1. Introduction
Interaction of metallic alloy with aggressive ions (Cl-, SO42-, NO3-) and O2 through the coating defects; establishes the corrosion products, diminishing the lifetime of steel or copper components and durability of assets. This corrosion is of inevitable in definite to the aerospace, automotive, structural engineering, oil and gas (energy) industries.1-3 In order to avert the financial drainage and to combat the coating deterioration, the application of protective coatings is one of the proficient methods. Most extant literature reported that Ni-W coatings were considered as ennobling alternate to chromium based (hexavalent) coatings. The governing factors those demerits the hard chrome is: low propulsion power of chromium (hexavalent) ions and its toxicity in the electrolyte bath, high consumption of electricity, low cathodic current density, and crucially a high chance of failure and in execution due to its residual tensile stress and cracks.4,5 Its superlative properties made it to applicable in many technological applications like faucets, springs, magnetic heads and relays6 and in particular as an electrode for H2electrocatalysis.7 The traditional macroscopic scale experimental evaluations (Tafel slope, A.C. Electrochemical impedance spectroscopy, Weight loss measurements. etc) have paved a way in revealing the electrochemical behavior and kinetics of corrosion process occurring at the juncture of metal or alloy/corrosive solution. Nevertheless, the above mentioned techniques have left some caveats, which are to be dealt with. Researchers of corrosion science crave for in knowing the (i) potential correlation between the intrinsic electronic properties and inhibition mechanism, (ii) nature of reactive groups that aids in the organic adsorptions onto the surface of metals/alloys, and (iii) orientation of additive molecule over the alloy surface, which dictates the surface binding propensity.8 To an answer of above all, sophisticated and efficient computational simulations (DFT, MC, MD,RDF, Fukui indices etc) came into light to be dealt with the structure-reactivity relationship of additive/inhibitor9,10 in revealing the mechanism and the key role of each atoms or functional groups of the additive for electrodeposition11,12 and predominantly to corroborate single point corrosion experimental data.13,14 The accurate and precise information (molecular properties) like polarizability, excitation energies, electron donating ability, etc throws up a light in encoding the design of typical additive15,16 and intervene in exact synthesis strategy of additive molecule, cutting down the time and cost associated with exhaustive experimentations.17Furthermore, greenness and novelty of the said computational techniques aids in avoiding the consumption or release of reductant malicious substances into the ecosystem, contrasting empirical pathways.18
On the continuation of our previous work,19 herein we have theoretically investigated the corrosion behaviors of salen-type ligand (OPD and PPD) which comprising of ortho- and para-phenylenediamine tethered hydroxy naphthalene, respectively on Ni-W alloys in H2SO4 medium. The unique features of these OPD and PPD containing ample electron-rich aromatic centers (13 π-electrons) along with two nitrogen and oxygen atoms and their delocalization could improve the metal-inhibitor interactions via coordination bonding. In addition, by introducing isomeric spacers on salen-type ligands that prominently affects the numerous factors, such as electron-donating ability, nature of bonding, hardness/softness, rate of adsorption, molecular orientations, the distance between two layers (Ni-W and ligand), etc., and thus undeniably challenging to determine those issues from experimentally. Keep in mind, we have systematically examined the aforementioned issues with the aid of DFT, MC, MD, and RDF and thus compared with experimental results.