Bode plots for steel immersed in 0.1 mol L⁻¹ Na₂SO₄ solution (pH 6.0) for 1 day (closed triangle), 3 days (closed circle), and 7 days (closed star) in the presence of tannin; for 1 day (open square), 3 days (open lozenge), and 7 days (open circle) in the absence of tannin.