Table 6: 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 2.5) in the presence and absence of tannin.

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

1 day with
tannin
1 . 9 × 1 0 3 48.11.76 4 . 7 3 × 1 0 6 0.91
3 days with
tannin
2 . 1 × 1 0 3 49.62.34 5 . 6 8 × 1 0 6 0.91
7 days with
tannin
1 . 4 × 1 0 3 Rs(R1Q1)57.43.08 4 . 4 3 × 1 0 6 0.89
1 day without
tannin
2 . 3 × 1 0 3 63.60.71 2 . 8 0 × 1 0 5 0.76
3 days without
tannin
2 . 1 × 1 0 3 49.80.70 9 . 2 1 × 1 0 5 0.66
Rs (Ω cm2)Q1 (F cm2)n1R1 (Ω cm2)Q2 (F cm2)n2R1  (kΩ cm2)
7 days without
tannin
4 . 5 × 1 0 3 Rs(R1Q1)(R2Q2)47.2 1 . 5 3 × 1 0 5 0.74112 5 . 4 9 × 1 0 4 0.80.68

𝑅 𝑠 represents the ohmic resistance between the reference and working electrodes; R1and 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 .