## 3 Results and Discussion
### Results of Inhibition Efficiency Test of IL1 and IL2 as Corrosion Inhibitor by the EIS Method  
Percent inhibition efficiency (IE%) of IL1 and IL2 were calculated by Equation (1) as per [PERSON] et al. [36]:  
\[\text{IE}\%=\frac{\text{R}_{\text{inhibitor}}-\text{R}_{\text{blank}}}{\text {R}_{\text{blank}}}\times 100\% \tag{1}\]  
where Rinhibitor is the resistance after the addition of corrosion inhibitor, whereas Rblank is the resistance before the addition of the corrosion inhibitor. In this study, the concentration range of the new ILs used was 20-120 ppm to get the optimum concentration for corrosion inhibition in brackish water. The optimum concentration of the IL1 and IL2 precursors, which have corrosion inhibition concentration potential, was reported previously to be in the range 80-100 ppm [11]. The resistance value after the addition of the inhibitor will be higher than the resistance value without the addition of the corrosion inhibitor as reported earlier by others [9; 29]. This shows that the resistance on the electric surface will be much higher if the carbon steel surface has been coated by the corrosion inhibitor. Consequently, the semicircular diameter of the Nyquist Z curve will enlarge after the addition of corrosion inhibitors [35]. The IE% of IL2 is higher than IL1 due to the presence of three butyl chains in IL2 which increase hydrophobicity and also decrease the interaction between the steel surface and the corrosion agent as reported by [PERSON] et al. [11] and also the longer alkyl chains can increase the inhibition efficiency in 1% NaCl. Based on Figure 2, the Zr to \(-\)Zi data with the addition of 80 ppm IL1 and 60 ppm IL2 showed the effect of the decreasing corrosion rate. The increased corrosion resistance in the system was indicated by the larger diameter of the semicircular curve. However, when the IL1 concentration was increased to 100 ppm and IL2 to 120 ppm, the results showed a slight decrease in the corrosion inhibition. This validated that the optimum range for corrosion inhibition for IL1 was around 80-100 ppm and for IL2, it was around 60-100 ppm, with the best corrosion inhibition concentration at 80 ppm for IL1 and 100 ppm for IL2. The results of the corrosion inhibition performance test using the EIS method showed that IL1 and IL2 have potential as corrosion inhibitors. The percent inhibition efficiency (IE%) of each of IL was higher than 80%, which is shown in Figure 3.  
The above results showed that new ILs have potency as a corrosion inhibitor, but the IE% of each IL was still only 80%. Therefore, the application method of IL-based corrosion inhibitors was modified to increase the IE% value with direct injection method by the direct coating of ILs on carbon steel surfaces. Two drops each of IL were weighed and the weight was noted for applying the direct coating method on the carbon steel surface and the coated carbon steel coupon was dried in the open air for 1 h. The coated carbon steel was added to the system containing 1% NaCl solution. The test was carried out at various temperatures ranging from 25 \({}^{\circ}\)C to 55 \({}^{\circ}\)C with the increase of 10 \({}^{\circ}\)C. After the measurements were completed with the low temperature range, the carbon steel coupon was replaced with the newly coated carbon steel coupon and the measurement was carried out at the higher temperatures. Since the ILs are hydrophobic, all ILs were not dissolved in water during the measurement. The IE% data of the coated carbon steels are shown in Table 2.  
Figure 3: Inhibition efficiency of ILs at various concentrations at a temperature of 25 \({}^{\circ}\)C.  
Figure 2: Nyquist Z curves of (**a**) IL1 and (**b**) IL2 at 25 \({}^{\circ}\)C.  
Data in Table 2 show that the IE% of IL1 and IL2 were increased by using the direct coating method. The IE% values of IL1 and IL2 at 25 \({}^{\circ}\)C were 99.3% and 99.2%. These data showed that the role of the long alkyl chain (propyl and butyl) in the two ILs increased their hydrophobicity properties [8; 21].