Corrosion Behavior and Adsorption Thermodynamics of Some Schiff Bases on Mild Steel Corrosion in Industrial Water Medium

The inhibition performance and adsorption behavior of (E)-2-(3-nitrobenzylidene) hydrazine carbothioamide (SB1) and (E)-2-(4-(dimethylamino) benzylidene) hydrazine carbothioamide (SB2) on mild steel corrosion in industrial water medium have been investigated by gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results revealed that inhibition efficiency depends on both the concentration of the inhibitors and temperature of the system. Increasing temperature reduces the inhibition efficiency of both inhibitors. Polarization studies indicated that these compounds behave as mixed type of inhibitors. The adsorption of both inhibitors was spontaneous and followed Langmuir adsorption isotherm. Thermodynamic parameters are calculated and discussed. The relation between inhibition efficiency and molecular structures of SB1 and SB2 was discussed by considering quantum chemical parameters. The surface adsorbed film was characterized by scanning electron microscopy (SEM). 1. Introduction Mild steel (MS) is the widely used material in the fabrication of heating and cooling water system in many industries. Therefore, study of corrosion inhibition of mild steel in industrial water medium is a subject of technological importance. Organic compounds which are extensively used in several industries as corrosion inhibitors in various environments [1–3]. The anticorrosive property of these inhibitors depends on the specific interaction between certain functional groups in the inhibitors with the active centers on the metal surface [4–7]. Hetero atoms such as nitrogen, oxygen, and sulphur present in the inhibitors play an important role in this interaction by donating lone pair of electrons [8–12]. Hence the organic compounds containing these hetero atoms and multiple bonds behave as efficient corrosion inhibitors due to the availability of π-electrons for interaction with the metal surface [13]. The compounds containing an azo methine group (–C=N–) in their skeleton (Schiff bases) are the best examples for this type of corrosion inhibitors. They can be synthesized by condensation of primary amines with carbonyl compounds [14]. Schiff bases were reported as effective corrosion inhibitors for mild steel, copper, and aluminum in various media [15–24]. Although most of the commercial corrosion inhibitors are synthesized by using aldehydes and amines as main components, usually these Schiff bases possess more inhibition efficiency than their constituent carbonyls and amines [25, 26]. The higher inhibition efficiencies of