# _Laasonia inermis_ as an Active Corrosion Inhibitor for Mild Steel in Hydrochloric Acid
## 3 Results and Discussion
### Adsorption Consideration of Lawsonia ihernis Plant Extract  
The adsorption behaviour of the _Lawsonia ihernis_ extract was studied by fitting the data obtained to Langmuir adsorption isotherms using the equation below:  
\[\frac{c}{\theta}=\frac{n}{K_{ads}}+\mathrm{nc} \tag{4}\]  
where C = the inhibitor's concentration; \(\theta\) = the degree of surface coverage; n = the number of moles of the inhibitor molecule; \(K_{ads}\) = the adsorption equilibrium constant.  
Figure 5 is a plot of \(\frac{c}{\theta}\) versus c, and the linearity of the graph shows that the data are well fitted to the Langmuir adsorption isotherm, although the graph in Figure 5 did not completely fit the Langmuir adsorption model. This could be as a result of significant adsorption between adsorbed molecules or as a result of the heterogeneity of the metal's surface or multi-layer adsorption on the metal's surface, which the Langmuir model could not adequately account for.  
Figure 5: \(Cont.\)  
The equilibrium constant value of 0.002 and the R\({}^{2}\) value of 0.8899 for 0.1 M HCl and 0.0819 and 0.9999 for 1 M HCl signify a perfect fit to the Langmuir adsorption isotherms, which shows strong adsorption. This shows that LI adsorbed appreciably on the metal's surface. The negative value of \(-15.787\) kJ/mol for \(\Delta\)[PERSON] suggests that the adsorption process of the _Lawsonia internis_ on mild steel surfaces in the 0.1 M HCl solution is an interplay between both physisorption and weak chemisorption mechanisms. Initially, there is the physical interaction of the components of the _Lawsonia internis_ plant extract (for example, hexadecanoic ethyl ester and hexadecanoic acid-2,3-dihydroxy ester) through van der Waals forces. However, with extended contact time, covalent bonding between the Fe component of the mild steel and the carboxyl or hydroxyl groups in the identified components takes place, leading to the chemisorption of the _Lawsonia internis_ components on the mild steel. More negative \(\Delta\)Gads values above \(-20\) kJ/mol depict a strong chemisorption interaction mechanism [21]. Negative values closer to zero indicate a physisorption mechanism [22]. The negative calculated standard free-energy value of \(-6.305\) kJ/mol obtained for 1 M HCl suggests that the adsorption process was energetically favourable, depicts a physisorption mechanism, and underscores the potential of the LI to inhibit the corrosion of the mild steel in acidic media.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Isotherm** & **Intercept** & **Slope** & **Kads** & **Ln Kads** & **R\({}^{2}\)** & \begin{tabular}{c} \(\Delta G_{ads}\) \\ **(K)mol\({}^{-1}\))** \\ \end{tabular} \\ \hline \
\begin{tabular}{c} Langmuir \\ Mild steel (0.1 M) \\ \end{tabular} & 513.3307 & 0.8728 & 0.002 & 6.267 & 0.8899 & \(-15.787\) \\
\begin{tabular}{c} Langmuir \\ Mild steel (1 M HCl) \\ \end{tabular} & 12.2050 & 1.0203 & 0.0819 & 2.503 & 0.9999 & \(-6.305\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Adsorption isotherm parameters obtained from the corrosion data for mild steel coupons in 0.1 M and 1 M HCl containing _Lawsonia internis_.  
Figure 5: Langmuir isotherms for mild steel in different concentrations of _Lawsonia internis_ for mild steel corrosion in (**a**) 0.1 M and (**b**) 1 M HCl at 30 \({}^{\circ}\)C.