Novel thiophene symmetrical Schiff base compounds as corrosion inhibitor for mild steel in acidic media
Research highlights
► The aim of the present investigation is to examine the inhibitory action of two Schiff bases. ► Compounds are mixed type (cathodic/anodic) inhibitors. ► Inhibition efficiency (% IE) increases with increasing concentration of compounds. ► Adsorption of compounds on mild steel surface obeys Langmuir’s isotherm.
Introduction
The use of inhibitors is one of the most practical methods for protection against corrosion especially in acidic media [1], [2]. Acid solutions are generally used in several industrial processes. HCl is widely used in picking solutions. Because of their aggressiveness, the use of corrosion inhibitors is considered as the most effective method for the protection of many metals and alloys against such acid attack [3], [4], reduces the dissolution rate of metals. Compounds containing functional groups with heteroatoms, which can donate one pair of electrons, are found to be very efficient as inhibitors against metal corrosion in many environments. Many heterocyclic compounds with polar groups and/or π electrons are efficient corrosion inhibitors in acidic solutions. Organic molecules of this type can adsorb on the metal surface and form a bond between their N-electron pair and/or π electron cloud and the metal surface, thereby reducing the corrosion in acidic solutions [5], [6], [7]. Some Schiff bases inhibitors have been previously reported as effective corrosion inhibitors for various metals in acid media, such as Schiff bases containing thiophene substituents [8], [9] and furoin thiosemicarbazone [10].
The aim of the present investigation is to examine the inhibitory action of two Schiff bases containing nitrogen and sulphur het-eroaromatic compound as substituents in their structures for the corrosion of mild steel in 1 M HCl solution. Effects of concentrations and molecular structures on the inhibition efficiencies of the selected Schiff bases have been studied systematically. Differences in behaviour of inhibitors were explained based on structural properties of investigated inhibitors.
Section snippets
Experimental
Compounds L1 and L2 were synthesized according to published methods [11], [12]. Diamine (1 mM) (hot ethanol, 20 mL) was added dropwise 2 mM of thiophene 3-carbaldehyde (Fluka Chemical Company). These products are soluble in ethanol/H2O and DMF solutions. The structures of these products were confirmed by elemental analysis (service d’Analyse, ICSN- CNRS, Gif sur Yvette (France)) and FT-IR spectroscopy. The molecular structures of L1 and L2 are shown in Fig. 1. Elemental analyses: Anal. Cal. For L1
Tafel polarisation measurements
Fig. 2, Fig. 3 show anodic and cathodic polarisation plots recorded on mild steel electrode in 1 M HCl in absence and presence of different concentrations of L1 and L2 inhibitors. Table 1 shows the electrochemical corrosion kinetic parameters, i.e., corrosion potential (Ecorr), cathodic and anodic Tafel slopes (bc, ba) and corrosion current density icorr obtained by extrapolation of the Tafel lines. As it was expected both anodic and cathodic reactions of mild steel electrode corrosion were
Conclusion
The corrosion behaviour of mild steel was investigated in 1 M HCl with and without addition of various concentrations of Schiff bases at different concentrations, using potentiodynamic and electrochemical impedance techniques. Polarisation curves reveal that both compounds are mixed type (cathodic/anodic) inhibitors and inhibition efficiency (% IE) increases with increasing concentration of compounds. The adsorption of Schiff bases on mild steel in 1 M HCl solution obeys Langmuir adsorption
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