In this work, it has been found that SRB could survive in the CO2-saturated simulated shale gas produced water at 60 °C (Fig. 2), and then leading to the corrosion acceleration of steel (Fig. 1). Temperature is a very important factor both influencing the biological activity of SRB as well as the corrosion process of steel [33, 34]. Most bacteria have a better growth when the temperature is about 30°C, and most bacteria will die when the temperature is over 60 °C. But some special bacteria can also grow well with a growth temperature of over 60 °C [35, 36]. The high temperature usually can destroy the biological activity of enzymes as well as the cell structure, then causing the death of bacteria. However, SRB used in this work can adapt to the high temperature and maintain a good biological activity. So, the used SRB has a good adaptive ability to the extreme environments, creating a good corrosion environment.
The anodic corrosion process is the dissolution of Fe, i.e., reaction (2). However, the cathodic reactions will be complex due to the presence of SRB as well as CO2. For SRB corrosion, the cathodic reaction is the reduction of sulfate[37, 38], as shown in reaction (3). And the cathodic reactions of CO2 corrosion include reactions (4) to (5) because the pH value of the test solution is 5.38. And the FeCO3 corrosion product will change to FeS due to the difference of their solubility, i.e., Reaction (7).
Fe ® Fe2+ + 2e− (2)
SO2− 4 + 9H+ + 8e−® HS− + 4H2O (3)
2H2CO3+ 2e−® HCO- 3 + H2 (4)
2HCO- 3 + 2e−® 2CO2- 3 + H2 (5)
FeCO3+ HS-® FeS + HCO- 3 (7)
For the abiotic control specimen, the low corrosion rate can be due to the addition of some organics, such as biocide, which plays the role of corrosion inhibitor. Corrosion inhibitor is an important corrosion protection method applied to the oil and gas field industry[39, 40]. But the corrosion behavior of steel as well as the adsorption behavior of inhibitive organics has altered after the introduction of SRB in the test solution. First, SRB cells and the inhibitive organics can have competitive adsorption on steel surfaces. The adsorption of SRB cells causes the formation of biofilm as well as the corrosion acceleration, while the adsorption of inhibitive organics will lead to the decline of steel corrosion rate. According to the weight loss (Fig. 1) and SEM images of biofilm (Fig. 8) as well as the surface morphologies without corrosion products, the adsorption of SRB cells has dominated the main position, and the adsorption of inhibitive organics is poor. Second, once SRB cells can adsorb on the steel surface well, the biofilm will be formed fast due to the good growth of SRB. SRB biofilm as a catalyst accelerating steel corrosion. CO2 is also an important factor influencing SRB corrosion. And the presence of CO2 can assist SRB to promote steel corrosion. So, the corrosion of steel in the biotic condition is more severe than the control. In addition, the temperature will also affect SRB corrosion behavior. And the increase of temperature usually can promote both the anodic and cathodic reactions. So, steel corrosion will be more severe at 60 °C, a high temperature.