Table 3: Fitting parameters used to simulate the EIS plots for steel immersed for 1, 3, and 7 days in 0.1 mol L−1 Na2SO4 solution (pH 6.0) in the presence and absence of tannin.

Immersion timeChi-squareEquivalent circuitFitting parameters
Rs (Ω cm2)R1 (kΩ cm2)Q1 (F cm2)n

1 day 1 . 3 × 1 0 3 69.21.851.48 × 10−40.80
3 days 0 . 9 × 1 0 3 Rs(R1Q1)61.42.133.01 × 10−40.73
7 days1.1 × 10−358.62.302.24 × 10−40.78
Rs(Ω cm2)Q1 (F cm2)n1R1 (kΩ cm2)Q2 (F cm2)n2R2 (kΩ cm2)
1 day with tannin3.1 × 10−364.12.34 × 10−60.640.3691.84 × 10−40.5753.7
3 days with tannin2.6 × 10−3Rs(Q1[R1(Q2R2)])62.3 3.10 × 10−60.600.3781.57 × 10−40.6366.5
7 days with tannin3.4 × 10−365.62.68 × 10−60.600.1127.83 × 10−50.736.56

𝑅 𝑠 represents the ohmic resistance between the reference and working electrodes; R1 and R2 represent the resistance or charge transfer; Q1   and Q2 are the impedance related to a constant phase element (CPE) and can be attributed to electrode surface or adsorbed species; n, n1, and n2 represent a CPE exponent. Capacitor for 𝑛 = 1 , a resistor for 𝑛 = 0 , and a diffusion process for 𝑛 = 0 . 5 .