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 time
Chi-square
Equivalent circuit
Fitting parameters
Rs (Ω cm2)
R1 (kΩ cm2)
Q1 (F cm2)
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
Rs(R1Q1)
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
Rs (Ω cm2)
Q1 (F cm2)
n1
R1 (Ω cm2)
Q2 (F cm2)
n2
R1 (kΩ cm2)
7 days without tannin
Rs(R1Q1)(R2Q2)
47.2
0.74
112
0.8
0.68
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 , a resistor for , and a diffusion process for .