Triphenylphosphine-functionalized graphene oxide: A novel corrosion inhibitor for carbon steel in acidic environments

2.1. Materials

The materials utilized in this study included graphite, hydrogen peroxide, potassium permanganate, sodium nitrate, and triphenylphosphine (TPP), sourced from Fluka, alongside hydrochloric acid, sulfuric acid, tetrahydrofuran (THF), and dimethylformamide (DMF), obtained from BDH. The chemical composition of the C1010 carbon steel alloy was as follows: C (0.13%), S (0.05%), P (0.04%), Mn (0.30%), Ni (0.30%), Si (0.37%), Cr (0.10%), Cu (0.30%), As (0.08%), with the balance being iron (Fe).

2.2. Methods

2.2.1. Synthesis of graphene oxide

The oxidation process was initiated by adding 2 g of graphite powder to 50 mL of sulfuric acid at low temperature, followed by stirring in an ice bath for 15 min. Subsequently, 1 g of sodium nitrate and 6 g of potassium permanganate were gradually added to the suspension while maintaining the temperature below 20°C. The mixture was then stirred in the ice bath for 2 h to ensure complete oxidation. After this period, the ice bath was replaced with a water bath, and the reaction temperature was raised to and maintained at 35°C for 30 min. At this stage, the mixture developed a pasty consistency and turned a deep red-brown color, indicating the formation of intermediate oxidation products. After that, deionized water 50 mL was slowly added to the reaction mixture, and the temperature was raised to approximately 90–98°C. The suspended mixture was diluted with 250 mL of warm deionized water. To terminate the reaction and reduce residual potassium permanganate species, ∼30 mL of 30% (H₂O₂) hydrogen peroxide was added dropwise until the solution changed color to bright yellow, confirming the formation of GO. The resulting mixture was allowed to stand, and the final product was collected and dried at 40°C for 24 h to yield graphene oxide powder, as illustrated in Figure 1 [15].

Close

Figure 1.

Synthesis of graphene oxide (GO) with (GP).

Export to PPT

2.3. Electrochemical measurement

The electrochemical properties of C1010 carbon steel alloy in a hydrochloric solution (1M) were investigated by the Tafel extrapolation method. A synthesized GP compound was evaluated as a corrosion inhibitor at concentrations of 1, 2, 3, 4, 5, and 6 ppm. The study assessed various electrochemical parameters, including corrosion rate (CR, in mpy), corrosion current density (I_corr, in µA·cm⁻²), charge transfer resistance (RCT, in Ω), corrosion potential (Ecorr, in mV), and the anodic and cathodic Tafel slopes (βa and βc, in mV·decade⁻¹). Electrochemical tests were conducted at temperatures of 298, 308, 318, and 328 K. The temperature dependence of corrosion rate and inhibition efficiency was evaluated through a series of experiments conducted at different temperatures, with and without the presence of inhibitors.

2.4. Instrumental

The structural and morphological properties of GO and its functionalized form GP were investigated using fourier transform infrared spectroscopy (FTIR) via KBr powder disc, X-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM). Electrochemical analyses were performed utilizing a DY2300 Series potentiostat/galvanostat system.

3.1. Characterization by infrared spectroscopy

The FTIR spectra of GO revealed characteristic absorption bands, including a broad band of O–H stretching vibration at 3386.39 cm⁻¹, a stretching vibration C=O peak at 1724.36 cm⁻¹, and peaks at 1622.80 cm⁻¹ and 1378.85 cm⁻¹ corresponding to stretching vibrations C=C and bending vibrations C–OH, accordingly, a distinct absorption band at 1029.8 cm⁻¹ Figure 2(a) further confirms the presence of epoxy (C–O) functionalities, consistent with the oxygenated nature of GO [12].In comparison, the FTIR spectrum of GP (Figure 2b) also exhibits a broad O–H stretching band at 3386.39 cm⁻¹. The absorption at 1724.05 cm⁻¹ is attributed to the C=O stretching vibrations of ester groups, suggesting successful functionalization. A peak at 1625.77 cm⁻¹ is ascribed to aromatic C=C stretching, while additional peaks at 1580.38 cm⁻¹ and 1157.91 cm⁻¹ correspond to C–O stretching and epoxy group vibrations, accordingly. Moreover, a distinct band at 2362.37 cm⁻¹ is associated with P–H bending vibrations, indicating the incorporation of phosphorus-containing functionalities [15,16].

