## 1 Introduction  
The corrosion is the main problem in cooling water systems. The corrosion may be metallic corrosion owing to the existence of water and/or microbial corrosion due to formation of bacterial biofilms [1]. The biofilm is regarded as a microbial community of embedded cells in a matrix of biopolymers called extracellular polymeric substances (EPS) formed on metallic and nonmetallic surfaces [2; 3; 4]. The favorable environmental conditions for formation of biofilms high doses of chloride ions and humidity [5]. The biofilms, which are responsible for the corrosion in cooling systems, are comprised of aerobic and anaerobic bacteria [6]. Due to its cost-effectiveness, the mild steel is considered as an important construction material in industry. The mild steel has low corrosion resistance compared with other metallic materials [7]. Hence, extensive work has been performed to mitigate the corrosion and microbial corrosion of mild steel using corrosion inhibitors and biocides, respectively [8]. There are many approaches for controlling the microbial corrosion including chemical, electrochemical, mechanical, and biological [9; 10]. The prevention of the formation of biofilms is dependent on destroying the microbial community, which can be achieved by controlling the environmental conditions such as pH, injected chemicals, and temperature [11].  
A variety of chemicals such as, e.g., organic and inorganic compounds, natural products, or ionic liquids, were employed as both biocides and corrosion inhibitors [12; 13; 14; 15; 16]. Ionic liquids were introducedas efficient biocides and corrosion inhibitors for many metals and alloys in different media [17; 18; 19; 20]. [PERSON] et al. [17] showed the potential of the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate as a dual function inhibitor for the bacterial growth and for the corrosion of steel in 3.5% NaCl solutionson the bacterial growth inhibition and on the corrosion inhibition of mild steel in 3.5% NaCl solutions. The antimicrobial impact of hydroxyl-imidazolium chloride ionic liquids was reported [20].  
In general, ionic liquids are comprised of large organic cations like, e.g., imidazolium, pyridinium, pyrrolinium, phosphonium, and organic or inorganic anions such as, e.g., halide, nitrate, sulfate, trifluoromethylsulfonate, bis(trifluoromethylsulfonyl) amide, and dicyanamide. The large structure of ionic liquids and the existence of heteroatoms such as N, S, O, or P, and the bulky structure of ionic liquids make them potential candidates as potential corrosion inhibitors.  
In this paper, two ionic liquids namely, 1-(2-hydroxyethyl)-3-methylimidazolium chloride ([OH-EMIm]Cl) and 1-ethyl-3-methyleimidazolium chloride ([EMIm]Cl) were evaluated as biocides to reduce the microbial growth and to control the formation of biofilms.Bacterial contamination can cause biofouling and corrosion to many infrastructures and industrial units such as, e.g., cooling water systems, underground storage tanks, and water distribution systems [21; 22; 23; 24; 25]. The influence of the employed ionic liquids on the growth of planktonic bacteria was investigated.Furthermore, the inhibition efficiency of the employed ionic liquids on the corrosion of mild steel in chloride solutions was demonstrated. Incorporation of hydroxyl group into the side chain of the imidazolium cation of the ionic liquids can influence the biocidal activity as well as the corrosion inhibition efficiency of the ionic liquid.