## 3 Results
### Corrosion Mechanism  
The proposed overall mechanism of _D. vulgaris_ induced corrosion is shown in Figure 12. _D. vulgaris_ reduce sulphate ion from the environment by enzyme through a serious of reactions [64; 65; 66]. At first, the enzyme ATP-sulfurylase also known as adenylylsulphate pyrophosphorylase or sulphate adenylyltransferase, which uses ATP and sulphate to create adenosine 5\({}^{\prime}\)-phosphosulfate (APS). APS is subsequently reduced to sulphute and release adenosine monophosphate (AMP). Sulphite is then further reduced to sulphide, while AMP is turned into adenosine diphosphate (ADP) using another molecule of ATP. The overall process, thus, involves an investment of two molecules of the energy carrier ATP, which must to be regained from the reduction [67]. The source of electron comes from anodic reactions including iron ion and oxidation reaction of lactate from environment. Thus, the redox reactions are as following [68; 69]:  
Anodic:  
\[4 Fe\to 4 Fe^{2+}+8e^{-}\,E_{o}=-447\;\mathrm{mV} \tag{5}\]  
\[CH_{3}CHOHCOO^{-}+H_{2}O\to CH_{3}COO^{-}+CO_{2}+4H^{+}+4e^{-}\,E_{o}=-430\; \mathrm{mV} \tag{6}\]  
Cathodic:  
\[SO_{4}^{2-}+9H^{+}+8e^{-}\to HS^{-}+4H_{2}O\,E_{o}=-217\;\mathrm{mV} \tag{7}\]  
Corrosion products formation:  
\[Fe^{2+}+S^{2-}\to FeS \tag{8}\]  
\[3 Fe^{2+}+6 OH^{-}\to 3 Fe(OH)_{2} \tag{9}\]  
\[Fe^{2+}+CO_{2}+OH^{-}\to FeCO_{3}+H^{+} \tag{10}\]  
The cathodic reaction was the consumption of electron by bacteria for their metabolism. The half cells potentials were calculated under standard condition of 25 \({}^{\circ}\)C, pH 7, 1 M concentration for solutes and 1 bar partial pressure for gases. Thus, the Gibbs energy between Equations (5) and (7), Equations (6)and (7) were \(-\)177.56 kJ. mol\({}^{-1}\) and \(-\)164.44 kJ. mol\({}^{-1}\). The negative Gibbs energy suggests that the two redox reactions are highly favourable and corrosion of stainless steel can occur. The change in environmental pH is basically caused by the reaction of bacterial metabolites with hydroxide ion OH\({}^{-}\) from the environment, thus a reduction in the concentration of hydroxide ions. Another reason that is due to the precipitation reaction of hydroxide ion with cation released from materials which also caused in decrease the concentration of hydroxide ion. The biogenic sulphide  
Figure 12: Schematic of changing pH mechanism and corrosion mechanism caused by SRB breakdown by two main steps including adhesion (**a**), bacterial metabolism (**b**) and corrosion process (**c**); bacterial metabolism including specific proteins in the outer membrane (OM), periplasm (PP) and cytoplasmic membrane (CM) allow electron go through for sulphate reduction process in the cytoplasm (CP); corrosion process including the production of hydrogen sulphide (H\({}_{2}\)S) produced by bacterial metabolism which consumes electron from iron and lactate (nutrient) and the combination of iron ion with sulphide and hydroxide to form corrosion products (FeS/Fe(OH)\({}_{2}\)). produced by bacteria metabolism caused the dissolution of the materials along with hydroxide from the environment, thus resulted in precipitation of metal sulphide and metal hydroxide as corrosion products. One of the important consequences of these electrochemical reactions is that the hydroxide ions are now part of the corrosion products and this results in lowering the pH by increasing the H\({}^{+}\) ions. This is clearly seen in this research as all the pH of the solutions reach a value of about 7.5. It appears that the pH regulation by bacteria is a common affair although more studies are needed in this area. [PERSON] et al. [70] found that _Vibrio tapetis_ also modified the pH of various laboratory media (6-8.7) to an average value of 7.71. In abiotic environments, higher pH results in a lower corrosion rate compared to at lower pH due to the protection from more stable passivity [10; 71; 72].