Close

Figure 2.

Infrared spectra for (a) graphene oxide, (b) GP.

Export to PPT

3.2. Characterization by X-Ray diffraction

The pattern of XRD in GO (Figure 3a) showed a characteristic peak at 2θ = 10.97°, corresponding to an interlayer spacing (d-spacing) of 8.06 Å. This increased interlayer distance confirmed the success of graphite oxidation and exfoliation during chemical synthesis. The disappearance of the characteristic graphite peak at 2θ≈26°further confirms the complete oxidation of graphite into GO [12]. In Figure 3b, the XRD pattern of pristine graphene GP shows multiple diffraction peaks. Notably, the peak at 2θ = 10.97°, with a corresponding d-spacing of 8.06 Å (0.806 nm), confirms the presence of GO. Upon functionalization of GO with TPP, the interlayer spacing increases further due to the intercalation and interaction between TPP molecules and GO sheets. This structural modification is evidenced by the appearance of new diffraction peaks at 2θ = 31.02°, 32.59°, 34.86°, 38.10°, 40.17°, 42.29°, 46.00°, and 53.25°, corresponding to interlayer spacing of 2.82 Å (0.282 nm), 2.74 Å (0.274 nm), 2.57 Å (0.257 nm), 2.36 Å (0.236 nm), 2.24 Å (0.224 nm), 2.13 Å (0.213 nm), 1.97 Å (0.197 nm), and 1.71 Å (0.171 nm), respectively [17]. These changes indicate a significant alteration in the layered structure of GO upon functionalization.

Close

Figure 3.

XRD for (a) GO, (b) GP.

Export to PPT

The crystallite size of the synthesized compounds was estimated using the Debye-Scherrer equation based on XRD pattern analysis Eq 1. [7]:

The crystallite size calculation involved parameters such as X-ray wavelength (λ), FWHM (β), shape factor (K = 0.9), and Bragg angle (θ). The calculated sizes were approximately 16 nm for GO and 6.10 nm for graphene powder GP.

To calculate the size of crystallite by another equation 2 (Williamson – Hall) [17]:

Figure 4 illustrates the measurement of the crystallite size of GP using the Williamson–Hall equation. In this method, the microstrain ($\text{£}$) of particles is also evaluated. The crystallite size is estimated from the linear plot of β cosθ (y-axis) versus sinθ (x-axis). The intercept of this plot corresponds to (Kλ/D), where the crystallite size (D) and the shape factor (K). According to the Williamson–Hall analysis, the size of the crystallite was determined to be 3.39 nm, while the calculated microstrain ($\text{£}$) for the GP sample was -0.57385 Figure 4 and Table 1.

Close

Figure 4.

Crystallite size for GP by using the Williamson-Hall equation.

Export to PPT

3.3. Field emission scanning electron microscopy (FE-SEM) technique

The morphology and microstructure of the materials were studied using FESEM. The FESEM image of GO is shown in Figure 5a, which displays distinctly sharp edges and a relatively uniform, smooth surface. The darker areas in the image suggest the presence of multiple stacked layers, exhibiting characteristic wrinkling and folding. Conversely, Figure 5b presents the FESEM image of graphene nanoplatelets GP, revealing prominent surface undulations and distortions, consistent with a multilayered morphology [5,17].

Close

Figure 5.

FESEM images for (a) GO, (b) GP.

Export to PPT

3.4. Polarization studies (Tafel plot)

The data in Table 2 reveals that GP’s inhibition efficiency increases with increasing inhibitor concentration. This enhancement in performance is reflected by a notable reduction in both (I_corr) corrosion current density and (CR) corrosion rate, accompanied by R_ct values significantly increased with the addition of GP, confirming the formation of a resistive film on the surface of the steel. These results reinforce the conclusion that GP hinders the charge transfer process, consistent with surface coverage and reduced active corrosion sites. The addition of GP resulted in a corrosion potential (E_corr) shift of less than 89 mV for C1010 carbon steel, characteristic of mixed-type inhibition. Changes in the Tafel slopes (βa and βc) indicated that GP influences both anodic and cathodic processes. The slight changes in slope values further confirm the mixed-type inhibition mechanism, primarily attributed to chemical adsorption of GP molecules on the surface of the steel, blocking active sites and reducing corrosion [18,19]. Figure 6-9, in present and absence GP inhibitor, In the absence of the inhibitor GP, as the temperature increases from 298 K to 328 K, the I_corr values significantly increase, indicating accelerated corrosion due to enhanced electrochemical activity. However, in the presence of GP, although I_corr increases slightly, the inhibitor remains effective even at elevated temperatures, whereas the efficiency remains stable or improves, which may be due to enhanced adsorption kinetics or stronger chemical interactions at elevated temperatures. This trend is consistently observed across all temperatures (308, 318, and 328K), showing that increasing the GP concentration enhances the inhibition efficiency. The inhibitor molecules adsorb onto the steel surface, forming a protective layer that reduces both anodic and cathodic reactions, thereby enhancing corrosion protection. For example, at 6 ppm, the inhibition efficiency remains high (from 94.88% to 96.06%), implying strong adsorption stability of GP molecules on the metal surface, likely through chemical interaction chemisorption. This suggests that GP acts as a temperature-resilient inhibitor, suitable for high-temperature acidizing operations. Increasing the concentration of GP from 1 ppm to 6 ppm at 298K resulted in a corresponding increase in inhibition efficiency. Inhibition efficiency improves with increasing GP concentration at all temperatures and remains above 88% even at 1 ppm, indicating high adsorption efficiency even at low doses. Moreover, the inhibition efficiency is raised at a certain constant concentration (at all studied concentrations) as temperature is increased, whereas the maximum inhibition efficiency is 96.40% while the minimum is 88.48%, indicating a chemical adsorption mechanism for the GP inhibitor on the C1010 surface.

Close

Figure 6.

Tafel plot with and without GP inhibitor at 298 k.

Export to PPT

Close

Figure 7.

Tafel plot with and without GP inhibitor at 308 k.

Export to PPT

Close

Figure 8.

Tafel plot with and without GP inhibitor at 318 k.

Export to PPT

Close

Figure 9.

Tafel plot with and without GP inhibitor at 328 k.

Export to PPT

3.5. Kinetics studies

The effect of temperature on corrosion kinetics was investigated by evaluating the activation energy (Ea), enthalpy of activation (ΔH*), entropy of activation (ΔS*), and Gibbs free energy of activation (ΔG*) using Arrhenius and transition state theory equations [19] as below Eq 3:

Where CR is the corrosion rate, E_a is the activation energy, and A is the frequency factor.

R is the molar gas constant; T is the temperature (K).

Equation (4) is used to determine the activation energy (E_a), enthalpy (ΔH*), entropy (ΔS*), and Gibbs free energy of activation (ΔG*) [20].

Avogadro’s number N (6.022 × 10²³ mol⁻¹ and), Planck’s constant h (6.626 × 10⁻³⁴ J·s), plotting ln(CR/T) against 1/T allows calculation of ΔH* (from the slope) and ΔS* (from the intercept). While the ΔG* was measure by the following equation (5):

Figures 10-12 illustrate the influence of GP on activation energy and thermodynamic parameters in comparison to the blank HCl solution. The calculated kinetic parameters (E_a, ΔH*, ΔS*, and ΔG*) have been presented in Table 3 for both inhibited and uninhibited conditions.

Close

Figure 10.

The activation of energy with and without the GP inhibitor.

Export to PPT

Close

Figure 11.

Thermodynamic functions of activation include ΔH* and ΔS* in the presence of HCl.

Export to PPT

Close

Figure 12.

Thermodynamic functions of activation include ΔH* and ΔS* in the presence of the GP inhibitor relative to corrosive acid, HCl.

Export to PPT

The kinetic analysis of carbon steel C1010 corrosion inhibition in a 1M hydrochloric solution with the GP inhibitor provided key insights into the inhibition mechanism. The increase in activation energy (E_a) with the inhibitor suggests the formation of a protective barrier that hinders metal dissolution, thereby suppressing corrosion. This rise in E_a with increasing inhibitor concentration is consistent with a chemical adsorption mechanism, which introduces an additional energy requirement for the corrosion reaction to happen. Furthermore, the enthalpy of activation (ΔH*) showed a clear increasing trend with the concentration of GP, suggesting that the process of inhibition is endothermic in nature. This implies that energy input is required for the formation of the transition state, further supporting the role of the inhibitor in stabilizing the metal surface. The entropy of activation (ΔS*) values were negative across all tested conditions, indicating a decrease in disorder during the formation of the activated complex at the metal/solution interface. As the inhibitor concentration increased, the adsorption of GP molecules onto the metal surface became more structured, leading to a more positive ΔG* value. This indicates lower spontaneity in the ability of corroded species to form a corrosion product and supports the hypothesis of chemical adsorption as the dominant inhibition mechanism. The most effective inhibition was observed at 6 ppm, which yielded the highest ΔG* value (71.76 kJ mol^-1 at 328 K), confirming the optimal surface coverage and strong protective effect of the inhibitor at this concentration. Overall, the kinetic and thermodynamic analyses confirm that the GP inhibitor acts by forming an energy barrier to corrosion through spontaneous, though ordered, adsorption onto the steel surface, thereby significantly reducing the corrosion rate of C1010 in acidic chloride environments.

3.6. Adsorption isotherms

The Langmuir model, which assumes monolayer adsorption on a homogeneous surface with no lateral interactions, provided a good fit to the experimental data, with R² values approaching unity Eq 6 [21].

Where C is the inhibitor concentration, θ is the coverage of the surface, and K_ads is the equilibrium adsorption constant in Table 4 , Data of Langmuir adsorption isotherm’s adsorbed GP inhibitors and Figure 13 shows the Langmuir plots obtained for the GP inhibitor at various temperatures in 1 M HCl solution. These plots reveal linear relationships, supporting the applicability of the Langmuir model.

Close

Figure 13.

Langmuir s adsorption isotherm for the GP inhibitor.

Export to PPT

The adsorption mechanism of the inhibitor is governed by chemisorption, which involves the formation of chemical bonds through charge sharing between the metal atoms and the inhibitor molecules. The calculated thermodynamic parameters of adsorption by using the adsorption equilibrium constant K_ads​obtained from Langmuir plots [18-20]. $\Delta G_{\text{ads}}^{°}$ was determined by using the equation (7) [22]:

The modified Van’t Hoff equation (8) was used to calculate the enthalpy of adsorption ($\Delta H_{ads}^{°}$), with R is the gas constant, T is the absolute temperature in Kelvin, and water concentration in the solution 55.5 mol L⁻¹.

Table 5 summarizes the thermodynamic parameters for the GP inhibitor, showing negative $\Delta G_{\text{ads}}^{°}$values across the entire temperature range, which confirms the spontaneity of the adsorption process. Furthermore, the magnitude of $\Delta G_{\text{ads}}^{°}$increased (became more negative) with rising temperature, suggesting that the adsorption process is chemical, where, $\Delta G_{\text{ads},}^{°}$ as in Table 5, approximately equal or less more than −40 kJ·mol⁻¹ imply chemisorption [23,24]. Additionally, the positive value of $\Delta H_{ads}^{°}$ confirms that the adsorption process has an endothermic nature. The positive $\Delta S_{\text{ads}}^{°}$ values indicate that the adsorption of inhibitor increases randomness at the metal-solution interface, likely due to the desorption of water molecules and solvated ions [25-29].

A plot of $\frac{\Delta G_{\text{ads}}^{°}}{T}$ against $\frac{1}{T}$ results in a straight line, from which $\Delta H_{\text{ads}}^{°}$can be determined [22] as the slope, as depicted in Figure 14.

Close

Figure 14.

The calculation of $\Delta H_{ads}^{°}$ for the adsorption of GO on the carbon steel surface (C1010) at 298 k.

Export to PPT

Under the experimental conditions, the adsorption heat can be approximated as the standard adsorption heat, allowing for the calculation of ∆G_ads values using the following equation (9) [30].