*3.3. Adsorption Study and Standard Adsorption Free Energy*

The corrosion inhibition adsorption process of the tested inhibitor on the N80 carbon steel surface was carried out by several adsorption isotherms, such as Temkin's, Frumkin's, Langmuir's, and El-Awady's adsorption isotherms. The Temkin's adsorption isotherm was found to give the best description of the adsorption behavior of the studied inhibitor. The Temkin's adsorption isotherm is defined by the following equations:

$$\theta = \frac{-2.303 \text{ logK}\_{\text{ads}}}{2a} - \frac{2.303 \text{ log C}}{2a} \tag{13}$$

where θ is the surface coverage (θ = *IE*%/100), *K*ads the adsorption-desorption equilibrium constant, *C* is the inhibitor concentration, *a* is the molecules interaction parameter. Positive values of *a* imply attractive forces between the inhibitor molecules, while negative values indicate repulsive forces between them.

*Kads* is related to the free energy of adsorption by the following equation:

$$
\Delta G\_{\text{ads}}^{\circ} = -RT\,\text{Ln}(K\_{\text{ads}}) \tag{14}
$$

where *R* is the gas constant (8.314 J K−<sup>1</sup> mol<sup>−</sup>1), *T* is the absolute temperature (K). The plot of surface coverage (θ) as a function of the logarithm of the inhibitor concentration at different CO2 partial pressures is shown in Figure 6.

**Figure 6.** Temkin's adsorption isotherm for carbon steel (N80) pipeline steel in CO2-saturated chloride at different pressures.

The plot of θ vs. log *C* yields a straight line and the regression coefficient ranges from 0.985 to 0.996. The calculated values of adsorption parameters Δ*G*◦ ads, *a* and *K* at different CO2 partial pressures are presented in Table 3 and the following notes can be written: (i) The values of Δ*G*◦ ads are negative for all three pressures, indicating that the adsorption of GA on the steel surface in the tested solution is a spontaneous process [7,21,22,28]. Furthermore, the value of Δ*G*◦ ads ranges between <sup>−</sup>10.64 to <sup>−</sup>8.37 kJ mol−<sup>1</sup> indicating that the adsorption of GA on the steel occurs through a physical adsorption process [7,21,22,28]; (ii) The values of "*a*" are negative for all three pressures, indicating that repulsion forces exist between the adsorbed inhibitor molecules in the adsorption layer, as also reported by other studies for the same tested inhibitor [7,22]; (iii) The values of *K*ads increases with an increase in CO2 partial pressure. It should be noted that *K*ads denotes the strength between adsorbate and adsorbent. It can be inferred that a large value of *K*ads implies a more efficient adsorption process and thus, a better corrosion inhibition efficiency [21,22]. The results suggest that the adsorption of GA increases with an increase of the environment pressure, leading to a greater surface coverage and consequently, a better protection performance.

**Table 3.** Parameters of the Temkin's adsorption isotherm calculated from weight loss measurements after 24 h of immersion time.

