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

Abstract Study of the synergistic inhibition properties of the combined admixture of 4-methyl-norvalin and 2-methoxy-4-formylphenol (non-toxic organic compounds) on the corrosion of P4 low carbon mold steel in 1.5 M H2SO4 and HCl solution was performed with weight loss measurement, potentiodynamic polarization and optical microscopy analysis. Results obtained showed the inhibition efficiency of the admixed compound performed above the value for effective corrosion inhibition at very low concentrations in H2SO4 solution. However, from mid concentration to the highest concentration studied, the inhibition efficiency of the compound was above 85% from electrochemical tests. In HCl solution the inhibition efficiency of the compound was well above 80% and 90% for weight loss and polarization tests at all concentration studied. Anodic corrosion inhibition mechanism was determined for the compound in H2SO4 according to Freundlich and Frumkin adsorption isotherms due to weak Van der Waals attraction between the inhibitor molecules and the steel’s surface. The mechanism of corrosion inhibition in HCl was calculated to be cathodic type according to Langmuir isotherm model.Morphological images of inhibited and non-inhibited steel surfaces significantly contrast each other though the extent of deterioration was much severe for the steel from H2SO4 solution.


Introduction
Corrosion, the chemical or electrochemical deterioration of engineering materials due to reaction with their environments has been a recurring problem for decades. The problem is responsible for the huge financial cost required for repair, replacement and maintenance of industrial parts, components, structures, equipment's etc. that are prone to deterioration [1,2]. Corrosion of metallic alloys is of importance due to their extensive industrial applications especially in environments where aqueous corrosion reactions are present. Carbon steel combines excellent mechanical properties with good formability, availability and low cost as a result it is the most widely used steel. However, it has very weak resistance to corrosion, thus the need for cost effective replacement with steels of higher corrosion resistance in addition to effective corrosion control method. P4 low carbon mold steel, a tool grade steel with extensive application in plastics processing industry has improved thermal conductivity, high levels of toughness and hardness, machinability and good corrosion resistance [3]. At temperature beyond 165 ∘ C hydrogen chloride gas permeates from PVC, combining with atmospheric moisture and resulting in HCl acid. This and other highly corrosive media can corrode the steel [4]. However, the presence of Cr and Mo at 5% and 0.4% composition within this steel offers improved corrosion resistance compared to the traditional carbon steels [5][6][7]. The limited presence of chromium causes the formation of passive protective film against the attack of corrosive anions while Mo offers improved resistance to localized corrosion especially pitting [8][9][10][11][12]. In addition to this, use of chemical compounds known as corrosion inhibition can complement the protective ability of the alloying elements on the low carbon mold steel. Corrosion inhibitors tend to counteract the electrochemical action of corrosive species in aqueous environments. Depending on the category of the inhibitor they act by film formation, selective precipitation and/ or alteration of the  corrosive environment [13]. Research on green corrosion inhibitors shows it acts through film formation on carbon steels with good inhibition protection [14][15][16][17]. In contribution to research on corrosion prevention through appropriate material selection and chemical control, this research aims to study the synergistic combination effect of 4-methyl-norvalin and 2-methoxy-4-formylphenol organic derivatives on the corrosion of P4 low carbon mold steel in dilute H 2 SO 4 and HCl solution.

Experimental method
P4 low carbon mold steel (PLCS) sourced from the open market gave nominal (wt.%) composition shown in Table 1, after analysis at the Materials Characterization Laboratory, Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria. The low carbon mold steel is of cylindrical configuration (diameter, 1.6 cm). The steel was machined, sectioned to average thickness of 0.6 cm, and grinded with silicon carbide papers (80, 320, 600, 800 and 1000 grits) before washing with distilled water and acetone. 4-methyl-norvalin purchased from Sigma Aldrich, USA is a white powdery substance with molecular formula of C 6 H 13 NO 2 and molar mass of 131.17 g/mol. Its molecular structure is shown in Figure 1(a). 2-methoxy-4-formylphenol is a phenolic aldehyde organic powdery compound with the molecular formula of C 8 H 8 O 3 and molar mass of 152.15g/mol. Its molecular structure is shown in Figure 1(b). Their combined admixture (MTH) in ratio 1:1 was prepared into 0.5g, 1g, 1.5g, 2g, 2.5g and 3g which were each added to 200 ml of 1.5 M H 2 SO 4 and HCl solution to give volumetric concentrations (%) of 0%, 0.25%, 0.5%, 0.75%, 1.0%, 1.25% and 1.5% MTH. Potentiodynamic polarization analysis was performed at 30 ∘ C with a threeelectrode system within glass cell containing 200 mL of the electrolyte solution at specific MTH concentrations. The electrodes which consist of resin embedded PLCS electrodes with exposed surface area of 1.13 cm 3 were connected to Digi-Ivy 2311 potentiostat. Polarization plots obtained at scan rate of 0.0015 V/s between potentials of −2 V and +1 V. Weight loss measurement of PLCS specimens was performed in 200 mL of the acid solution for 240 h at 24 h interval. Graphs of corrosion rate and inhibition efficiency versus exposure time were plotted from the data obtained during the exposure hours. Optical images of corroded and inhibited PLCS morphology were analysed after weight loss with Omax trinocular metallurgical microscope to produced photomicrographs with magnification of ×40.   ure 3(b)] throughout the exposure hours. The inhibiting action of MTH also proves to be more stable in HCl than in H 2 SO 4 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 H 2 SO 4 , it nevertheless signifies active surface deterioration of the steel.

Potentiodynamic polarization studies
Corrosion polarization studies performed on PLCS in 1.5M H 2 SO 4 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 H 2 SO 4 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 SO 2− 4 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.
The polarization plot in Figure 4 shows similar cathodic polarization behavior confirming that the mecha- nisms 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 H 2 SO 4 acid media. Cathodic shift in corrosion potential was observed with increase in MTH concentration, compared to the anodic shift in H 2 SO 4 . 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 H 2 SO 4 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 H 2 SO 4 and cathodic type in HCl [18].

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 H 2 SO 4 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.
log θ = n log C + log K ads (2) 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]. log 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.
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, ∆G o ads evaluated 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 H 2 SO 4 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  show the spontaneity and stability of the adsorption mechanism. In H 2 SO 4 solution the ∆G o ads values obtained depicts physisorption adsorption mechanism. ∆G o ads values 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.

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 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

Conclusion
The combined admixture of 4-methyl-norvalin and 2methoxy-4-formylphenol effectively inhibited the corrosion of P4 low carbon mold steel in H 2 SO 4 and HCl solution. Observation of the inhibition efficiency values with respect to exposure time shows the inhibition performance of the organic compound was concentration and time dependent in H 2 SO 4 while in HCl its performance is independent of these parameters. Calculated results from statistical derivations of the adsorption isotherms shows the adsorption of the inhibitor molecules on the adsorbent steel surface were exceptionally weak indicating surface coverage through film formation is the inhibition mode in H 2 SO 4 while in HCl statistical derivation shows physiochemical interaction in addition to weak chemical bonding occurred between the inhibitor molecules and the ionized steel surface.