The constituents present in Cordia dichotoma are found to be effective for mild steel corrosion inhibition activity in acidic media [21].

Experimental

Materials and methods

To determine the inhibitive property of EECdS on AA6063 in acidic media, chemical and electrochemical methods were employed. All the used aggressive solutions (0.5 M HCl) were made of AR grade reagent and deionized water. All sets of experiments were performed in triplicate, to confirm the reproducibility of the results. Preparation of test solutions, cleaning of Al coupons and preparation of CdS ethanolic extract were done as per standard practices [22-24].

Preparation of Al coupons

Industrial AA6063 of 98.99% purity has been used for test coupons. For weight loss experiments, mechanically press cut rectangular coupons (3 cm × 2 cm × 0.16 cm), with a hole of about 0.12 mm of diameter near the upper edge, were used. These were carefully examined to check for rough edges, and the surface treatment of the coupons was carried out by using various grades of emery paper (110 to 410), until the surface was free from scratches and other apparent defects. Then, they were degreased in acetone, finally washed with bi-distilled water, and dried in an oven. Thereafter, these specimens were kept in desiccators, until they cooled to room temperature, and then accurately weighed using a digital balance. They were further subjected to heating, cooling and weighing, until a constant weight was obtained, and then kept in desiccators. The surface treatment was immediately carried out before each corrosion test [25].

Ethanolic extract of cordia dichotoma seeds (EECdS)

The stock solutions of Cordia dichotoma seeds were prepared by soaking a weighed amount, dried at room temperature and finely powdered, in a properly corked container containing distilled ethanol. EECdS mass was found to be 25.0 g/L of the plant compound. On the soaking period completion, the ethanolic solutions were refluxed for 24 h, to concentrate the inhibiting chemicals, and then filtered to remove any suspended impurities. This stock solution of the extract was stored in a clean corked bottle, and used as inhibitor.

Preparation of test solutions

The aggressive solutions were measured to 100 mL in seven separate beakers, labeled as S0, S1, S2, S3, S4, S5 and S6. EECdS was added in order to increasing concentration, as to have 0.025, 0.075, 0.175, 0.275, 0.542 and 1.05 g/L, respectively, in S1, S2, S3, S4, S5 and S6 beakers, while no extract was added to S0, i.e., the first beaker solution. All the beakers were kept unstirred, and covered with Teflon tape throughout the experiment.

Fourier-transform infrared (FTIR) spectroscopy

FTIR analysis was carried out to identify the functional groups present in EECdS, and those in the corrosion products. The spectra were recorded with the 8400s Shimadzu, Japan spectrometer, by scanning the samples through a wave number range of 400 to 4000 cm^-1. The samples were prepared for the analysis using KBr [26].

Electrochemical method

Electrochemical measurements were carried out using a three electrode chemical cell attached with a PGSTAT 12 potentiostat [Autolab make with GPES software (EcoChemie)] connected to a personal computer. Fresh electrodes and solutions were used for each experimentation sets [27]. The aerated solutions were kept in an unstirred position during the measurements, at a temperature of 298±1 K.

The AA6063 specimen embedded in Teflon tape with an exposed area of 0.34 cm² was used as working electrode, while the counter electrode was a platinum wire, and the reference electrode was saturated Ag/AgCl/KCl. Initially, the working electrode was put in the test solution, and left for 10 min at the open circuit potential.

The EIS measurements were carried out using AC signals of amplitude 20 mV peak-to peak at the open circuit potential, in the frequency range of 10 000 Hz to 0.5 Hz. After the impedance spectrum was obtained, the polarization curves were immediately recorded by changing the electrode potential.

Potentiostatic polarization measurements

Using the above explained spectrum, the effect of EECdS concentration on electrochemical parameters values, such as corrosion current density (I_corr), corrosion potential (E_corr) and polarization resistance (R_p), was calculated. The so obtained corrosion rate was used to find out EEgCdS inhibition efficiency value in an acidic environment (0.5 M HCl.

Electrochemical Impedance Spectroscopy (EIS) measurements

EIS plots were analyzed in an acidic medium (0.5 M HCl) with and without inhibitor, for AA6063 coupons. The measurements for EIS studies were made with optimum EECdS concentrations in inhibited solutions. The inhibition efficiency was calculated by using the charge transfer resistance (R_ct) values obtained for uninhibited and inhibited (optimum concentration) solutions. The measurements were carried out at room temperature (298±1 K).

Scanning electron microscopy

AA6063 surface morphology, before and after immersion in the corrosion media, was examined using a SEM machine model- Quanta 3D FEG [28-31.

