Black Wattle Tannin As Steel Corrosion Inhibitor

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 cm^2)	R1 (kΩ cm^2)		Q1 (F cm^2)		n
1 day	1.3×10^−3		69.2	1.85		1.48×10^−4		0.80
3 days	0.9×10^−3	Rs(R1Q1)	61.4	2.13		3.01×10^−4		0.73
7 days	1.1× 10^−3		58.6	2.30		2.24×10^−4		0.78
			Rs(Ω cm^2)	Q1 (F cm^2)	n1	R1 (kΩ cm^2)	Q2 (F cm^2)	n2	R2 (kΩ cm^2)
1 day with tannin	3.1× 10^−3		64.1	2.34×10^−6	0.64	0.369	1.84×10^−4	0.57	53.7
3 days with tannin	2.6×10^−3	Rs(Q1[R1(Q2R2)])	62.3	3.10×10^−6	0.60	0.378	1.57×10^−4	0.63	66.5
7 days with tannin	3.4×10^−3		65.6	2.68×10^−6	0.60	0.112	7.83×10^−5	0.73	6.56

𝑅_𝑠 represents the ohmic resistance between the reference and working electrodes; R₁ and 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.