## 3 Results
### Electrochemical Testing  
The OCP value of the working electrode in different pH environments declined rapidly in the first 8 days due to the formation of biofilm on the surface of the working electrode making the electron transfer between the working electrode surface and solution more difficult (Figure 5). The OCP value then increased slightly in all pH experiments which indicates the beginning of corrosion of the samples. The EIS spectra of working electrodes immersed in different pH environments (7-10) after different exposure times is presented in Figure 6. The Nyquist plots reveal a capacitive arc representing the resistance of the film formed on the electrode surface. The film can be a passive film, biofilm or even corrosion product layer. The higher the radius of the capacitive arc, the higher the resistance of the electrode to corrosion. Different equivalent electric circuit (EEC) models have been proposed for interpreting impedance spectra of materials in microbial environment. The model \(\mathrm{R_{s}}[\mathrm{Q_{CPE}}[\mathrm{R_{b}}[\mathrm{C_{\mathit{d}I}} \mathrm{R_{\mathit{c}I}}]]]\) has been used for studying microbial corrosion [18; 46]. Passive film along with biofilm formation on the surface of materials can act as a double layer capacitance [18; 46]. In this research, this model was used to fit the experimental data as it can present the double layer capacitance. \(\mathrm{R_{s}}\) represents  
Figure 4: CISM images of bacterial attachment (2D) and biofilm formation (3D) after 2 days immersion of coupons at pH 7 (**a,e**), (**b**) pH 8, (**b,f**) pH 9, (**c,g**) pH 10 (**d,h**). Figure 5: OCP curves of DSS 2205 in pH 7, 8, 9 and 10 environments over time for samples at different starting pH. the resistance of solution, \(\rm R_{b}\) is the resistance of biofilm/passive film formed on material surfaces, \(\rm R_{ct}\) is the charge transfer resistance, \(C_{dl}\) is the capacitance of electrical double layer. A constant phase element (CPE) was introduced to the model as it presents a deviation from a true capacitive behaviour. It is interesting to note that after stabilization at open circuit potential for 2 h, the working electrode in pH 10 had the highest impedance as it has the highest radius of capacitive arc even though it had the lowest biofilm thickness on the surface. The overall impedance of working electrodes increases with immersion time (0-14 days) and this reveals the thickening of protective film on the surface of the electrodes. This can be due to the biofilm on the surface acting as a barrier to protect the surface of the material. However, after 14 days, the impedance starts to decrease, especially at pH 9 which reveals that the protective ability was lost. This indicates the corrosion started occurring after 14 days exposure, and this is in good agreement with OCP variation (Figure 5). After 28 days exposure, the impedance of the working electrodes at different pH values changed dramatically compared to the first 2 days of exposure. The working electrode immersed in pH 9 solution had the lowest impedance after 28 days immersion indicating the thinnest protective film on the surface and therefore more susceptible to pitting. This can be seen in the OCP value after 28 days immersion (Figure 5) when the working electrode in pH 9 environment had the highest OCP value. The total resistance (\(\rm R_{tot}\)) was defined by the total resistance of biofilm/passive film layer formed on working electrodes surfaces and charge transfer resistance (\(\rm R_{tot}=R_{b}+R_{ct}\)), where \(\rm R_{b}\) and \(\rm R_{ct}\) were parameters obtained from EIS fitting procedure. The total resistance of all electrodes increased rapidly, reaching a peak after 14 days at pH 7, 8 and 9, due to the formation of biofilm on the surface (Figure 7). The working electrode impedance at pH 10 reached a maximum after 18 days of immersion. This is longer than the time taken to reach the maximum impedance in the lower pH environments.