Results and discussion

FTIR analysis of the extract and corrosion product

It was found that the OH stretch at 3600 cm^-1 shifted to 3610 cm^-1, the C-H stretch at 2960 cm^-1 shifted to 2800 cm^-1, and cyclic five member aldehyde and ketone stretchs at 1900 cm^-1 shifted to 1860 cm^-1. EECdS spectra values indicated that there was an interaction between EECdS and the metal surface, and that these functional groups were involved in the formation of a bond with AA6063. Moreover, it can be affirmed that the functional groups have coordinated with the Al³⁺ formed on the metal surface, resulting in the Al³⁺ extract complex formation on the metal surface, which promoted the coupon surface corrosion inhibition [28-29].

Weight loss measurements

This trend may result from the fact that the adsorption and surface coverage increase with higher inhibitor concentrations. Thus, the surface is efficiently protected from the medium. The adsorption of such compounds onto the metal surface creates a barrier for charge and mass transfer, leading to a decrease in the interaction between the metal and the corrosion solution. The presence of this barrier film inhibits the metal surface corrosion [32-33].

It was observed that the corrosion rates were lower than the value obtained for the blank solution, indicating that EECdS addition inhibits AA6063 corrosion. The corrosion rate decreased as the inhibitor concentration increased. This behavior is the result of EECdS increased adsorption and coverage onto the metal surface. The corrosion rate, with respect to the immersion time period in an uninhibited solution, sharply decrease up to 48 h; after that, for an immersion time period of 72 h, a slightly increase was observed. This means that, at higher immersion time periods, a corrosion product was formed. At lower inhibitor concentrations, the same trend was observed, but at higher inhibitor concentrations, the corrosion rate decreased with an increased period of contact. The minimum corrosion rate was found to be 0.0832 mmy^-1, at 72 h, for the highest inhibitor concentration (1.05 g/L), thus clearly indicating that, at higher immersion time periods, the corrosion rate was observed to be independent from EECdS concentration.

Variation in inhibition efficiency with immersion time and inhibitor concentration

It was observed that IE% increased with higher inhibitor concentrations, and the maximum IE% value was found to be 90.63%, for a concentration of 1.05 g/L. The increase in IE% with higher concentrations indicates an increase in the number of components adsorbed onto the metal surface, by blocking the active sites.

Effect of temperature on the inhibition efficiency and corrosion rate

The effect of temperature variation is very complex, because many changes occur on the metal surface, such as inhibitor desorption, rapid etching and, moreover, it may undergo decomposition at high temperatures. Hence, the temperature (in the range of 293-353 K) effect on the adsorption and activation parameters was also studied. Weight loss measurements were carried out in the absence and presence of various inhibitor (EECdS) concentrations, at an immersion time period of 72 h.

These findings confirm that the corrosion rate is dependent on temperature and concentration. The minimum corrosion rate was observed at 293 K, at the highest inhibitor concentration (1.05 g/L.

The obtained data reveal that the highest IE% found at room temperature was 90.63%, which decreased with an increase in temperature. It was found that the inhibition efficiency increased with higher EECdS concentrations, at all temperatures, which means IE is dependent on the concentration. But at a temperature of 323 K, an exception was found, where the second highest concentration yielded maximum inhibition efficiency.

The negative sign of the B constant reaction values indicates that the corrosion rate process is inversely proportional to the inhibitor concentration, meaning that the inhibitor becomes more effective with increasing concentrations. The high negative B value observed at room temperature, when the change in .corr with the inhibitor concentration became steep (high negative value for constant B), reflected good inhibitive properties for EECdS. The k value increased with rising temperatures. At each temperature, a linear variation was observed in EECdS presence, thus confirming a first order kinetics [34].

Kinetic parameters of activation energy

The values show that EECdS addition led to an increase in the apparent activation energy to a value greater than that of the uninhibited solution, indicating a physical adsorption of the extract onto the metal surface. These results show a decrease in metal dissolution, which is due to a hindrance of the specimen surface by the formation of the metal complex layer [35-37].

The negative sign of enthalpy (ΔH) reveals that the inhibitor molecules adsorption is an exothermic process. The shift towards positive values of entropy (ΔS) implies that the activated complex in the rate determining step represents dissociation rather than association, meaning that disordering increases on going from the reactant to the activated complex.

The average difference in E_a+-ΔH is 2.7 kJ/mol, approximately equal to RT (2.63 kJ/mol), indicating that the corrosion reaction is unimolecular, and that the inhibitor equally acted on E_a+ and ΔH.

