Inhibition Behaviour of Some Isonicotinic Acid Hydrazides on the Corrosion of Mild Steel in Hydrochloric Acid Solution

New corrosion inhibitors, namely, isonicotinic acid (1H-indol-3-yl-methylene)hydrazide (INIMH) and isonicotinic acid (1H-pyrrol-2-yl-methylene)hydrazide (INPMH), have been synthesized, and their inhibitive characteristics for the corrosion of mild steel in 0.5？M HCl were investigated by mass loss and electrochemical techniques. The structures of the synthesized compounds were confirmed using spectral studies. Potentiodynamic polarization studies revealed that the investigated inhibitors are of mixed type. Various thermodynamic parameters were evaluated. Langmuir adsorption isotherm was found to be the best description for both inhibitors. FTIR spectra, energy dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) were performed to characterize the passive film on the metal surface. 1. Introduction Mild steel is the most important engineering material particularly for structural, instrumental, industrial, and automobile applications. Corrosion problem occurs in these industries and can cause disastrous damage to metal and alloy structures causing economic consequences in terms of repair, replacement, and product losses. Pickling is a treatment on metallic surfaces in order to remove tightly adherent oxide films, stains, rust, or scale resulting from hot forming, heat treating, welding, and other high temperature operations [1]. The two acids commonly used in pickling are hydrochloric acid or sulphuric acid or it may be a combination of acids and may also contain nitric or hydrofluoric acids. Once the acid is rinsed out, the freshly cleaned steel will be very prone to oxidation (corrode) unless we apply a corrosion inhibitor of some type [2]. Therefore, a wide variety of corrosion inhibitors ranging from rare earth elements [3, 4] to organic compounds [5–8] have been used. The use of inhibitors is one of the practical methods for preventing corrosion of mild steel especially in acid media [9]. Inhibitors protect the metal by adsorbing on the surface and retard metal corrosion in aggressive environment. Selection of an appropriate inhibitor for specific environment and metal is of great importance. Generally, the heterocyclic organic compounds having higher basicity and electron density on the heteroatoms like oxygen, nitrogen, and sulphur have a tendency to resist corrosion [10, 11]. It has been known that efficient inhibitors should possess plentiful pi-electrons and unshared electron pairs on either nitrogen atoms or sulfur atoms of the inhibitors to the d-orbital of iron. The adsorption characteristics of organic molecules are also