## 3 Results
### Effect of pH on Microbial Corrosion Behaviour  
This study found that pH affected SRB metabolism, which in turn affected the microbial corrosion of the material at different pH. For the first two days' exposure, as can be seen from Nyquist plots (Figure 6a,b), the impedance of the working electrode in pH 10 environment was the highest, even though it had the lowest bacteria attachment as presented in Figure 4 and in the change in double layer capacitance (Figure 8). This can be explained as follows:  
* For the first 2 h of exposure, the increase in biofilm thickness was too low to act as a barrier to prevent diffusion of metal ions to the environment. Therefore, the impedance of materials depended on their passive film. It has been shown in previous literature that passive film has semiconducting properties [60; 61; 62]. The films are characterised by the presence of electrical barriers developed at the film-electrolyte interface and at the junction [61]. The donors or acceptors in semiconducting passive layers are defects, including cationic and anionic vacancies or cationic interstitials. These vacancies act as the dopants. For instance, cation interstitials imparting n-type character and cation vacancies yielding p-type character of semiconducting properties of passive film. Thus, the presence of these dopants prevent the diffusion of ion from metal substrate and the penetration of damaging anion such as Cl\({}^{-}\) from electrolyte [61; 62]. As the pH of the environment increases, the density of dopants increases, thereby increasing the corrosion resistance [10], represented by higher radius impedance of working electrode in pH 10 solution than others (Figure 6a).
* There was a gradually enrichment of chromium content on the passive film layer as pH increased. Thus, the space charge layer increases with the chromium content [10] which results in higher pitting resistance.  
The impedance of the electrodes continued increasing during 14 days of immersion as the biofilm grew presented as increasing impedance radius (Figure 7) and total resistance (Figure 8). The electrodes impedance decreased after 14 days exposure indicating that corrosion of the material started around this time.  
Hydrogen sulphide is one of the strongest promoters of hydrogen entry into the steel surface [15; 63] where the passive film can be destroyed. The formation of biogenic metal sulphide film as a reaction of metal ion and sulphide can temporarily lead to the protection of iron against corrosion [6]. Table 3 shows the presence of sulphide in the matrix of corrosion products and biofilm formed on coupon surfaces. This film acts as a barrier by impeding the diffusion of metal ion to environment. This can be seen in Figure 7 where the total resistance of all working electrodes in different pH environments lowered after 18 days exposure as the rate of resistance drop decreased. At higher pH, the dissolution rate of H\({}_{2}\)S is higher than at lower pH environment [31], and this resulted in higher dissolved biogenic sulphide which damaged to passive film of materials. This can be seen in Figure 2 where dissolved sulphide at pH 9 and 10 environments were higher than at pH 7 and 8 environments. Sulphides strongly depassivate stainless steels favouring development of pitting on the surface. Thereby, corrosion of materials was higher at pH 9 environment than at lower pH environments. From Figure 6c-g, it can be clearly seen that the radius of impedance of working electrode in pH 9 environment decreased with time exposure which indicates a reduction in corrosion protection. The working electrode at pH 10 environment also exhibited this characteristic. In this case, after the lag phase, the sulphide production sharply increased leading to the loss in corrosion resistance of the electrode. After 28 days of immersion, the impedance of the working electrode in pH 10 environment was lower than in pH 7 and 8.