## 3 Results
### SEM Images of COUPONS immersed in Different pH Environments  
The SEM images of coupons immersed in different pH environments after biofilm removal are presented in Figure 11. Pits were evident as dark circular marks on the surface of the coupons due to corrosion. The samples before the corrosion experiments were found to have no pits or voids (no photos available). Visual inspection revealed that the number and size of pits were greater in samples immersed in pH 9 than the others, indicating a higher corrosion rate in the pH 9 environment. This is in good agreement with both potentiodynamic polarization results (higher current density of coupons at pH 9) and ICPMS results (higher dissolved metal concentrations at pH 9). Element concentrations of the coupons are shown in Table 3. Sulphur was detected on the surface of the coupons in the matrix of corrosion products and biofilm and was highest in coupons immersed at pH 9. Sulphur generally is known to form metal sulphide as a corrosion product [15; 33; 49].  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\mathbf{I_{corr}}\) **(mA-cm\({}^{-2}\))** & \(\mathbf{E_{corr}}\) **(mV/Ag/AgCl)** \\ \hline pH 7 & 0.166 & \(-\)707 \\ pH 8 & 0.228 & \(-\)685 \\ pH 9 & 0.867 & \(-\)542 \\ pH 10 & 0.282 & \(-\)641 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Corrosion density and potential of working electrode in different pH environments.  
Figure 9: Potentialodynamic polarization curve of working electrode in different pH environments.  
Figure 11: Cont.  
Figure 10: Concentrations of chromium, manganese, iron, nickel and molybdenum in the medium following immersion of coupons for 28 days in different pH environments.  
microniches to protect micro-organisms from harsh environments [53; 54; 55]. It has been shown that pH as well as other gradients exist in a biofilm [50; 51]. The presence of SRB in high alkaline environments can also be explained by the adaptation of bacteria to their environment [52]. The adaptive mechanisms can include: (i) increased proton capture and retention by elevating transporter (monovalent cation/proton antiporters) and enzyme (e.g., ATP synthase) activities, (ii) increased metabolic acid production through metabolic changes, and (iii) changes in cell surface properties [56; 57; 58].  
Bacteria generally modify their surrounding pH through metabolic activities [56]. SRB metabolism which results in the production of hydrogen sulphide and can decrease the concentration of OH\({}^{-}\) which lowers the pH of the environment. In this research, SRB at pH 10 were active but only after an extended lag phase. Speciation of H\({}_{2}\)S in the aqueous phase is governed by the reversible ionization reaction shown below [31]:  
\[H_{2}S_{aq}+OH^{-}\to HS_{aq}^{-}+H_{2}O \tag{3}\]  
\[H_{2}S_{aq}+2 OH^{-}\to S_{aq}^{2-}+2H_{2}O \tag{4}\]  
Another product carbon dioxide (CO\({}_{2}\)) was produced by the oxidation of lactate from environment (Equation (7)) could combine with OH\({}^{-}\) to form bicarbonate (HCO\({}_{3}\)\({}^{-}\)) and carbonate (CO\({}_{3}\)\({}^{2-}\)), thus decreased the environmental pH [1; 6]. It has been shown that the dissolution of hydrogen sulphide was pH dependent [31]. The data provided by U.S. Center for Environmental Research Information shows that at pH 7, roughly 50% of hydrogen sulphide is dissolved in solution, while at pH 9, more than 99% of hydrogen sulphide is dissolved in solution [59]. Thus, at higher pH, the dissolution rate of hydrogen sulphide is higher [31; 59] and this was supported in this study which showed higher concentrations of dissolved sulphide at pH 9 and 10 than at pH 7 and 8.  
At pH 10, there was less biofilm formation on the material surface compared to in other pH environments, due to the significant lag phase, (Figures 4 and 8). At pH 7, 8 and 9, the extent of bacterial adhesion was nearly the same as each other and the biofilm thickness was around 75 um (Figure 4).