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} Na_{2}SO_{4} solution (pH 6.0) 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

69.2

1.85

1.4810^{−4}

0.80

3 days

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

61.4

2.13

3.0110^{−4}

0.73

7 days

1.1 10^{−3}

58.6

2.30

2.2410^{−4}

0.78

R_{s}(Ω cm^{2})

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

n_{1}

R_{1} (kΩ cm^{2})

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

n_{2}

R_{2} (kΩ cm^{2})

1 day with tannin

3.1 10^{−3}

64.1

2.3410^{−6}

0.64

0.369

1.8410^{−4}

0.57

53.7

3 days with tannin

2.610^{−3}

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

62.3

3.1010^{−6}

0.60

0.378

1.5710^{−4}

0.63

66.5

7 days with tannin

3.410^{−3}

65.6

2.6810^{−6}

0.60

0.112

7.8310^{−5}

0.73

6.56

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 .