Thermodynamic parameters

where 55.5 is the water concentration of the solution in mL/L.

ΔGads negative values ensure the spontaneity of the adsorption process, and the stability of the adsorbed layer onto the specimen surface. Generally, ΔGads values around -20 kJ/mol, or lower, are consistent with physisorption, while those around -40 kJ/mol or higher involve chemisorption. In the present case of study, ΔGads values were lower than -20 kJ/mol, indicating physical adsorption of the molecule onto the AA6063 surface.

Adsorption isotherm

All the values are low, which suggests physisorption of EECdS and AA6063 active constituents.

where C is EECdS concentration, θ is the degree of surface coverage, K_ad is the equilibrium constant of the adsorption process, and K_ad = K^1/y. In this model, the number of y active sites has been included. 1/y values lower than one imply multilayer adsorption, while values greater than one suggest that the inhibitor molecules occupied more than one active site.

The charge transfer resistance (R_ct) values have been calculated from the difference in the impedance at low and high frequencies. The double layer capacitance values were estimated from the frequency (f), at which the imaginary component of the impedance (-Z ") is maximum.

where R_ct ^inh and R_ct⁰ are the charge transfer resistances, in the inhibitor absence and presence, respectively.

From the impedance parameters, it can be seen that AA6063 corrosion was decreased in the inhibitor presence, as the charge transfer resistance was significantly increased. The decrease in C_dl values indicates the EECdS molecule adsorption onto the metal surface. Decrease in C_dl can result from a decrease in the local dielectric constant, or from an increase in the electrical double layer thickness, suggesting that EECdS functions by adsorption at the metal-solution interface [43-44]. The changes in R_ct and C_dl values are caused by the gradual replacement of water molecules by the organic molecules adsorption onto the metal surface, decreasing the extent of the dissolution reaction. The semicircle obtained from the present system indicates that the inhibitor molecules adsorption is charge transfer controlled [45-46]. The IE% was found to be 85.15 %, at the highest concentration from R_ct.

Scanning electron microscopy (SEM) analysis

The surface morphologies reinforce the inference that the inhibitor prevents the metal surface decay. It is clearly seen that the metal surface dipped in 0.5 M HCl without inhibitor had a more pronounced depth of pitting, compared to the one with EECdS.

From the mass loss measurements, it was found that EECdS is a good AA6063 corrosion inhibitor in an acidic environment (0.5 M HCl). At lower temperatures, the inhibitor has obeyed Langmuir adsorption isotherm, but, at higher temperatures, deviation was observed. The experimental data followed the El- Awady's kinetic/thermodynamic model. A strong and spontaneous interaction between the extract and the AA6063 surface was indicated by the activation energy values and thermodynamic parameters. The presence of an adsorbed layer of extract onto the specimen's surface was justified by FTIR and SEM measurements. FTIR values showed that strong links between the metal and the extract surface were formed, due to CdS constituents (oxygen and nitrogen). EIS measurements reconfirm the other observations. SEM photographs showed the formation of a protective layer on the specimen surface, which inhibits AA606 corrosion.

Conclusions

From the study it has been concluded that:

1. EECdS acts as a good inhibitor for AA6063 alloy acid corrosion. EECdS inhibition efficiency increases with higher concentrations at all temperatures, while decrease in IE is detected with increasing temperatures, depending on the inhibitor concentration and temperature.

2. EECdS inhibits corrosion up to 90.13% in an acidic medium, with the highest concentration (1.05 g/L), at room temperature.

3. EECdS was found to obey Langmuir adsorption isotherm from the experimental data fit at room temperature, while at elevated temperatures, deviation from ideal Langmuir adsorption isotherm was observed. The experimental data were also fitted into the El-Awady's kinetic/thermodynamic model.

4. The free energy of adsorption value indicates strong, spontaneous and physical EECdS adsorption onto the AA6063 surface.

5. The EECdS extract presence increases the activation energy of the corrosion process, and adsorption heat gave a negative value.

6. The thermodynamic data for the inhibitor adsorption and AA6063 acid corrosion suggested the physical adsorption occurrence of the inhibitor species.

7. Entropy's (.S) shift towards positive values indicated the increase in disordering, on moving from the reactant to the activated complex.

8. FTIR spectra showed that the organic molecules present in the inhibitor were involved in the inhibition process.

9. The electrochemical measurements reconfirmed the results of other techniques.

10. SEM photographs further confirmed the formation of a protective film onto the Al alloy surface by the green inhibitor.

References