Copper, a highly versatile metal, is prized for its exceptional conductivity, malleability, and affordability, leading to its extensive use across numerous industries, including machinery manufacturing, marine, defense, construction, and transportation [1,2]. However, a significant challenge arises from its susceptibility to corrosion in various environments, such as acidic, alkaline, and saline solutions. Corrosion, an electrochemical process, gradually degrades metals, resulting in substantial economic losses [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. Corrosion causes significant economic losses in the chemical industry due to equipment damage, downtime, and safety hazards. Copper piping and storage tanks are used to transport and store various chemicals. Corrosion can lead to leaks and contamination, compromising the integrity of chemical processes [12].

While various protective measures exist, including coatings and cathodic protection, the use of corrosion inhibitors (CIs) offers a cost-effective and convenient solution. Corrosion inhibitors (CIs), typically organic compounds containing heteroatoms such as nitrogen, sulfur, phosphorus, and oxygen, function by interacting with the metal surface to form a protective barrier, thereby mitigating the corrosion process [13,[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. Extensive research has explored the inhibitory properties of numerous organic compounds, demonstrating the crucial role of heteroatoms and their associated electron-rich environments in enhancing corrosion resistance [[28], [29], [30], [31], [32]].

Even though metals are not entirely immune to corrosion, these agents (inhibitors) can interact with the metal surface and form a protective layer, thereby mitigating this phenomenon. By interacting with the metal surface, these substances create a barrier that effectively lessens the corrosive attack [19]. A comprehensive body of research has explored the inhibitory potential of a diverse array of aliphatic and aromatic organic compounds. Notably, all organic molecules incorporating heteroatoms – including nitrogen, sulfur, phosphorus, and oxygen – have demonstrated remarkable efficacy in inhibiting corrosion [24].

Some examples of compounds that have been experimentally and theoretically studied include carbazone, thiourea [32], pyrazole derivatives [33], pyrimidine, 1,5-benzodiazepin-2-one [34], hydrazide [35], coumarin [32], poly(ethyleneimine) [36], 5-(3-aminophenyl)-tetrazole [37], 3-amino-1,2,4-triazole-5-thiol [38], pyrimidinone [39], 2-[(2-methylquinolin-8-yl) oxy], acetohydrazide [40], and 5-(4-methylpiperazine)-methylquinoline-8-ol [41].

Computational chemistry has emerged as a valuable and indispensable tool in the study of corrosion inhibition, offering a rapid, cost-effective, and reliable means of investigating the mechanisms by which organic molecules impede this process [[42], [43]]. This approach provides crucial insights into the inhibition mechanisms that are often inaccessible through experimental methods alone. Theoretical investigations of corrosion inhibition mechanisms often employ a combination of density functional theory (DFT) methods and molecular mechanics-based approaches, such as molecular dynamics (MD) simulations [[34], [35], [36]]. These computational techniques offer a powerful framework for understanding the inhibitory behavior of various molecules. Quantum chemical calculations, in particular, serve as invaluable instruments for elucidating the molecular processes underlying corrosion inhibition. They provide a deeper understanding of adsorption phenomena and the associated electronic effects, offering valuable insights into the intricate interplay between the inhibitor molecule and the corroding surface.

Hydrazides, a family of organic compounds, possess a unique structure featuring a nitrogen-nitrogen bond (R-NH-NH2). Within this framework, R represents an acyl, sulfonyl, or phosphoryl group. Hydrazides and their derivatives exhibit a diverse array of biological and chemical properties. Their potential as therapeutic agents is particularly intriguing, with demonstrated antibacterial, anti-inflammatory, anti-cancer, anti-platelet, anti-malarial, analgesic, and antioxidant activities. Beyond their medicinal promise, hydrazides have proven to be effective corrosion inhibitors, offering robust protection against the degradation of numerous metals, including the ubiquitous mild steel. Thus, this class of compounds emerges as an attractive area of research, promising significant contributions to both human health and mitigating corrosion in industrial applications [[44], [45], [46]]. Ahmed et al. reported iron corrosion in sodium chloride solutions (3.5 % by weight) and its inhibition with oxalohydrazide (OXH) (or oxalyldihydrazide). They found that the presence of EH in the chloride solution significantly reduces the acuteness of the chloride, especially the pitting corrosion. They found that the presence of OXH in the chloride solution significantly reduces the severity of chloride-induced pitting corrosion [47]. Mabrouk et al. synthesized a novel inhibitor, N1, N2-bis [1-(pyridine-2-yl) ethylidene] oxalohydrazide (NOL), and investigated its corrosion inhibition properties on stainless steel 304L immersed in 1 M hydrochloric acid. The researchers employed experimental techniques, including mass loss, potentiodynamic polarization, and electrochemical impedance spectroscopy, to assess the inhibitor's performance. Results indicated a concentration-dependent enhancement of inhibition efficiency, reaching a maximum of 95.7 % at the highest NOL concentration tested. Complementary theoretical studies were conducted to elucidate the adsorption behavior of NOL on the metal surface under acidic conditions [48].

