The adsorption thermodynamic parameters Hads
Fig. 2 shows the influence of temperature on the weight loss for aluminum in 0.5 M HCl in the absence and presence of 10 ppm of MC12 and PC12. As can be seen from the figure, the weight loss (and hence the rate of corrosion) of aluminum sample enhances with the immersion time and the addition of both inhibitors retards the rate of dissolution at all ranges of the investigated temperatures. The data show that the weight loss increases with increasing temperature. This can be due to the decrease in the strength of adsorption process with increasing temperature suggesting that physical adsorption may be the type of adsorption of the inhibitor on the sample surface. It has been reported that (EL-Deeb and Mohamed, 2011, Sayyah et al., 2001 and Abd El Rehim et al., 2010), for A 77-01 corrosion of metals, the logarithm of the corrosion rate (in mg cm−2 min−1) is a linear function with 1/T (following Arrhenius equation type):equation(2)log(rate)=-Ea/2.303RT+Awhere Ea is the apparent effective activation energy, R is the universal gas constant and A is the Arrhenius pre-exponential factor. The corrosion rate for each concentration from 1 to 10 ppm of the monomeric and its polymeric surfactant was calculated at different temperatures and the logarithm of the corrosion rate was plotted against 1/T for each concentration and the value of Ea was calculated and tabulated in Table 4. An alternative formula of the Arrhenius equation is the transition state equation:equation(3)Rate=RT/Nhexp(ΔS°/R)exp(-ΔH°/RT)where h is the Planck’s constant, N is the Avogadro’s number, ΔS° is the entropy of the activation, and ΔH° is the enthalpy of activation. The plot of log (Rate/T) vs. 1/T gives a straight line with a slope of (−ΔH°/2.303 R), from which the value of ΔH° was calculated and listed in Table 4. These values indicate that the presence of the additives increases the activation energy and the activation enthalpy, for the corrosion process. The addition of inhibitors modified the values of the activation energy and the activation enthalpy. This may be attributed to the adsorption of inhibitors on the aluminum surface and this adsorption makes an energy barrier and this energy barrier of the corrosion reaction increases as the concentration of the inhibitor is increased.