Adsorption behavior and corrosion inhibitive characteristics of newly synthesized cyano-benzylidene xanthenes on copper/sodium hydroxide interface: Electrochemical, X-ray photoelectron spectroscopy and theoretical studies

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Abstract

Elegant process for synthesis of 3-(7H-dibenzo[c,h]xanthen-7-yl)benzaldehyde (3), as new starting material to create a set of novel xanthene analogues, 2-(3-(7H-dibenzo[c,h]xanthen-7-yl)benzylidene)malononitrile (4), 3-(3-(7H-dibenzo[c,h]xanthen-7-yl)phenyl)-2-cyanoacrylic acid (5), and Ethyl-3-(3-(7H-dibenzo[c,h]xanthen-7-yl)phenyl)-2-cyanoacrylate (6), was achieved starting with available materials under mild conditions. Various concentrations (ca. 0.1–1.0 mM) of the synthesized cyano-benzylidene xanthene derivatives, namely compounds 36, were tested as inhibitors to control copper corrosion in alkaline solutions employing polarization and electrochemical impedance spectroscopy (EIS) measurements. Results revealed that the four studied xanthenes derivatives served as efficient (mixed-type) inhibitors. The inhibition efficiency increased with increase in inhibitor concentration.The inhibition performance of studied compounds varied according to their chemical structures. The best inhibitor, compound (5), achieved a maximum inhibition efficiency of 98.7% (calculated from corrosion current densities) and ~ 95% (estimated from charge-transfer resistance values) at a concentration of 1.0 mM. The morphology of the corroded and inhibited copper surfaces was studied by scanning electron microscopy (SEM). The adsorption of the inhibitor molecules was confirmed by high-resolution X-ray photoelectron spectroscopy (XPS) profiles. XPS data were used to compare the inhibition efficiencies exhibited by studied compounds. The oxidation rate of the Cu surface was found to be frivolous, referring to high inhibition efficiency, only in the presence of inhibitor (5), and Cu0 share is 87% of all copper components. The shares of Cu0 were significantly reduced to 43%, 26% and 20% for inhibitors (3), (4) and (6), respectively. These findings go parallel with the results obtained from electrochemical measurements. The quantum-chemical calculations of the investigated molecules were performed to support electrochemical findings, and their correlations with the inhibition efficiency of the synthesized compounds were discussed.

Introduction

Copper (Cu), one of the most abundant metals in the earth’s crust, is useful and highly applied in a pure or alloying form. The most important, and well-known, Cu alloys are brass, Cu-Ni and bronze. High electrical and thermal conductivity, mechanical workability, and malleability are the most favourable characteristics of copper [1] that made it widely used in various industrial applications [2], [3]. Cu is also atmospheric and aqueous corrosion-resistant due to the formation of protective layers of passive Cu-oxides and/or nonconductive corrosion products on its surface [4], [5].

However, Cu corrodes readily in aqueous media containing oxygen and some anions such as chloride, sulfate, and hydroxide [6], [7]. Cu corrosion and subsequent formation of corrosion products have undesired impacts on the performance of the applied Cu-based systems, reducing their efficiency [8]. For these reasons, numerous corrosion-research groups worldwide devoted themselves toward elaborating new organic compounds as efficient inhibitors for Cu corrosion in a variety of harsh aqueous environments. For instance, azoles [3], [9], [10], [11], [12], [13], [14], [15], thiols [16], [17], amino acids [18], [19], [20], surfactants [21], [22], [23], [24], [25], and phytic acid and its salts [26], [27], [28], and many others [29], [30], served as typical organic inhibitors for Cu corrosion. Most of these efficient inhibitors are organic compounds containing N, S, O, and P as heteroatoms that are capable of donating electrons to the vacant d-orbitals of the Cu atoms, forming coordinate covalent bonds [29], [30]. At the same time, such compounds can accept free electrons from the Cu atom using their anti-bonding molecular orbitals forming feedback bonds [29], [30].

Xanthene and its derivatives (xanthenes) are one of the most important classes of heterocyclic compounds exhibiting various pharmacological characteristics such as antibacterial, antiviral, anti-inflammatory and fungicide [31], [32], [33], [34]. Xanthenes also acted as effective corrosion inhibitors for the acid corrosion of mild steel [35], [36], [37]. To the best of our knowledge, there are no articles on xanthenes as inhibitors for the aqueous corrosion of copper. From here comes the objective of the present work, which explores the use of our newly synthesized xanthene derivatives as promising inhibitors for the corrosion of Cu in NaOH solutions.

The choice of xanthenes for this work was based on the consideration that it contains several benzene rings that are fused together. The π electrons of these rings are delocalized over all their carbon atoms thus, constituting together a pool of electrons. In addition, the new xanthene derivatives synthesized here contain polar functional groups, namely − CN, −COOEt, and –COOH including nitrogen and oxygen heteroatoms. All these features of the chemical structure of our newly synthesized xanthene derivatives can induce greater adsorption of the inhibitor molecule onto the surface of copper [38].

The objective here is to study the inhibition performance of such newly synthesized xanthene derivatives towards the alkaline corrosion of Cu. Measurements were conducted in naturally aerated 1.0 M NaOH solution using various electrochemical techniques. Morphology and chemistry of the corroded and inhibited Cu surfaces were investigated by SEM and XPS examinations. Theoretical calculations, employing several quantum chemical methods and molecular modelling techniques, were also performed to support the experimental findings.

Section snippets

Materials and methods

All chemicals and reagents, supplied by Sigma Aldrich Co., Darmstadt, Germany, were analytical grade or chemically pure.IR spectra were conducted in the Micro Analytical Center (MAC) at Taif University (Taif, Saudi Arabia).Other spectral analyses, namely 1H NMR, 13C NMR, MS and elemental analysis were performed at Mansoura University (Mansoura, Egypt). Methodology and instrumentations used were previously reported [39], and discussed in depth in Section S1 (Supplementary Information).

Synthesis of 3-(7H-dibenzo[c,h]xanthen-7-yl)benzaldehyde (3).

A mixture

Chemistry

Multi component’s condensation in the presence of a Brönsted or Lewis acid is the key synthetic method employed for xanthene preparation [42]. Recently, the multi-component processes (MCPs) have gained significant economic and environmental interest, as they have been shown to be a very elegant and quick way of accessing complex structures from basic building blocks in a single synthetic operation, and display high atomic efficiency and high selectivity [43]. Thus, isophthaldehyde(1) was

Conclusions

Four novel xanthene derivatives (36) were successfully prepared, fully characterized and further investigated as inhibitors for the corrosion of Cu in alkaline solutions. Measurements were performed in 1.0 M NaOH without and with different concentrations (ca. 0.1–1.0 mM) of the inhibitors examined using various electrochemical techniques. The efficiency of each studied inhibitor towards the alkaline corrosion of Cu was found to enhance with inhibitor concentration, reaching its maximum value

CRediT authorship contribution statement

Mohamed E. Khalifa: Conceptualization, Methodology, Supervision, Investigation, Writing - original draft, Writing - review & editing. Islam H. El Azab: Methodology, Data curation, Investigation, Software. Adil A. Gobouri: Methodology, Data curation, Investigation. Gaber A.M. Mersal: Investigation, Writing - review & editing. Sarah Alharthi: Investigation, Writing - review & editing. Murat Saracoglu: Software, Writing - review & editing. Fatma Kandemirli: Software, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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