## 3 Results and Discussion
### Corrosive Potential of Microbial Communities  
Before conducting immersion corrosion tests of metal coupons in produced waters, the initial composition of the water samples was studied. Two samples of produced water from wells near Leningorsorsk (PW_L1 and PW_L2) were characterized by substantially lower salinity compared to samples obtained from wells located in the Nurlat region. The pH values of all samples were in the range of 7.3-7.5, and the total dry matter content of water samples was comparable and ranged from 2.4% to 2.5%. Sample PW_L1 contained: Na\({}^{+}\)--8.65 \(\pm\) 0  
Characterizing the microbial communities associated with oilfield waters is fundamental to understanding, predicting, and preventing MIC, as well as improving the sustainability of the oil industry. Therefore, a separate task in this work was to assess the degree of corrosion of steel coupons by microbial communities in the produced waters collected from wells near Leningorsk (PW_L1 and PW_L2). The corrosion rate based on the weight loss of steel coupons after 45 days of immersion in the production water is shown in Figure 5.  
A study of mass loss in systems with studied produced water samples showed that metal coupons in systems with the additional provision of electron donors and growth factors (sodium lactate and yeast extract) are subject to stronger corrosion effects compared to systems with their absence. Both samples with additional nutrients (PW_L1_stimul and PW_L2_stimul) showed higher corrosion rates (from 0.101 to 0.225 mm year\({}^{-1}\)). In unstimulated systems, the corrosion rate varied from 0.0098 to 0.010 mm year\({}^{-1}\), which is considered low corrosiveness. Probably, the maintenance of microbial activity as well as the accumulation of their metabolic products contributed to the accelerated biodegradation of coupons in systems with additives of organic compounds. The destruction of metal surfaces by bacteria occurs due to the initial formation and subsequent adhesion of biofilms, which, because of physical and chemical reactions, modify this interface [48]. SEM images of coupons of selected points with signs of corrosion are presented in Figure 6. These images of coupons immersed in both produced waters indicate localized corrosion, probably in the form of pitting. However, images of steel immersed in the produced waters with the addition of nutrients showed stronger localized corrosion compared to experiments without them. After microbial consortia formed biofilms on the metal surface, they began to produce high levels of sulfide and other compounds that potentially stimulated the corrosion process. Within the various microorganisms associated with MIC, sulfate-reducing bacteria have a significant effect on the corrosion of metals and usually contribute to pitting corrosion. This is a more serious type of corrosion than uniform corrosion because of the rapid penetration into the material structure and its further deep destruction [48]. Figure 6 also proves that the corrosion products of coupons submerged in PW_L1 were very different from those submerged in PW_L2, which can be explained by the different chemical composition of the waters and their microbial diversity. Thus, SEM-EDS analysis of the corrosion products formed on the surface of the steel coupons after 45 days was conducted. The EDS analysis of the original coupon showed the presence of the chemical elements: Fe, C, Mn, and Si. The EDS analysis of the coupons immersed in PW_L1 (without stimulation) showed that the main corrosion products included the following elements:  
Figure 5: Corrosion rate based on weight loss of steel coupons. Experiments were performed in the absence (unstimul) and presence (stimul) of additional nutrients. Means that do not share a letter are significantly different (ANOVA, Tukey method, \(\alpha\) = 0.05). Fe, O, S, C, Mn, Ca, and Si. The EDS analysis of the coupons immersed in PW_L1 (with stimulation) showed that the main corrosion products consisted of the elements: Fe, O, C, S, Mn, Ca, Si, Cl, and Al.