3.1.3. EIS Measurements

To better study the corrosion resistance, EIS measurements were performed for samples B1–B4. The OCP values for the samples varied around −0.62 V (±0.05 V).

Figure 7 shows the Nyquist plots (a) and Bode plots (b) for the samples exposed to SW and SW+bacteria (MIC) for 14 days. The semicircles of B3 and B4 (in SW) in the Nyquist plots are larger than those of B1 and B2 in SW+bacteria (MIC). The amplitude of the impedance for the samples (B3–B4) in SW is higher than in the SW+bacteria (B1–B2 MIC) at low frequency side (0.01 Hz). The phase peaks shifted to the low-frequency side for the samples in the SW with bacteria, which suggests that the capacitive behaviour is significant.

**Figure 7.** *Cont*.

**Figure 7.** Nyquist (**a**) and Bode (**b**) impedance plots for samples exposed to SW (B3, B4) and to SW + bacteria (B1, B2 MIC) for 14 days.

Figure 8 shows a comparison of Nyquist (a) and Bode (b) impedance plots of samples exposed to SW+bacteria (B1 MIC) and exposed to SW (B3) for different time durations. The impedance module at 0.01 Hz decreased over time in both conditions. The phase angle peak of B1 (MIC) shifted in the low-frequency direction with the increase of duration.

The impedance of the samples at a frequency of 0.01 Hz measured with different exposure durations is presented in Table 5. The electrochemical impedance value at the low-frequency side is related to the corrosion resistance of the steel. The samples exposed to the seawater with microorganisms have smaller impedance values and larger phase angle than without microorganisms.


**Table 5.** Impedance values and phase angles at 0.01 Hz.

**Figure 8.** A comparison of Nyquist (**a**) Bode (**b**) impedance plots of samples exposed to SW + bacteria (B1 (MIC)) and exposed to SW (B3) for different time durations.

The impedance module can be influenced by resistance, capacitance and even inductance in a corrosion cell. To further analyse the capacitive and resistive behaviour of the corrosion cells, the impedance data were fitted with an equivalent circuit presented in Figure 9. The capacitive elements are submitted by constant phase elements (CPE) Qc and Qdl. The impedance of a CPE can be calculated by the equation:

$$\mathbf{Z}\_{\rm CPE} = \mathbf{Y}\_0^{-1} \text{(j\omega)}^{-n} \tag{7}$$

where Y0 is the admittance constant of the CPE (in sn/Ω); ω is the angular frequency (rad/s); n is the CPE exponent, and n = α/(π/2) (α is the constant phase angle of the CPE). When n = 1, the CPE becomes a pure capacitor [27].

Rel: electrolyte resistance, Qc: constant phase element for the oxide layer, Rcp: pore resistance, Qdl: constant phase element for the double layer, Rct: charge transfer resistance. 

**Figure 9.** Equivalent circuit used for fitting the impedance data.

The fitting results are presented in Table 6. The fitting results show that the resistance attributed to the surface layer (Rcp) is very small, compared to the charge transfer resistance Rct. Thus, the polarization resistance is in the same order of the Rct. After exposure for 28 days the corrosion resistance is approaching the same level (3.5 <sup>k</sup>Ω·cm2) for the samples in SW and in SW+bacteria (MIC). The corrosion resistance was calculated using apparent surface area of the samples, since the real active corrosion area was unknown.

**Table 6.** Parameters and fitting results of the impedance data using an equivalent circuit, R in Ω·cm<sup>2</sup> and C in (F·cm<sup>−</sup>2).


The capacitance was also calculated (C = Y01/n·R(1 − n)/n, [28]), using apparent surface area. The steel samples in SW+bacteria have a larger capacitance of the double layer, than in SW. A larger capacitance results in a lower impedance module at the low-frequency side (Table 5).
