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 time
Chi-square
Equivalent circuit
Fitting parameters
Rs (Ω cm2)
R1 (kΩ cm2)
Q1 (F cm2)
n
1 day
69.2
1.85
1.4810−4
0.80
3 days
Rs(R1Q1)
61.4
2.13
3.0110−4
0.73
7 days
1.1 10−3
58.6
2.30
2.2410−4
0.78
Rs(Ω cm2)
Q1 (F cm2)
n1
R1 (kΩ cm2)
Q2 (F cm2)
n2
R2 (kΩ cm2)
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
Rs(Q1[R1(Q2R2)])
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; R1 and R2 represent the resistance or charge transfer; Q1and 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 .