## 5 Microbial Corrosion by SRB
### Microbial Corrosion in Acidic Environment
#### 5.1.2 Microbial Activities  
As addressed above about sulphate reduction activities of SRB, in acidic environment, sulphate reduction gains more available energy than at neutral pH. The lower the pH, the more energetically advantageous, the higher the sulphide produced. Biogenic sulphide ions/hydrogen sulphides are reported to be detrimental to corrosion resistance of steel as they react readily with anodic ferrous ions to cause the precipitation of iron sulphides. It has been proposed that corrosion proceeds through the depolarisation of the cathodic area by adsorption of the polarising H2 into the crystal lattice of the iron sulphide species which causes sulphide stress cracking (SSC), or the establishment of an aggressive galvanic cell of iron/iron sulphide [104], both of which result in the acceleration of corrosion. Consequently, microbial corrosion caused by SRB was higher at low pH environments. Additionally, higher reduction rate of sulphate requires higher oxidation of carbon sources and metal which are the main source of electron donor [64]. As and when bacteria attach to a material surface, the matrix of formation of biofilm on metals and the precipitation of metal sulphide/metal carbonate can prevent the diffusion of carbon source like lactate source from environment [64]. This could consequently increase the dissolution rate ofmetals. Overall, corrosion of metals was accelerated at low pH, which was proved by laboratory experiment in both carbon steels and duplex stainless steels. On the other hand, the layer of the biofilm and metal sulphide/carbonate precipitation can be advantageous for corrosion resistance of material under acidic environments as they prevent the diffusion of proton and chloride from environment [105]. Indeed, the protection ability of biofilm as a barrier for metal has been reported in several types of bacteria in extreme pH environments [106; 107; 108]. [PERSON] et al. [109] addressed that aerobic _Pseudomonas cichorii_ was able to inhibit the corrosion of mild steel in corrosive phosphate-buffered basal salt solution (BSS). Analysis of surface film using Fourier transform infrared (FTIR) spectroscopy revealed that formation of an iron oxide/iron phosphate layer within biofilm matrix may contribute to the corrosion reduction. [PERSON] et al. [106] reported that a protective biofilm of _Bacillus subtilis WB600_ formed on aluminium alloy Al 2024 as the alloy was passive in the presence of bacteria and was prone to corrosion without the bacteria. [PERSON] and [PERSON] [4] studied the corrosion behaviour of Al 2024, cartridge brass, and mild steel in AS containing _Shewanella ana_ and also revealed the protective behaviour of the biofilm [107]. The experimental results in pH 4 environment from ref. [17] are interesting. The growth of bacteria was arrested for the first 5 days and resumed when the bulk pH increases to around 4.6. Corrosion rate was found to slow down when the bacteria growth due to the presence of biofilm along with the precipitation of corrosion products.This acted as a barrier to prevent the diffusion of proton and chloride from environment. Overall, in a very extreme corrosive environment such as low pH with presence of chloride, the layer where minerals and corrosion products are entrapped in the copolymer matrix of biofilm can be advantageous for resistance of materials. Additionally, the presence of organic compounds in the environment at low pH can also impact corrosion resistance of materials [110; 111]. Acetate is the final metabolic products in the oxidation of lactate/pyruvate caused by SRB. It was reported that acetate can cause higher corrosion in materials than formate, pyruvate and lactate in the environment without the presence of SRB [111]. Additionally, authors reported that in _D. desulfuricans_ containing environment, high amount of acetate was produced by bacteria metabolism and hence, resulted in high corrosion rate of iron coupons.