Inhibition effect of the synergistic properties of 4-methyl-norvalin and 2-methoxy-4-formylphenol on the electrochemical deterioration of P4 low carbon mold steel

3.1 Weight-loss analysis

Inhibition study of specific concentrations of MTH inhibiting compound on PLCS corrosion in 1.5M H₂SO₄ and HCl solution was performed for 240 h. Data for weight-loss (ω_L), corrosion rate (C_R), and MTH inhibition efficiency (ξ_F) after at 240 h of exposure are shown in Table 2. Figures 2(a) and 2(b) show the plots of PLCS corrosion rate and MTH inhibition efficiency versus exposure time in H₂SO₄ and HCl solution. The corrosion rate of PLCS at 0% MTH in H₂SO₄ solution [Figure 2(a)] was significantly higher than the values obtained for PLCS at specific MTH/H₂SO₄ solutions due to the electrochemical action of SO₄^2-species within the electrolyte. The corrosion rate value at 24 h (10.96 mm/y) increased significantly to 22.98 mm/y at 120 h, after which it remained generally stable till 168 h before declining to 18.70 mm/y at 240 h. The presence of MTH compound at lower volumetric concentrations (0.25% - 0.75% MTH) significantly reduced the corrosion rate values of PLCS in comparison to values obtained at 0% MTH. However, the corrosion rates of PLCS at these concentrations continued to increase significantly from onset to values between 15.07 and 10.66 mm/y at 240 h. These values show MTH compound between 0.25% and 0.75% MTH has no inhibiting action on the corrosion of PLCS other than to interfere with the corrosion reaction mechanisms. Further increase in MTH volumetric content to values between 1% and 1.5% drastically decreased the corrosion rate of PLCS to values between 2.95 mm/y and 2.47 mm/y at 24 h, and 0.99 mm/y and 0.82 mm/y at 240 h. This observation is due to the presence of sufficient molecules of MTH to counteract the electrochemical action of SO₄^2-anions on PLCS surface. The observation also shows MTH inhibiting action in H₂SO₄ is concentration dependent. Figure 3(a) shows the inhibiting action of MTH in H₂SO₄ at 1% to 1.5% MTH concentration is generally stable after 72 h of exposure till 240 h. The MTH compound performed more effectively in HCl solution with inhibition efficiency above 90% at all concentrations studied [Figure 3(b)] throughout the exposure hours. The inhibiting action of MTH also proves to be more stable in HCl than in H₂SO₄ throughout the exposure hours. It is also observed that the performance of MTH compound is independent of concentration. PLCS at 0% MTH/HCl [Figure 2(b)] deteriorated rapidly with the corrosion rate reaching 1.280 mm/y at 240 h. Though this value is quite lower than the value obtained in H₂SO₄, it nevertheless signifies active surface deterioration of the steel.

Figure 2

Plots of PLCS corrosion rate versus exposure time at 0% - 1.5% MTH inhibiting compound (a) in 1.5M H₂SO₄ and (b) in 1.5M HCl

Figure 3

Plots of PLCS inhibition efficiency versus exposure time at 0% - 1.5% MTH inhibiting compound (a) in 1.5M H₂SO₄ and (b) in 1.5M HCl

3.2 Potentiodynamic polarization studies

Corrosion polarization studies performed on PLCS in 1.5M H₂SO₄ and HCl solution at 0%, 0.25%, 0.5%, 0.75%, 1%, 1.25% and 1.5% MTH inhibiting compound resulted in the potentiodynamic polarization plots shown in Figure 4 and 5. Table 3 shows the polarization data. In H₂SO₄ solution, addition of MTH at 0.25% concentration shifts the corrosion potential of PLCS in the anodic direction from −0.287V_Ag/AgCl (0% MTH) to −0.243V_Ag/AgCl. This phenomenon is due to partial surface coverage of PLCS by MTH inhibitor molecules which counteracts the electrochemical action of SO₄^2-anionic species. The change in corrosion potential corresponds to change in corrosion rate from 9.48 mm/y (0% MTH) to 6.80 mm/y (0.25% MTH) resulting in very low inhibition efficiency of 28.32%. Further increase in MTH concentration caused alternating shifts in corrosion potential within anodic inhibition processes. Increased in inhibition efficiency also occurred, attaining an optimal value of 92.87 at 1.5% MTH concentration. The visible decrease in corrosion rate values corresponds to decrease in corrosion current and increase in polarization resistance with increase in inhibitor concentration. Observation of the Tafel slopes shows variation in MTH concentration has not significant influence on the cathodic reaction process compared to the anodic counterpart. This supports the discussion of anodic inhibition from observation of changes in corrosion potential values.

Figure 4

Polarization plots for PLCS at 0% - 1.5% MTH/1.5 M H₂SO₄ solutions

Figure 5

Polarization plots for PLCS at 0% - 1.5% MTH/1.5 M HCl solutions

The polarization plot in Figure 4 shows similar cathodic polarization behavior confirming that the mechanisms of hydrogen evolution and oxygen reduction reaction are not affected. Similar anodic polarization behavior before passivation processes also occurred. The passivation plot of PLCS at 0% MTH passivated briefly from 0.49V to 1.11V, producing a short passivation range before failure. This phenomenon is associated with pitting corrosion behavior due to the formation of microscopic pits at sites where the passive films breaks. The passive film on the plot shows acute instability from the visible current transients. The passivation region of the polarization plots of PLCS from 0.25% MTH to 1.5% MTH increased significantly compared to the plot for PLCS at 0.25% MTH due to formation of protective film by MTH molecules on PLCS surface. This hindered the formation of corrosion pits. The increase in passivation region signifies higher corrosion resistance of PLCS in the presence of MTH concentrations. The visible current transients on the passivation plot of PLCS at 0% MTH synonymous with unstable passivation decreased significantly at 0.25% MTH, beyond which it completely disappeared due to stability of the passive film.

