Electronic effect vs. Molecular size effect: Experimental and computational based designing of potential corrosion inhibitors

Predicting of corrosion inhibition potential of organic compounds from the outcomes of experimental and computational studies is gaining meticulous interest. A proper understanding of substituents or electronic effect helps in designing of successful inhibitors before their experimental trials and expensive as well as hazardous syntheses. Recently, worldwide research in corrosion science is intended for the designing of such effective molecules based on the knowledge derived from previously published reports. Addition of polar substituents in the molecular structure of organic compounds enhances their hydrophilicity thereby the corrosion inhibition potential. Nevertheless, despite of their solubility enhancement property, some of the substituents, especially electron withdrawing (EW) substituents, decrease the corrosion inhibition potential. Generally, the addition of polar substituents brings two types of effect, namely, electronic or substituents effect that concerns the change in electron density at donor sites and molecular size effect which talks about the effect of molecular size on metal surface coverage capabilities. The present review article is intended to describe in what conditions electronic effect and/or molecular size effect will operate. The effect on solubility, orientation and hydrophilicity/ hydrophobicity of organic corrosion inhibitors is described herein in association with their corrosion inhibition performance.