Electrochemical Behavior of X70 Steel Corrosion under Thin Electrolyte Layer of Na₂SO₄

The corrosion behavior of X70 steel under thin electrolyte layer was studied in 0.20 mol/L Na2SO4 by cathodic polarization and electrochemical impedance spectroscopy. It was found that the cathodic diffusion limiting currents increased with the decrease of the layer thickness. The critical thickness for maximum cathodic diffusion limiting current was about 50 μm. In the range of 40 μm to 50 μm, the cathodic diffusion limiting current slightly decreased. The further decrease in the electrolyte layer thickness increased the cathodic diffusion limiting current to some extent again. Under thin electrolyte layer, the corrosion process of X70 steel changed with electrolyte layer thinning. When the thickness of electrolyte layer was less than 45 μm, the control step for corrosion of X70 steel consisted of charge transfer and diffusion. The corrosion rate of the steel increased firstly and then decreased with the decrease of the electrolyte layer thickness. The corrosion rate under 100 μm layer first increased, then decreased, and then increased again. The corrosion rate of 200 μm layer kept steady; and the corrosion rate under 400 μm layer and in the bulk solution first decreased, and then increased.