Researchers, such as Quraishi et al., have investigated the corrosion-inhibiting effects of some aromatic hydrazides on mild steel in hydrochloric acid. At all concentrations tested in this investigation, all of these substances inhibited mild steel corrosion in an HCl absolution. Additionally, it has been noted that when concentrations of all these chemicals rise, so does their inhibitory efficiency [11]. A study investigated the inhibitory performance and adsorptive behavior of 4-hydroxy-N-[(E)-(1H-indole-2-ylmethylidene)] benzohydrazide (HIBH) on mild steel in a 1 M HCl solution. HIBH demonstrated exceptional corrosion inhibition properties for mild steel immersed in 1 M hydrochloric acid solution. Its performance improved significantly with increases in both concentration and temperature. At optimal concentrations, HIBH achieved inhibition efficiencies exceeding 90 % across all studied temperatures [[49], [50]]. Mazlan et al. investigated the effect of a series of hydrazide derivatives as efficient corrosion inhibitors (CIs) on the iron metal surface (1 1 0) using MD simulations. Their results showed that both increasing temperature and using a low concentration of inhibitor (0.04 M) enhanced inhibitory efficacy. In this study, DFT calculations showed a good correlation with experimental and molecular dynamics simulation. Molecular quantum chemical calculations revealed that the reactive sites mainly from N and O-atoms affected the inhibition efficiency of CIs on the corrosion of Fe (1 1 0). The adsorption mechanism in terms of energy, aggregation, and self-assembly of CIs on mild steel surface in bulk aqueous met the Langmuir isotherm expectation [35]. In a study, Yu et al. investigated the anti-corrosion performance of three pyrazolopyrimidine derivatives, namely 4-aminopyrazolopyrimidine (APP), 4-hydroxypyrazolopyrimidine (HPP), and 4-mercaptopyrazolopyrimidine (MPP), using molecular dynamics simulation and DFT calculations. All inhibitors are oriented nearly parallel to the copper surface. This parallel orientation minimizes copper corrosion by maximizing the bonding area between inhibitor molecules and the copper surface. In this study, the calculated interaction energies for MPP, HPP, and APP were −608, −280.4, and −263.3 kJ/mol, respectively. The order of copper inhibition power among these inhibitors was MPP > HPP > APP [4]. In 2011, Kokalj et al. used density functional theory calculations to study the gas-phase adsorption of triazole, benzotriazole, and naphthotriazole molecules in copper. The compounds exhibited the following order of adsorption strength on the copper surface: triazole > benzotriazole > naphthotriazole [51]. Also, it has been reported that imidazole, 1,2,4-triazole, and their 1-methyl derivatives are copper corrosion inhibitors in NaCl solution through corrosion experiments and DFT calculations of their bonding to copper (111). Deprotonated triazole exhibited significantly stronger adsorption than 1-methyltriazole, whereas deprotonated imidazole showed only a more exothermic free energy of adsorption in the aqueous phase compared to neutral 1-methyl-imidazole. These findings suggest that triazole should be a more effective inhibitor than 1-methyl-triazole, while imidazole and 1-methyl-imidazole might exhibit similar efficacies [52]. In another study, the corrosion inhibitor performance of 4-hydrocoumarin derivatives was investigated in detail by Hadisaputra et al. from an electronic perspective in copper metal. Their findings revealed that the order of corrosion inhibition efficiency is OCH3 > CH3 > H > Cl [53].

This study aims to investigate the influence of methyl group substitution on the corrosion inhibition efficiency of oxalohydrazide (OXH) on copper surfaces using DFT calculations. Specifically, we will determine the adsorption energies and preferred adsorption configurations of OXH and its methylated derivatives, analyze key physicochemical factors, and examine the electronic interactions between the inhibitors and the copper surface. While previous studies have explored the corrosion inhibition properties of various hydrazide derivatives on different metals, including mild steel and stainless steel, a comprehensive theoretical investigation of the inhibitory mechanism of oxalohydrazide (OXH) and its methylated derivatives on copper surfaces is lacking. This study addresses this gap by employing DFT calculations to systematically investigate the influence of methyl group substitution on the corrosion inhibition efficiency of OXH on copper. Specifically, we determine the adsorption energies (ΔEadsorption) and identify the most stable adsorption configurations for OXH and a series of nine methylated derivatives (N′-methyloxalohydrazide, N1-methyloxalohydrazide, N1,N′1-dimethyloxalohydrazide, N′1,N′2-dimethyloxalohydrazide, N1,N2-dimethyloxalohydrazide, N1,N′1,N2-tetramethyloxalohydrazide, N1, N′1,N′2-trimethyloxalohydrazide, and N1,N′1,N2,N′2-tetramethyloxalohydrazide) (Fig. 1) on copper. Furthermore, we analyze key physicochemical factors, including electronegativity (χ), gap energy (Egap), charge transfer (ΔN), ionization potential (I), electron affinity (A), hardness (η), and Fukui functions, to elucidate the corrosion inhibition mechanism. Finally, we compare the densities of states (DOS) of the complex structures (Cu surface + inhibitors) with that of the bare copper surface to gain deeper insights into the inhibitory effects of OXH and its methylated derivatives at different substitution positions. This systematic theoretical approach provides a novel contribution to the understanding of the structure-activity relationship for hydrazide-based corrosion inhibitors on copper and offers valuable guidance for the design of more effective corrosion protection strategies.

Oxalohydrazide (OXH) was chosen as the target molecule for this investigation due to a combination of factors. Its structural simplicity allows for a systematic study of substituent effects. The presence of multiple heteroatoms (N and O) suggests a strong potential for surface interaction. This study focuses on elucidating the fundamental mechanisms of inhibition at the molecular level, where computational methods offer a powerful tool. Furthermore, previous work [47] has indicated its effectiveness in inhibiting iron corrosion, suggesting potential applicability to other metal systems, such as copper.