Corrosion of mild steel in low conductive media simulating natural waters

Abstract

The corrosion of a mild steel was examined in two aerated neutral aqueous solutions, defined as reference solution (0.2 g L−1NaCl) and as Σ-solution (1.3 g L−1NaCl + 0.63 g L−1NaHCO3 + 0.27 g L−1Na2SO4). Their composition was chosen on the basis of the physical and chemical properties of certain natural waters. The solutions simulated the least (reference solution) and the most (Σ-solution) aggressive waters of the Sebou river in Morocco, as determined after a four-year examination (1991–94), at 13 pump stations located along the river. Various experimental methods were used to determine the corrosion mechanism. Cathodic range voltammetry using a rotating disc allowed the kinetics of oxygen reduction process to be determined. Since the conductivity of the solutions were low, the potential was corrected for ohmic drop estimated through the high frequency limit in the Nyquist diagrams (electrochemical impedance spectroscopy) as well as the current interrupter method. After correction, the polarization curves revealed a diffusion plateau attributed to dissolved oxygen reduction. At the plateau, a two-step mechanism was derived involving oxygen diffusion through the hydrodynamic layer and through a porous inner layer formed by the corrosion products. This inner layer could not be observed by SEM, but both EIS and EHD (electrohydrodynamic impedance) confirmed the presence of a thin porous dielectric layer. At the open circuit potential, the corrosion rate was determined by the diffusion rate of dissolved oxygen in the Σ-solution, and by charge transfer in the reference solution. This shows that the corrosion mechanism strongly depends on the electrolyte and its conductivity.