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Effects of Sodium Chloride on Acidic Nanoscale Pores Between Steel and Cement

Olsen, Richard, Leirvik, Kim Nes, Kvamme, Bjørn, Kuznetsova, Tatiana
The Journal of Physical Chemistry C 2016 v.120 no.51 pp. 29264-29271
Gibbs free energy, adsorption, bicarbonates, calcium carbonate, calcium oxide, carbon dioxide, carbon sequestration, cement, clay, corrosion, desorption, ions, iron oxides, molecular dynamics, pH, saline water, simulation models, sodium, sodium chloride, steel, temperature, wells
Carbon capture and storage is an attractive method of reducing carbon dioxide release to the atmosphere. One possibility is to inject carbon dioxide into underground aquifers. Injection wells consist of steel casings supported by cement (or plugged by cement, if abandoned), which contains large quantities of calcium oxide that can react with carbon dioxide to form calcium carbonate. Cement also contains various other clay materials similar to calcium carbonate. Pores between steel and cement make an ideal environment for corrosion due to saltwater and even more so due to the acidic environment, which may reach pH < 3, depending on temperature and pressure. Thus, surfaces of the pores transform into calcium carbonate and iron oxide, potentially reducing the integrity of the wells. Sodium and chloride ions will arrange in distinct layers reaching approximately one nanometer outside the surface and will interact with the acidic environment. To properly understand how adsorbed sodium chloride affects corrosion processes at the nanoscale we employed molecular dynamics simulations, where effects on adsorption free energies and density profiles were investigated. It was found that water densities within one nanometer from the surfaces were reduced as a consequence of adsorbed sodium chloride, thus lowering the absolute value of water adsorption free energy. Furthermore, even small amounts of hydronium and bicarbonate could alter the qualitative behavior of sodium and chloride density profiles. However, the strongest effects of adsorbed sodium and chloride ions were seen on free energies of hydronium and bicarbonate. Absolute values of adsorption free energies, as well as desorption free energy barriers, decreased at both surfaces for positive ionic species and increased for negative ionic species, suggesting that the primary mechanism was of an electrostatic nature. It was found that all major changes in free energy, due to adsorbed sodium chloride, were caused by interactions with the first layer of adsorbed sodium and the first layer of adsorbed chloride.