# Crude Glycerol as an Innovative Corrosion Inhibitor
## 3 Results and Discussion
### Crude Glycerol Inhibition Ability  
Crude glycerol was used as received without any further treatment and mixed with pre-specific amounts of 0.5 M HCl solutions. Different concentrations of glycerol/acid solution were prepared: 0.1%, 0.3%, 0.5%, 0.8%, and 1% by weight. The corrosion inhibition efficiency and the inhibition surface coverage were determined. Figure 2 illustrates the inhibition efficiency (IE %) as a function of immersion time for all inhibitor concentrations. At any given time, the inhibition efficiency increased with the concentration of inhibitor added to the mix. This can be attributed to the formation of a protective glycerol layer on the surface of the steel specimens. For all concentrations, it was observed that maximum inhibition efficiency was achieved at 70 h of residence time, beyond which the efficiency at a given concentration either remained unchanged or dropped slightly. Additionally, the overall maximum inhibition efficiency (98%) was observed at 70 h residence time and 1% inhibitor concentration. This latter value remained unchanged until the experiment was stopped after 120 h.  
Figure 1: Steel specimen interaction as a function of time in 0.5 M HCl in terms of (**a**) weight loss in grams, (**b**) percentage weight loss, and (**c**) corrosion rate. Inhibition efficiency (IE %) and the degree of surface coverage (\(\theta\)) of the glycerol inhibitor for the corrosion study of steel in 0.5 M HCl were calculated using Equation (2) [35; 36; 37; 38; 39]:  
\[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]  
where \(W_{o}\) and \(W\) are the average weight losses in the absence and presence of the crude glycerol inhibitor, respectively. The inhibition efficiency increased with an increase in crude glycerol concentration. The primary action of inhibitors in acid solution is believed to be adsorption on the metal surface. This involves the assumption that the corrosion reactions are prevented from occurring over the active sites of the metal surface covered by adsorbed inhibitor species and the corrosion reaction occurred mainly on the inhibitor-free sites [40; 41]. In this work, the concentration of the crude glycerol was kept below 1% due to the basic nature of crude glycerol, which may reduce the performance of the acid solution in the studied system. The fraction of a covered surface (\(\theta\)) is determined as a function of concentration at specific constant temperature, and the adsorption isotherm was subsequently evaluated at equilibrium condition. The concentration \(C\) is plotted against the surface coverage as shown in Figure 3. As expected, the surface coverage increased with concentration. Figure 2: Inhibition efficiency of various glycerol concentrations as a function of immersion time. The three most common adsorption isotherms, namely the Langmuir, Freundlich, and Temkin isotherms, were tested to fit the data and adequately describe the process in this study. The Langmuir isotherm suggested the dependence of the fraction of the surface covered \(\theta\) on the concentration of the inhibitor \(C\) according to Equation (3):  
\[C_{e}/q_{e}=1/k_{l}\;q_{m}+C_{e}/q_{m} \tag{3}\]  
where \(C_{e}\) is the equilibrium inhibitor concentration, \(k_{l}\) the Langmuir equilibrium constant, representing the degree of adsorption (i.e., the higher the value of \(k_{l}\) indicates that the inhibitor is strongly adsorbed on the metal surface). The Langmuir isotherm plot of \(C_{e}/q_{e}\) versus \(C_{e}\) is a straight line with positive slopes, as shown in Figure 4.  
Figure 4: Langmuir adsorption isotherm for the inhibition effect of crude glycerol. Figure 3: Concentration as a function of surface coverage for the inhibition effect of crude glycerol. The experimental data were fitted to Langmuir adsorption isotherm model, and the results are presented in Figure 4. The model was then used to predict the adsorption constant. The adsorptive equilibrium constant was estimated to be 0.483 and 0.5077 g/L after 70 and 120 h, respectively.