The electrochemical measurements were performed using a potentiostat/galva- nostat (ACM) connected to a computer. A three electrode cell assembly, consisting of a C-steel rod embedded in araldite as the working electrode (WE), a platinum sheet as the counter electrode (CE) and a saturated calomel electrode as the reference electrode (RE), was used for the electrochemical measurements. The temperature of the electrolyte was maintained at the required temperature using a water bath. Before immersion in the test solutions, the WE was polished with a polishing machine using emery paper from 600 to 1200 grade until a mirror image was obtained. Then, the WE was washed with distilled water then immersed in acetone for 1 minute in an ultrasonic cleaner. The WE electrode was prepared directly before electrochemical measurements then immersed in the test solution at open circuit potential for one hour until a steady state potential was obtained before impedance and polarization measurements were performed. All experiments were performed in aerated solutions. From the polarization data, were calculated like the degree of surface coverage (θ), the percentage inhibition efficiency (% IE), corrosion rate and charge transfer resistance [13] .

The reaction of PET waste was depolymerized with Thiodiglycol, at weight ratio of PET to Thiodiglycol 1:8 (wt% of PET: wt% of Thiodiglycol) using 0.5% of Zinc acetate as catalyst (by weight based on weight of PET). The reaction mixtures were heated under vigorous stirring in nitrogen atmosphere at temperature about 160˚C - 180˚C for 10 h and at 140˚C for 2 h. The temperature of the reaction was then lowered to 100˚C for 1 h. The mixture was allowed to cool to room temperature. at the end of the reaction, distilled water was added in excess to the reaction mixture with vigorous agitation to precipitate oligomer of Bis(2-((2-hydroxyethyl)thio)ethyl terephthalate (BHET) out of the product, black liquid viscous of (BHET) was obtained [14] [15] , the suggested mechanism was shown in (Figure 1), The chemical structure of (BHET) was confirmed from their FTIR (Figure 2) and 1HNMR (Figure 3) spectroscopy.

Figure 2 shows the FTIR spectra of (BHET). From the FTIR spectra a strong peak at wavelength (3396 cm⁻¹) attributable to group (O-H), while the peak at (1718 cm⁻¹) is attributed to the ester carbonyl groups [16] and the strong peak at (1105 cm⁻¹) attributable to bond (C-O) ester association. The peak at 810 cm⁻¹ for (BHET) is assigned to −CH out of plane bending of substituted phenyl.

Figure 3 shows the ¹HNMR of (BHET), the peak at (8.11 ppm) is singlet for proton aromatic ring (a). The OH group (b) show a singlet peak at (4.87 ppm), and peak triple (c) which is attributed to the group of methylene nearest carbonyl ester at (4.53 ppm), and peak (d) powerful Triplet attributable to the

group of methylene near oxygen atom at (3.66 ppm), and the Triplet (e) peak attributed to the symmetric methylene group nearest to the sulfur atom at (3.02 ppm), while the peak (f) at (2.84 ppm) back to the methylene group asymmetric nearest sulfur atom, and the peak at 8.5 ppm is triplet for proton of amide. The peak (g) at (2.59 ppm) is quintet for solvent (d⁶-DMSO) [17] [18] .

Typical potentiodynamic polarization curves for the C-steel in 0.1 M HCl in the presence and absence of different concentrations of BHET are shown in (Fig- ures 4-11) The respective Tafel parameters, inhibition efficiency (% IE), surface coverage (θ), corrosion rate and charge transfer resistance are provided in Table 1. It is clear that the shapes of the Tafel plots for the inhibited electrodes are different from those of uninhibited electrodes. The presence of the inhibitor decreases the current density but does not change other aspects of the behavior.

It is evident from Table 1 that the adsorption of the inhibitor shifted the corrosion potential (E_corr) in the negative direction. The addition of BHET decreases both of the Tafel slopes (ßa and ßc), the anodic and cathodic Tafel slopes. This indicates that BHET is a mixed-type inhibitor affecting the iron dissolution and hydrogen evolution [19] [20] . The reduction of the positive and negative currents in the presence of BHET can be explained by the blocking of active sites by the formation of a protective film on the surface of the electrode [21] [22] . The values of the surface coverage and inhibition efficiency reached its maximum at a BHET concentration of 40 PPM at 298 k. As can be seen from Table 1, BHET inhibitor greatly reduces corrosion current with a slight shift in the corrosion potential. If the displacement in the corrosion potential is more than 85 mV, with respect to the corrosion potential of the blank solution, the inhibitor can be designated as a cathodic or anodic type [22] . In the present study, and through the difference corrosion potential values of corrosion inhibitor in the presence or absence of the inhibitor, which indicated that the studied inhibitor is a mixed- type inhibitor which is in agreement with some other studies [20] .

Table 1 shows the values of (Rct) increases with inhibitor, and the value of Rct indicates that of the efficiency inhibitor to preventing the erosion of carbon- steel in the acidic media. The highest resistance to charge transfer (R_ct(inh.₎) value at (298 K) and was (442.1 Ω) at a concentration (30 ppm) of the inhibitor. Using BHET inhibitor at (328 K) gave the highest value of (R_ct(inh.₎) of 39.21 Ω at a concentration (50 ppm), while the value reached to 5.456 Ω in the absence of inhibitor at the same conditions [23] .

Temperature is an important parameter when studying metal dissolution. It is known that the effect of temperature on the acid-metal reaction is highly complex. The corrosion rate in acid solutions, for example, increases exponentially with an increasing temperature because hydrogen evolution decreases. Many changes may occur on the metal surface, such as adsorption, desorption, rearrangement or decomposition of the inhibitor [24] , Only a few inhibitors are effective at high temperature as they are at low temperature [25] .

The effect of increasing temperature (from 298 - 328 K) on the corrosion rate of C-steel in 0.1 M HCl, and its effects on inhibition action of 30 ppm BHET are shown in Table 2. The inhibitory effect (% IE) decreased from 97.1% at 308 K to 75.9% at 328 K.

The values of the activation energy Ea of the corrosion process in 0.1 M HCl in the presence and absence of BHET was calculated using the Arrhenius equation: [26]

where W is the corrosion rate (mpy), A is the Arrhenius constant, Ea is the activation energy, R is the gas constant and T is the absolute temperature.

Figure 12 shows Arrhenius plots of the ln CR vs 1/T for carbon steel in the corrosive medium with and without addition of 30 ppm of BHET. Straight lines are obtained with a slope of (−Ea/R). The value of Ea can be obtained from the slope of the straight line which was found to be 0.0337 kJ∙mol⁻¹ and 0.0619 kJ∙mol⁻¹ in the absence and presence of 30 ppm of BHET, respectively. The

higher value of Ea in the presence of BHET than its absence indicates a strong inhibitive action of the BHET by increasing the energy barrier for the corrosion process [27] . And the higher Ea value in the inhibited solution can be correlated with the increased thickness of the double layer.

A plot of ln(W) against 1/T shown in Figure 13 which gives straight lines with a slope of (−ΔH/R) and an intercept of [(ln(R/Nh)) + (ΔS/R)] to which the values of ΔH and ΔS are calculated and are given in Table 3.

The enthalpy and entropy of activation (ΔH and ΔS) can be calculated by given equation:

where h is Plank constant and N is Avogadro’s number.

The positive signs of enthalpies (ΔH) reflect the endothermic nature of dissolution process [28] [29] . Large and negative values of entropies (ΔS) show that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex.