Corrosion inhibition of type 430 stainless steel in an acidic solution using a synthesized tetra-pyridinium ring-containing compound

2.1 Inhibitors synthesis

First, N-methyl-2-chloro-pyridinium iodide was prepared by mixing 2-chloro-pyridine (5 g) with methyl iodide (3 mL) in ethanol (20 mL) for 3 h. The mixture was cooled and then treated with excess ether. The solid salt was filtered, washed with ether, and then dried at 80 °C for 6 h. A solution of 4,4′-bipyridyl (1.0 g) in methanol (10 mL) was added drop wise to a stirred solution of N-methyl-2-chloro-pyridinium iodide (5 g) in methanol (20 mL) under nitrogen atmosphere. The mixture was refluxed for 2 h. The red solid product of 1,1′-bis (1-methyl pyridinium-2-yl)-4,4′-dipyridinium dichloride di-iodide (TPy) was filtered, washed with acetone and dried at 100 °C for 6 h. 1,1′-dimethyl-4,4′-dipyridinium di-iodide (DPy) was synthesized according to Emara (1991). The DPy has a melting point from 295–8 °C and TPy has above 300 °C, the elemental analyses of these compounds are indicated in Table 1 and Fig. 1 shows the chemical structures.

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Figure 1

Chemical structure of the inhibitors.

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¹HNMR data of TPy: δ = 9.4 (d, 4H), 9.3 (d, 2H), 9.0 (t, 2H), 8.63 (d, 2H), 8.5 (t, 2H), 8.07 (d, 4H), 4.3 (s, 6H, 2 CH₃).

2.2 Materials and methods

A standard corrosion glass cell was used for the polarization measurements. The material of the working electrode is a sheet with an area of 1 cm² from ferritic type 430 SS, it was cut from cold rolled annealed sheet (produced by Nilaco, Japan and containing 17–18.5% Cr and <1500 ppm C). The counter and reference electrodes are platinum sheet and silver–silver chloride (Ag/AgCl, 3 M KCl), respectively. The electrolyte solution was prepared from concentrated analytical reagent H₂SO₄ and bi-distilled water. The inhibitor solution was prepared by dissolving the appropriate weight in 0.5 M H₂SO₄ solution. All experiments were conducted thermostatically at a given temperature and in an aerated condition without stirring. Prior to each experiment the working electrode was wet polished with emery papers up to grade 1000, rinsed with bi-distilled water, acetone, bi-distilled water and left in air for 30 min. Then, it was transferred to the glass cell which was filled by 200 ml of the acid solution.

Electrochemical experiments were recorded using a potentiostat of type Autolab PGSTAT30, coupled to a computer equipped with GPES software for potential and polarization measurements. Potentiodynamic measurements were performed with a scan rate of 0.001 V s⁻¹. Scanning electron microscope (SEM) images were obtained by using Joel JSM-6360LVSEM.

2.3 Weight-loss method

Type 430 stainless steel sheets with an area of 10 cm², in duplicate, were immersed in aerated 350 mL of 0.5 M H₂SO₄ solution without and with various concentrations of the studied inhibitors for 24 h at 30 °C. The weights of the steel specimens before and after immersion were determined using an analytical balance Mettler AE 166. The corrosion rate in mg dm⁻² day⁻¹ (mdd) was calculated and used in calculation of the inhibition efficiency (IE) according to the equation:

2.4 Quantum chemical calculations

The molecular structures of the studied compounds were geometrically optimized using the Density Functional Theory (DFT) method with B3LYP level and 6-31G^∗ basis set, and the Spartan′10 V.1.0.1 program package (Lee et al., 1988). B3LYP, a version of the DFT method that uses Becke’s three parameter functional (B3) and includes a mixture of HF with DFT exchange terms associated with the gradient corrected correlation functional of Lee, Yang and Parr (LYP) (Shao et al., 2011), was used in this paper to carry out quantum calculations.

3.1 Polarization measurements

The effects of the synthesized compounds under study on the corrosion and polarization behaviors of the SS were studied in 0.5 M H₂SO₄ at 30 °C. Addition of the inhibitor shifted the steel potential (−0.495 V) to the positive side, and this shift in potential increased by increasing inhibitor concentration (−0.449 V and −0.438 V in the case of 1 × 10⁻³ M of DPy and TPy, respectively), as shown in Table 2. This result indicated that the compounds under study are mainly anodic inhibitors.

Cathodic–anodic polarization of the SS in the absence and the presence of the inhibitor in 0.5 M H₂SO₄ at 30 °C has been performed, as shown in Fig. 2A for the TPy inhibitor. The anodic polarization branch was extended up to the transpassive potential. Complete analysis of the polarization curve, including cathodic–anodic Tafel lines and active–passive range, was done and the parameters are recorded in Table 2 for the two inhibitors. From the corrosion current (i_cor), the inhibition efficiency (IE) was calculated using Eq. (2):

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Figure 2

(A) Cathodic–anodic polarization of the SS in 0.5 M H₂SO₄ in the absence and presence of different concentrations of TPy at 30 °C, (B) magnification view of the cathodic–anodic Tafel region.

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The values of the cathodic slope (b_c) are slightly higher than those of mild steel (Wahdan et al., 2002), primarily the result of the presence of oxide film on the SS surface. The constancy of the b_c values indicates that the mechanism of hydrogen evolution is not changed in the absence and the presence of the inhibitor. The values of b_a decreased at high concentrations of the inhibitor, indicating change in the mechanism of metal dissolution.

The influence of TPy on the passive behavior of SS in H₂SO₄ solution is shown in Fig. 2A. The passivation parameters in the absence and presence of DPy and TPy are recorded in Table 2. The SS showed an active–passive behavior in the studied solutions. Presence of the inhibitor at any concentration did not affect the primary passive potential (E_pp) of the SS. However, the current corresponding to E_pp, the critical current density (i_cit), decreased gradually with an increase in the inhibitor concentration. i_crt is the maximum active anodic current of the SS after which a suppression occurs due to a surface oxide film formation. Presence of the inhibitor was found to decrease i_crt, facilitating oxide film formation, which is reflected in a more stable passive range and a lower passive current (lower oxide dissolution), as shown in Fig. 2B. Thus, suppression efficiency (SE) of the inhibitor is significant and could be obtained using Eq. (1) after replacement by i_crt. The SE of TPy is higher than that of DPy within the studied inhibitor concentration range, and is 46.5% and 66.8% in the presence of the maximum inhibitor concentration (1 mM) of DPy and TPy, respectively.

3.2 Weight loss measurements

Weight loss measurements of the SS in 0.5 M H₂SO₄ in the absence and the presence of different concentrations of the studied inhibitors were carried out at 30 °C and the results are reported in Table 3. As shown from the results, the corrosion rate of the SS decreased in the presence of the additives and the inhibition efficiency increased with an increase in the concentration of the inhibitor. The TPy compound provides greater inhibition efficiencies than those of DPy within the concentration range under study. The values of inhibition efficiencies calculated from the weight loss method are in good agreement with those calculated from the corrosion current by the polarization method.

3.3 Surface investigation

The SEM images of the SS samples after 12 h immersion in 0.5 M H₂SO₄ in the absence and the presence of 1 mM of the inhibitors under study are shown in Fig. 3. In the pure acid solution the steel suffered from general corrosion as shown in Fig. 3A, where highly attack on the entire surface is observed. The presence of the inhibitor almost prevented the corrosion; in case of the DPy inhibitor very slight attack appears in the medal of the image (Fig. 3B). In case of the TPy inhibitor the image (Fig. 3C) is free from any attack and the polish scratch of the sample is still appearing. These results confirm the polarization and weight loss methods.

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Figure 3

SEM images of SS samples after 12 h immersion in 0.5 M H₂SO₄ (A) and in the presence of 1 × 10⁻³ M inhibitor at room temperature; DPy (B) and TPy (C).

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3.4 Adsorption isotherm

The organic compounds inhibit the corrosion through its surface adsorption on the metal surface. The fraction of surface coverage (θ) by inhibitor molecules can be calculated from the equation:

Rearrangement gives:

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Figure 4

Langmuir isotherm for monolayer TPy inhibitor chemisorptions.

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The constant K is related to the standard free energy of adsorption $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ by the equation:

3.5 Effect of temperature

The effects of temperature on the inhibitive action of the studied inhibitors were performed. The polarization electrochemical measurements of the SS were carried out in 0.5 M H₂SO₄ and in the absence and presence of 1 × 10⁻³ M inhibitor at different temperatures (25–40 °C), and the results are recorded in Table 4. As expected the corrosion current increased with an increase in temperature; however the IE of both used inhibitors increased with temperature, indicating that the adsorption process of the inhibitors on the SS surface is endothermic. The SE of the two inhibitors did not largely change with temperature. These results reflect a strong adsorption of the studied compounds on the steel surface.

The dependence of corrosion current on the temperature can be expressed with Arrhenius equation:

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Figure 5

Arrhenius plots for SS in H₂SO₄ in the absence (○) and presence of 1 × 10⁻³ M inhibitor; DPy (□) and TPy (Δ).

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3.6 Quantum chemical calculations

Quantum chemical calculations were performed to explain the relationship between the molecular structure and the inhibitive action of the inhibitors under study. Fig. 6 presents quantum chemical parameters computed for the studied inhibitors, namely energies of the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and the energy gap (ΔE = E_LUMO−E_HOMO). Fig. 6 also presents the electrostatic potential map, the total optimized energy, and the dipole moment of the two inhibitors. As shown in Fig. 6, the HOMO in DPy is localized mainly around the π-framework. In contrast, the HOMO in TPy is localized largely on nitrogen atoms and around the aromatic system, which enhances electron donation from the TPy inhibitor to the metal surface. The data in Fig. 6 reveal that TPy has higher values for both E_HOMO and E_LUMO and a small ΔE (1.5 eV) value. The corresponding value of the energy gap (ΔE) for DPy is 5.5 eV. In principle a decrease in the energy gap leads to easier polarization of the molecule and greater adsorption on the surface (Fouda and Ellithy, 2009). Thus, a small ΔE in TPy facilitates adsorption, and enhances the efficiency of inhibition.

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Figure 6

Frontier molecular orbital of the inhibitors.

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The charge distribution in a molecule can provide critical insight into its physical and chemical properties. The map of the studied inhibitors is displayed in Fig. 7. The electrostatic potential map indicates the size and shape as well as charge distribution of the studied compound. The results from DFT calculations show that TPy has a more negative charge (red color) than DPy. Additionally, the planarity of TPy is more pronounced than DPy. These data clearly show that the negative charge density localized on N atoms and the center aromatic system in TPy is much greater than that in the other inhibitor, indicating that N atoms and π-aromatic are the negative charge centers that donate electrons to the metal in the steel to form a coordinate bond and π-metal interactions, respectively. The dipole moment values support this conclusion. We suggest that the higher value of the dipole moment for TPy (1.7 D) leads to an increase in inhibition, which could be related to the dipole–dipole interaction of molecules and metal surface. Fig. 6 shows a higher dipole moment for TPy in comparison to that of DPy inhibitor. These results explain the good adsorption on the steel surface and, thus, the inhibition efficiency for these compounds and particularly for TPy compared with that of the DPy inhibitor. These theoretical calculations are in good agreement with the experimental results.

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Figure 7

Electrostatic potential map of the inhibitors (red = negative charge and blue = positive charge).

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3.7 Inhibition process

In this Study, electrostatic adsorption is mostly the first stage of inhibition process which occurred between the cationic species (pyridinium ions) and the negative charged metal surface (Abd El-Maksoud, 2004). The orientation of the adsorbed organic molecules at the metal surface is significant. It has been reported that the surfactant molecules adsorb by its hydrophilic head directing its hydrophobic tail to the solution side (Zyauya and Dawson, 1994). But, the adsorption of the heterocyclic compounds occurs with the aromatic rings sometimes parallel to the metal surface. In case of the pyridinium compounds, Saleh explained that the pyridinium ring is in position parallel to the metal surface (Saleh, 2006). It is suggested that the molecules of multi-pyridinium rings are physically adsorbed via the positively charged nitrogen atoms and the aromatic rings are parallel to the surface and the methyl group tail is extended in the solution (Obaid et al., 2013). Thus the tetra-pyridinium (TPy) molecule will cover a larger area of metal surface than that of the dipyridinium (DPy) molecule. This is evident by larger inhibition efficiency of the first compound particularly at the lower inhibitor concentration (partially covered of the electrode surface).

The parallel position of the pyridine rings to the metal surface allows for transfer of the π-electrons of these rings to the empty d-orbital of the metal to form coordinate type of link. The chemisorption of TPy and DPy compounds is evident from the increase in inhibition efficiency with temperature, the lower value of E_a and the larger negative value of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ in the presence of the inhibitor compound. The DFT calculation confirmed the higher electronic donation and thus interaction for the molecules of these compounds with the metal surface, and the TPy molecules have more ability for chemisorption than that of DPy molecules.