ABSTRACT
Thiourea and its derivatives have been studied for several decades, for their corrosion inhibition characteristics with steels. However, their interaction mechanisms with steel are not well understood. Such an understanding may be vital for design of novel thiourea derivatives as corrosion inhibitors for steels. We have carried out state of the art density functional theory (DFT) computations and experiments (weightloss, polarization and electrochemical impedance) to understand the effect of thioureas on corrosion of steel. The interactions of thiourea and dibutyl thiourea separately on an iron surface have been investigated using DFT. DFT indicates strong interactions between Fe atoms of the surface and C, S - atoms of the molecule while the N-atoms do not show any significant proclivity towards interactions with the Fe-surface. Among the hollow and bridge sites on the Fe-surface, S-atom was found to preferentially occupy the hollow site which facilitates formation of four strong Fe-S bonds which possibly explains the strong corrosion inhibition by thioureas on steels. Besides, DFT computations provided insights for design of much more promising molecules for corrosion inhibition. Experimentally, dibutyl thiourea was found to have an extraordinary inhibition effect for steel in 1N HCl in accordance with DFT computations.
INTRODUCTION
Of all metals and alloys that are used widely in different industries, commercial and domestic applications, iron alloys have their own stature for their versatile usage. Corrosion is one of the important factors that affect the lifetime and service of the metallic component. The deterioration of the material as it forms thermodynamically most stable form due to its interaction with the surrounding conditions is corrosion. It is necessary to make an effective and efficient usage of materials and resources for sustainable lifestyle, often reminded by the ever-advancing Earth Overshoot Day. Metal and its processing not only involves the use of precious resources like fuel, water, time and energy but also adds to the pollution of ecology. Therefore, any increment in the lifetime of metal in service will reduce the deleterious effects on resources, and improve the efficiency of usage of resources. There are several ways to extend the life i.e. to reduce the corrosion rate of the metallic component, of which, usage of inhibitors is the point of interest in this work. Inhibitors are the molecules that delay the corrosion of metal through one or more mechanisms such as inhibiting either of the cathodic and anodic reaction, either by adsorption on to the active sites; or by elevating the activation energy for reaction; or by reducing the availability of reactive species for the reactions; or even by simply blocking the reaction sites.
