Investigation of UV-Induced geometrical isomerisation of helichrysum kraussii extracts and their inhibition potentials for acid corrosion of mild steel, zinc and aluminium

Abstract

Corrosion is the destruction of material due to chemical or electrochemical processes. Corrosion can be controlled by using corrosion inhibitors. Recently, plants have been explored as possible green corrosion inhibitors because they are environmentally friendly, easy to dispose, easily accessible and cheaper to extract or convert to inhibitors. Plant's metabolites or chemical constituents undergo geometrical isomerisation under the influence of UV light radiation. These changes might affect the extract's ability to inhibit corrosion. In this study, the effect of the UV light radiation on the Helichrysum kraussii (H. kraussii) plant extract was investigated through comparison with the non-UV-radiated H. kraussii plant extracts for mild steel, zinc and aluminium metal corrosion in 1.0 M HCl and H2SO4 at 30-60 ℃. This investigation was made possible by using techniques such as gravimetric weight loss analysis, Liquid chromatography-mass spectrometry (LCMS), Fourier transform infrared spectroscopy (FTIR), electrochemical analysis, contact angle, Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), and quantum molecular dynamic simulations. LCMS confirmed the formation of geometrical isomerisation when the H.kraussii extract was exposed to UV radiation. From the gravimetric analysis data, the corrosion rate (CR) was calculated and found to be decreasing with an increase in the concentration of the inhibitor extracts, with the highest concentration (1500ppm) at 30 ℃ having a corrosion rate of 3.92x10-4 and 1.70x10-4 g.cm2.h-1 for UV-radiated and non-radiated extract in 1.0 M H2SO4 for mild steel, respectively. Similar trends have been observed for the zinc and aluminium metals in HCl and H2SO4 solutions. The percentage inhibition efficiency (%IE) was found to increase with the increase in the two extract concentrations in the acidic media. In the presence of non-radiated extract, the % IE for mild steel in 1.0 M HCl was found to be 99.601% for 1500 ppm at 30 ℃. However, in the presence of UV-radiated H. kraussii extract, it was found to be 98.007%. The activation energy (Ea) values were found to increase with an increase in the concentration of both extract inhibitors. This suggests that the corrosion inhibition process of the three studied metals is through physisorption. The values of the change in Gibbs free energy (Δ𝐺𝑎𝑑𝑠𝑜) for both the extracts were found to be negative, indicating that the reaction between the inhibitor molecules and the metal surface was spontaneous. From the electrochemical impedance spectroscopy (EIS), the semi-circle diameter for mild steel and zinc metals and the circle diameter for the aluminium metal in the Nyquist plot was found to increase with an increase in the inhibitor concentration. The charge transfer resistance (Rct) values were found to increase with an increase in the extract inhibitor concentrations. For mild steel in 1.0 M H2SO4, the Rct values ranged between 8.751-26.52 Ωm2 and 10.20-26.41 Ωm2 for the UV-radiated and non-radiated extract, respectively. The introduction of the two extract inhibitors in the HCl and H2SO4 solution managed to reduce the corrosion current density (icorr) for mild steel, zinc and aluminium metals. The icorr values for mild steel in 1.0 M H2SO4 ranged between 397.202 -1395.884 mA.cm-2 and 343.358 – 1572.977 mA.cm-2 for the UV-radiated and non-radiated extracts. FTIR performed on the coated mild steel, zinc, and aluminium surface showed functional groups such as C=C, O-H, and N-H for both extracts, which are known to be functional groups of the most effective corrosion inhibitors. Contact angles were used to study the surface morphology of the metals, and it was observed that introducing the extract inhibitors in aggressive solutions changed the metal's surface from hydrophilic to hydrophobic surface. Computational simulations have been studied to demonstrate the most conceivable adsorption arrangements or configurations between the metal surface and the inhibitor. Parallel configuration of the inhibitor molecules on the metal surface was found to be the best configuration, giving high adsorption energy. SEM showed that introducing the two extract inhibitors in the acidic solution protected the metal surface from developing deep pits caused by the acid attack. EDS showed that corrosion took place on the metal surface and that the extract inhibitors were adsorbed on the metal surface based on the functional groups such as oxygen, calcium and nitrogen obtained on the metal surface. X-ray diffraction showed the presence of oxide on the metal surface confirming that the corrosion process took place. Based on the results obtained, it was found that the non-radiated H. kraussii extract was found to be more effective in inhibiting the corrosion of mild steel, zinc and aluminium metals in 1.0 M HCl and H2SO4 than the UV-radiated H. kraussii extract. This suggests that the UV radiation decreases the inhibition efficiency of the H. kraussii extract as an efficient corrosion inhibitor for mild steel, zinc and aluminium metals in 1.0 M HCl and H2SO4. Therefore, it is recommended that the H. kraussii extract be used only as a corrosion inhibitor for mild steel, zinc, and aluminium metals in 1.0 M HCl and H2SO4 in its original state without being exposed to UV radiation.