**3. Results**

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## *3.1. Electrochemical Measurements*

#### 3.1.1. PDP Curve Measurements

Figure 2 shows polarization curves in semilogarithmic plots for both polished samples P1 and P2. Small (Tafel) slopes in the anodic polarization parts and large slopes in the cathodic polarization parts are observed, approximately 50 mV more active than the corrosion potentials. This means that the corrosion is controlled by the cathodic reactions for the steel in the seawater.

The anodic reaction is iron dissolution [25],

$$\text{Fe} \Rightarrow \text{Fe}^{2+} + 2\text{e}^- \tag{1}$$

The cathodic reactions include here mainly the reduction of dissolved oxygen,

$$\text{O}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^- \Rightarrow 4\text{OH}^- \tag{2}$$

and, possibly, hydrogen evolution in anaerobic environments, e.g., under depsoits and biofilms,

$$2\text{H}^+ + 2\text{e}^- \Rightarrow \text{H}\_2\tag{3}$$

$$2\text{H}\_2\text{O} + 2\text{e}^- \Rightarrow \text{H}\_2 + 2\text{OH}^- \tag{4}$$

Corrosion current density icorr can be calculated from the Tafel slopes in the polarization curves using the Stern–Geary equation [26],

$$\mathbf{i\_{corr}} = \mathbf{b\_a} \times \mathbf{b\_c} \;/\; \text{(2.3-Rp} \cdot (\mathbf{b\_a} + \mathbf{b\_c})\tag{5}$$

where ba and bc are anodic and cathodic slopes, respectively, in the polarization curves; Rp is the polarization resistance.

The general corrosion rates (CR) can be calculated from the corrosion current densities using the following equation,

$$\text{CR} = 326\mathcal{T} \cdot (\text{i}\_{\text{corr}} \cdot \text{M}\_{\text{eq}}) / \text{p (mm/y)} \tag{6}$$

where icorr is current density (A/cm2), Meq equivalent mass (g), and ρ density of the materials (g/cm3).

The corrosion potentials and corrosion current densities are presented in Table 4. The average corrosion rate is about 0.3 mm/y. The corresponding corrosion resistance is about 1030 Ω·cm<sup>2</sup> (as a reference for further comparison with LPR results).

**Table 4.** Corrosion potentials and corrosion rates for the polished samples in seawater.


#### 3.1.2. LPR Measurements

The OCP values as a function of time for the steel in the different electrolytes are shown in Figure 3. The OCP values for the samples in the seawater without bacteria are around −0.6 V. With bacteria, the OCP values moved from −0.67 V to −0.6 V within 7 days. This suggests that the corrosion systems are unstable in the initial period (within 1 week).

**Figure 3.** Open circuit potential (OCP) values as a function of time measured for R4 steel samples in seawater with and without bacteria.

A typical linear polarization (LP) curve for a sample (S5) in SW exposed for 14 days is shown in Figure 4. The LPR (Rp = ΔE/ΔI) was calculated from the line in ±5 mV near the corrosion potential (zero current).

A typical LP curve for a sample (S10) in the SW + bacteria is shown in Figure 5. The linear range of the LP curve near the corrosion potential (−0.61 V) is much narrow compared to that of the S5 in SW near the corrosion potential (zero current). In the presence of bacteria, biofilms formed at the sample surfaces, which exhibit capacitive behaviour. In this case, the corrosion resistance was estimated in the anodic part.

**Figure 4.** A typical linear polarization curve for a sample (S5) in seawater (SW), exposed for 14 days.

**Figure 5.** A typical polarization curve for a sample (S10) in SW+bacteria (MIC), exposed for 14 days.

Figure 6 shows the polarization resistance after various exposure times measured for R4 steel samples in SW and in SW+bacteria (MIC). The polarization resistance for the samples in the seawater without bacteria (S5-6 SW) increased from 2.4 kΩ·cm<sup>2</sup> to 3.6 kΩ·cm2, while for the samples in the seawater with bacteria (MIC) it decreased from 10 kΩ·cm<sup>2</sup>

to 1 kΩ·cm<sup>2</sup> in 21 days. This indicates that the sample surfaces exposed to seawater with bacteria became more active in 3 weeks, compared to those exposed to seawater only. Originally the designed LPR test duration was for 21 days. Samples S12 and S13 were added and extended the test duration to 28 days to see if their LPR would further decrease in the SW + bacteria. (No sample was added to SW for 28 days, since the LPR in SW did not change much within 21 days).
