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} Na_{2}SO_{4} solution (pH 2.5) in the presence and absence of tannin.

Immersion time

Chi-square

Equivalent circuit

Fitting parameters

R_{s} (Ω cm^{2})

R_{1} (kΩ cm^{2})

Q_{1} (F cm^{2})

n

1 day with tannin

48.1

1.76

0.91

3 days with tannin

49.6

2.34

0.91

7 days with tannin

R_{s}(R_{1}Q_{1})

57.4

3.08

0.89

1 day without tannin

63.6

0.71

0.76

3 days without tannin

49.8

0.70

0.66

R_{s} (Ω cm^{2})

Q_{1} (F cm^{2})

n_{1}

R_{1} (Ω cm^{2})

Q_{2} (F cm^{2})

n_{2}

R_{1} (kΩ cm^{2})

7 days without tannin

R_{s}(R_{1}Q_{1})(R_{2}Q_{2})

47.2

0.74

112

0.8

0.68

represents the ohmic resistance between the reference and working electrodes; R_{1}and R_{2} represent the resistance or charge transfer; Q_{1} and Q_{2} are the impedance related to a constant phase element (CPE) and can be attributed to electrode surface or adsorbed species; n, n_{1}, and n_{2} represent a CPE exponent. Capacitor for , a resistor for , and a diffusion process for .