# Crude Glycerol as an Innovative Corrosion Inhibitor
## 3 Results and Discussion
### Crude Glycerol Inhibition Ability
The deviation from unity slope indicates that the Langmuir isotherm may not be strictly applied in this case. Hence, other adsorption isotherms need to be tested. The experimental results were also fitted to a Freundlich adsorption isotherm. The Freundlich adsorption isotherm is given according to Equation (4):  
\[log\ q_{e}=log\ k_{f}+1/n\ log\ C_{e} \tag{4}\]  
where \(k_{f}\) and \(n\) are Freundlich constants; \(q_{e}\) = amount of adsorbent per unit mass of adsorbate (g/g); \(C_{e}\) = equilibrium concentration (mg/L). The \(log\ q_{e}\) versus \(log\ C_{e}\) plot allows for a determination of the Freundlich constants. The results of adsorption isotherm models are shown in \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]0  
According to Equation (4), when \(log\ q_{e}\) is plotted against \(log\ C_{e}\), it gives a straight line with intercept (\(log\ k_{f}\)) and slope (\(1/n\)), as can be observed from \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]1  
The values of \(n\) = 0.362 for inhibition after 70 h and 0.378 after 120 h of inhibition. The Temkin adsorption isotherm was also applied by plotting \(q_{e}\) versus \(ln\ C_{e}\) after 70 and 120 h. According to the Temkin isotherm model (Equation (5)), the plot of surface coverage (\(q_{e}\)) obtained from the weight loss method versus \(ln\ C_{e}\) at different concentrations of the inhibitors is not a straight line as shown in \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]2  
\[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]3  
where \(B\) = (RT/b) constant related to heat of sorption (J/mol) obtained from the Temkin plot (\(q_{e}\) versus \(ln\ C_{e}\)); \(A\) (intercept) = Temkin isotherm equilibrium binding constant; b (slope) = Temkin isotherm constant; R = universal gas constant; T = absolute temperature. \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]4 Freundlich adsorption isotherm for the inhibition effect of crude glycerol. A summary of the adsorption isotherm models is given in \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]5  
From the table, it can be concluded that the corrosion inhibition using crude glycerol is best-described using Langmuir adsorption isotherm compared to the other two models. In this study, the corrosion of steel in 0.5 M HCl, the weight at time \(t\) denoted as \(W_{1}\) when \(\log W_{1}\) is plotted versus time, a linear correlation is observed as given by \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]6 This suggests first-order reaction kinetics. \[\text{IE }\%=\theta\times 100=[(W_{o}-W)/W_{o}]\times 100 \tag{2}\]7  
where \(W_{o}\) is the weight before immersion, \(K\) is the rate constant, and \(t\) is the time in h. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**adsorption Isotherm Model** & **Contact Time (h)** & \(R^{2}\) & \(K_{add}\)**g/L, Slope** & **Intercept** \\ \hline Langmuir & 70 & 0.999 & 0.848 & 2.069 \\  & 120 & 0.994 & 0.869 & 1.970 \\ \hline Freundlich & 70 & 0.965 & 0.362 & 0.342 \\  & 120 & 0.952 & 0.378 & 0.273 \\ \hline Temkin & 70 & 0.853 & 0.0085 & 0.1848 \\  & 120 & 0.8532 & 0.0085 & 0.1859 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the adsorption isotherm models for the inhibition of crude glycerol. Figure 6: The Temkin adsorption isotherm for the inhibition effect of crude glycerol. The value of the rate constant \(K\) is obtained from the slope of the straight line. The half-life value, \(t_{1/2}\) of the metal in the test solution, can be determined using Equation (7) [42; 43; 44]:  
\[t_{1/2}\ =\ \frac{0.693}{K}. \tag{7}\]  
The equilibrium constant of the adsorption process is related to the Gibbs free energy of adsorption \(\Delta G_{ads}\) as shown in Equation (8) [44]. \[\Delta\mathrm{G}=-2.303\mathrm{RT}\ \log(55.5\ \mathrm{K}) \tag{8}\]  
here \(R\) is the gas constant, \(T\) is the absolute temperature, and \(K\) is the adsorption equilibrium constant.