Role of a clay sediment deposit on the passivity of carbon steel in 0.1 mol dm−3 NaHCO3 solutions

The role of clays in the corrosion processes of carbon steel was investigated. Four different minerals, silica, kaolinite, chlorite and montmorillonite, were considered. They were deposited by sedimentation on carbon steel electrodes. The electrochemical behaviour of steel electrodes covered with mineral deposits was studied in 0.1 mol L−1 NaHCO3 solutions by cyclic voltammetry (4 cycles between −0.8 V/SCE and −0.1 V/SCE) and compared to that of a bare electrode. Potentiostatic experiments were performed at −0.62 V/SCE, close to the corrosion potential, and the interfacial processes were investigated by electrochemical impedance spectroscopy. Analysis of the corrosion products accumulated at the inner side of the deposited mineral layer was performed by μ-Raman spectroscopy. As it could be expected, the deposits hindered the transport of dissolved oxygen and Fe(II) species, inducing changes in the interfacial solution and modification of the composition of the rust layer. Moreover, specific interactions between minerals and dissolved Fe species also influenced the corrosion processes.