysis (figure) revealed that this film is made up of layers of a variety of compositions. Below the outer part made up of siderite (FeCO3), a magnetite (Fe3O4) layer developed in some zones, contributing to the capacity to protect against corrosion(2).
This improved understanding of the pseudo-passivation mechanisms, at the confined interface between the surface of a carbon steel and a corrosion deposit, is being used as the basis for a study currently under way on the influence of some contaminant compounds dissolved in the aqueous phase, such as oxygen.
a - For example: CO2 capture, transport and conversion, underground gas storage, conversion of biomass into fuels and chemicals.
b - Electrochemical impedance measurement, surface chemical analysis and local pH measurements.
c - Scanning electron microscopy analysis via electron backscatter diffraction.
(1) R. De Motte, R. Mingant, J. Kittel, F. Ropital, P. Combrade, S. Necib, V. Deydier, D. Crusset, Near, Electrochemica Acta, 290 (2018) 605-615.
(2) R. De Motte, R. Mingant, J. Kittel, F. Ropital, P. Combrade, S. Necib, V. Deydier, D. Crusset, The Use of Electrochemical Impedance Spectroscopy and Surface Analysis to Study the Formation of a Protective Film, submitted to Corrosion Science.