Equivalent electric circuits proposed to simulate the experimental data of steel immersed in 0.1 mol L⁻¹ Na₂SO₄ solution (pH 2.5) for (a) 1, 3, and 7 days in the presence of tannin, 1 and 3 days in the absence of tannin; for (b) 7 days in the absence of tannin.