Inhibitive action of some plant extracts on the corrosion of steel in acidic media
Introduction
The known hazard effects of most synthetic corrosion inhibitors are the motivation for the use of some natural products. Recently, Plant extracts have again become important as an environmentally acceptable, readily available and renewable source for a wide range of needed inhibitors. Plant extracts are viewed as an incredibly rich source of naturally synthesized chemical compounds that can be extracted by simple procedures with low cost. However, synergistic (and antagonistic) effects are often expected with these mixtures of inhibitors that may affect their inhibition efficiency. Several investigations have been reported using such economic plant extracts. El Hosary et al. [1] studied the corrosion inhibition of aluminium and zinc in 2 N HCl using naturally occurring Hibiscus subdariffa (Karkode) extract. The inhibition of corrosion of steel, aluminium and copper in HCl, H2SO4 and citric acid by molasses was also studied [2] and 83% and 13% inhibition efficiencies were obtained for HCl and H2SO4 solutions, respectively, containing 0.75% molasses. Loto reported the inhibitive action of Vernonia amygdalina (bitter leaf) on the corrosion of mild steel in 0.5 M HCl at 28 °C [3]. Avwiri and coworkers studied the inhibitive action of V. amygdalina on the corrosion of aluminium alloys in HCl and HNO3 at concentrations of 0.2 and 0.4 g/L at 29 °C [4]. They showed that the solution extract of the leaves serves as an excellent inhibitor. The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 20%, 50% and 88% aqueous orthophosphoric acid has been investigated by weight loss and electrochemical impedance spectroscopy (EIS). Plant extract was found to reduce the corrosion of steel more effectively in 88% than in 20% phosphoric acid [5]. An inhibition efficiency of 75.11% was observed with the extract of the leaves of Nypa fruticans Wurmb [6] for the corrosion of mild steel in hydrochloric acid solutions. El-Etre et al. examine a some naturally occurring substances as corrosion inhibitors for different metals in various environments [7], [8], [9], [10], [11].
Earlier, Barannik and Putilova [12] showed that the actual inhibitors in the plant extracts are usually alkaloids and other organic nitrogen bases, as well as carbohydrates, proteins and their acid hydrolysis products. The existing data show that most organic inhibitors act by adsorption at the metal/solution interface. This phenomenon could take place via (i) electrostatic attraction between the charged metal and the charged inhibitor molecules, (ii) dipole-type interaction between unshared electron pairs in the inhibitor with the metal, (iii) π electrons-interaction with the metal, and (iv) a combination of all of the above [13]. The adsorption process depends on the electronic characteristics of the inhibitor, the nature of the surface, the temperature and pressure of the reaction, steric effect, multilayer adsorption and a varying degree of surface site activity.
The aim of the present work is to test extracts of Chamomile, Halfabar, Black cumin, and Kidney bean as inhibitors for the acidic corrosion of steel and to discuss their inhibition mechanism.
Section snippets
Electrochemical tests
Electrochemical impedance and polarization curve measurements were achieved using Gill AC instrument. The frequency range for EIS measurements was 0.1–1 × 103 Hz with applied potential signal amplitude of 10 mV around the rest potential. The data were obtained in a three-electrode mode; platinum sheet and saturated calomel electrodes were used as counter and reference electrodes. The material used for constructing the working electrode was steel that had the following chemical composition (wt.%):
Potentiodynamic polarization results
Typical potentiodynamic polarization curves for steel in 1 M sulphuric acid in the absence and presence of different Halfabar extract concentrations are shown in Fig. 1. As seen, addition of Halfabar extracts affects both anodic dissolution of steel and cathodic reduction reactions indicating that the extracts could be classified as mixed-type inhibitors. In general, for acid solutions, when dissolved oxygen is present both hydrogen evolution and oxygen reduction reactions will be possible.
Conclusions
The plant extracts of Black cumin, Kidney bean, Chamomile and Halfabar can be used as excellent corrosion inhibitors for steel in acidic medium. To obtain the maximum protection efficiency, critical plant extract concentration should be determined. The inhibition mechanism depends on the formation of a stable plant extract-complex on the steel surface.
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