The corrosion polarization behavior of PLCS in HCl/MTH solution [Figure 5] contrasts its behavior in H₂SO₄ acid media. Cathodic shift in corrosion potential was observed with increase in MTH concentration, compared to the anodic shift in H₂SO₄. The shift is associated with selective precipitation of MTH molecules on PLCS surface whereby the cathodic reaction mechanism of the corrosion process is inhibited. Changes in cathodic Tafel slope values confirm this proposition. While the corrosion rate of PLCS in HCl is relatively smaller than the value obtained in H₂SO₄ at 0% MTH, it significantly differs from the values obtained at specific concentrations of MTH. The largest difference in corrosion potential of PLCS (in both acids) from 0% MTH shows MTH ia an anodic type inhibitor in H₂SO₄ and cathodic type in HCl [18].

3.3 Adsorption isotherm and corrosion thermodynamics

The nature of intermolecular reaction among protonated MTH molecules in interaction with PLCS surface can be further understood through mathematical models known as adsorption isotherms. Adsorption isotherms are graphical plots between the amounts of MTH molecules adsorbed on the surface of PLCS at constant temperature. The plots explain the sorption type or adhesion of MTH molecules to PLCS surface with respect to corrosion inhibition. In H₂SO₄ solution the intermolecular reaction and adsorption of protonated MTH obeyed the Freundlich isotherm according to equation 1 and 2 with correlation coefficient of 0.8680, and the Frumkin isotherms with correlation coefficients of 0.8530 according to equation 3. Figure 6(a) and 6(b) show the Freundlich and Frumkin isotherm plots.

Figure 6

(a) Freundlich isotherm plot, and (b) Frumkin isotherm plot

n is a constant with respect to the characteristics of adsorbed MTH molecule, K_ads is the adsorption-desorption equilibrium constant representing the intermolecular strength of the adsorbed layer. Freundlich isotherm describes the relationship between adsorbed molecules, their interaction (repulsion or attraction) and influence on the adsorption mechanism [19, 20].

Frumkin isotherm assumes total coverage of PLCS surface at high MTH concentrations for non-homogeneous materials and the effect of lateral interaction is significant. This involves considering the functional sites on the adsorbent surface.

In HCl, MTH obeyed the Langmuir isotherm with correlation coefficient of 0.9977 according to equation 4. Figure 7 shows the Langmuir isotherm plots.

Figure 7

Langmuir isotherm plot

The Langmuir isotherm assumes the intermolecular reaction on the metal surface is constant, the value Gibbs free energy is independent of the degree of molecular coverage and there is no lateral interaction effect resulting from the molecular reaction of adsorbates on the value of Gibbs free energy [21].

The physiochemical nature and adsorption strength of MTH intermolecular reaction on PLCS surface was determined from thermodynamic calculations. Data on Gibbs free energy, ΔGadsoevaluated from equation 5 is shown in Table 4 [22].

where 55.5 is the molar concentration of water in the solution, R is the universal gas constant, T is the absolute temperature and K_ads is the equilibrium constant of adsorption. K_ads for MTH adsorption on PLCS in H₂SO₄ solution was determined from the Freundlich isotherm due to its higher correlation coefficient value while K_ads for MTH adsorption on PLCS in HCl solution was determined from the Langmuir isotherm. The negative values of ΔGadsoshow the spontaneity and stability of the adsorption mechanism. In H₂SO₄ solution the ΔGadsovalues obtained depicts physisorption adsorption mechanism. ΔGadsovalues obtained for MTH inhibition in HCl solution shows physiochemical adsorption mechanism whereby molecular attraction and corrosion inhibition is through weak Van der Waals forces, and mild electrostatic attraction and covalent bonding.

3.4 Optical microscopy studies

Microscopic observations of PLCS (mag. ×40) before corrosion and after corrosion at 0% and 1.5%MTH in both acids are shown from Figure 8 to Figure 10(b). The morphological deterioration of PLCS at 0% MTH in H₂SO₄ and HCl solution where quite extensive due to the debilitating action of SO₄^2-and Cl⁻ anions [Figure 9(a) and 9(b)]. However, extent of deterioration in Figure 9(a) is much more than Figure 9(b) which confirms the corrosion rate results from potentiodynamic polarization test. While the deterioration in Figure 9(a) appears to be general surface deterioration in addition to the presence of numerous micro pits, the deterioration in Figure 9(b) shows the presence of fewer but more defined micro pits due to the electrochemical action of chlorides whose corrosive actions tends to be localized on PLCS surface. At 1.5% MTH the deteriorating effect of the anionic species on PLCS subsided due to the inhibiting action of protonated MTH molecules as shown in Figure 10(a) and Figure 10(b). The visible micro pits prevalent in Figure 9(a) substantially reduced in Figure 10(a) in addition to a less deteriorated morphology, while the micro pits shown in Figure 9(b) where generally absent in Figure 10(b). The few corrosion pits in Figure 10(b) appears shallow due to the inhibition action of MTH which arrested the formation of corrosion pits in Figure 9(b).

Figure 8

Morphology of PLCS before corrosion test

Figure 9

Morphology of PLCS after corrosion at 0% MTH (a) 1.5 M H₂SO₄ and (b) 1.5 M HCl

Figure 10

Morphology of PLCS after corrosion at 1.5% MTH (a) 1.5 M H₂SO₄ and (b) 1.5 M HCl