Ajayi, Olorunfemi Michael
(2019)
Corrosion inhibition of mild steel in 15 wt% HCl solution by synthetic and green compounds.
PhD thesis, University of Nottingham.
Abstract
Registration, Evaluation, Authorization, and restriction of CHemicals (REACH) legislation, in conjunction with Paris Commission (PARCOM) recommendations (1991), are driving research into environmentally friendly alternatives to currently-used corrosion inhibitors. The main focus of this thesis is on compounds of plant-origin because they contain components which can be extracted by simple procedures. These plant extracts are relatively benign in their environmental impact compared to the highly toxic compounds, including propargyl alcohol (PA) and 2- mercaptobenzimidazole (MI), which are currently in use to mitigate the corrosion of oil and gas wells during acidizing treatment.
The four ‘green’ corrosion inhibitors, chosen for this research, were extracts from durum wheat (DW), aloe vera gel (AV), inulin (IN; extracted from Jerusalem artichoke), and broccoli (BC).
The corrosion inhibition performance of PA, MI, DW, BC, IN and AV for mild steel (MS), immersed in 15wt% HCl solution in the absence and presence of the inhibitors, has been investigated. Weight loss and electrochemical (open circuit potential (OCP), potentiodynamic polarization and linear polarization resistance) were used to determine the corrosion inhibition performance of the inhibitors with a variation of concentration of the inhibitors, solution temperature (20-60°C) and immersion time (5- 192 h). Also, gas chromatography-mass spectrometry (GC- MS) and Fourier transform infrared (FTIR) spectroscopy were further employed to characterize all the chemical compounds to determine their main constituents and functional groups. The as-received MS surfaces were analyzed after immersion in uninhibited and inhibited solutions using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and 3D profilometry. X-ray photoelectron spectroscopy (XPS) was performed to determine the nature and composition of the protective film formed at the MS/HCl solution interface.
Potentiodynamic polarization results showed that all the studied inhibitors suppressed both the anodic dissolution of MS and cathodic hydrogen evolution – this implies that they can be classified as mixed- type corrosion inhibitors in the corrosive environment investigated.
Concentration studies showed that the corrosion rate of MS decreased with increase in the concentration of all the inhibitors until a point where a further increase in strength did not markedly change the corrosion rate. AV required significant concentration (100 g/L) in the electrolyte to be effective. The study also showed that the corrosion inhibition performance of PA was stable with an increase in temperature (over 99% IE), which indicated that the inhibition performance of PA in 15 wt% HCl was not temperature dependent. However, the corrosion inhibition performance of MI, DW, and BC decreased with an increase in solution temperature.
The inhibition performance of IN and AV decreased significantly with the increase in temperature especially at 60°C after 24 h immersion. At 20°C, the corrosion inhibition performance of PA (99%), MI (over 84%) and IN (80%) remained stable with prolonged immersion time (up to192 h). However, DW, BC, and AV decreased slightly at the onset but were later stable throughout the immersion time.
The adsorption of both the synthetic and ‘green’ inhibitors onto the MS surface followed the Langmuir adsorption isotherm. Thermodynamic parameters suggested that PA, MI and DW interacted with the MS surface at 20°C via both physical and chemical adsorption, while BC, IN and AV were physically adsorbed onto the MS surface.
SEM/EDS examinations revealed that the MS surfaces treated with different inhibitors were less corroded compared to the uninhibited surface. 3D profilometry showed that MS surfaces, after immersion in 15wt% HCl in the presence of various inhibitors, have lower surface roughness values. MS exposed to PA inhibited solution had the lowest surface roughness. XPS studies showed that the studied inhibitors adsorbed at the MS/solution interface, then formed a protective layer.
The corrosion inhibition performance of the synthetic inhibitors (PA and MI) were better than the ‘green’ inhibitors for the protection of MS in 15wt % HCl. However, DW and BC performed better than other ‘green’ inhibitors in all the experimental conditions studied. Hence, they may be promising candidate inhibitors. PA performed much better than all the inhibitors investigated, while AV poorly protected MS against corrosion in the environment/conditions studied.
The economic analysis of the inhibitors showed that DW and BC were cheaper than the commercial inhibitors (PA and MI), while IN and AV were costlier. The high concentration (100 ml/L HCl) and the cost of AV are pointers that it may not currently be economically viable on an industrial scale.
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