## 5 Microbial Corrosion by SRB
### Microbial Corrosion in Alkaline Environment
#### 5.2.2 Microbial Activities  
As addressed in Section 4.2, sulphate reduction gains an energetic advantage at high pH, thus the reduction of sulphate is favourable and requires high electron donor consequently. The precipitation of metal sulphide/metal carbonate as corrosion products can act as a barrier to prevent the diffusion of carbon sources from environment to bacteria. This results in high oxidation of metals which is similar to acidic conditions in that bacteria might be under carbon starvation by the barrier of corrosion products, hence accelerates corrosion process of materials.  
The production of biogenic sulphide is mainly sulphide ions in alkaline environment with the dissolution of hydrogen sulphide. Sulphide ion was reported to account for accelerating corrosion as sulphide is aggressive to passive film on stainless steel [122; 123; 124]. Sulphide causes the susceptibility of materials to corrosion by various ways, including cathodic depolarisation [125] and changes to the local pH upon hydrolysis reactions of dissolved metal ions and initiates pitting [126], supporting active dissolution of steel [127], and increasing electron uptake through metal sulphides [128]. The increase in sulphide ion concentrations led to the increase in corrosion rate of materials. An example is in ref. [129], which was included in Table 1. The corrosion rate of carbon steel increased by 86.7% when environmental pH increases from 7.5 to 9.5. Duplex stainless steel was also reported to accelerate the corrosion rate from 1.9 (\(\mu\)m year\({}^{-1}\)) at pH 7.5 to 10.1 (\(\mu\)m year\({}^{-1}\)) at pH 9 [18]. As mentioned above, the enhancement of protective film of materials leads to the increase of corrosion resistance of materials as environmental pH increases. However, in the SRB-containing environment, corrosion of materials increases with the increase of pH due to the high production of biogenic sulphide ions.  
The oxidation of carbon sources as electron donor for sulphate reduction produces carbonate under alkaline conditions as addressed in Section 4.2. Carbonate has been reported to cause scale in equipment such as heat exchanger, wellbore tubulars and components and transportation pipe lines [130; 131]. The scale of carbonate precipitation such as iron carbonate was reported to act as a protective layer to prevent corrosion of materials [132; 133; 134]. Previous literature reported the application of calcium carbonate on corrosion prevention of materials [132; 135]. However, the concentration of calcium should be taken into account as high concentrations of Ca\({}^{2+}\) could lead to the change of corrosion products from protective iron carbonate to non-protective calcium carbonate, and increase the corrosion rate [136].  
On the other hand, microbial corrosion of materials in alkaline environment also depends on the growth of bacteria. _D. vulgaris_ was reported to have an arrested growth at pH 10 solution [18]. Sulphide production was very low for the first 6 days until the environmental pH decreased and further bacteria growth was achieved. Thus, for the first few days, materials were protected from corrosion from the formation of protective layer and later the corrosion was accelerated by SRB metabolism.  
Overall, microbial corrosion of materials was attributed by different competitive factors. Although corrosion prevention of materials from the protective hydroxide film and the formation of corrosion products layer can act as protective layer to prevent corrosion process, SRB metabolism and high sulphide concentration still cause high corrosion rate of materials. However, corrosion caused by SRB in pH extreme environment of non-ferrous materials, e.g., amphoteric materials such as aluminium, zinc and anti-bacteria such as copper have received less attention and require further studies.