Inhibition of copper corrosion in chloride solutions by amino-mercapto-thiadiazol and methyl-mercapto-thiadiazol: an impedance spectroscopy and a quantum-chemical investigation

Abstract

The efficiency of 2-amino-5-mercapto-1,3,4-thiadiazole and 2-methyl-5-mercapto-1,3,4-thiadiazole as copper corrosion inhibitors in neutral chloride environments was investigated. The study was carried out by means of a density-functional quantum-chemical approach and by impedance spectroscopy. The theoretical method was used to obtain a microscopic information about the surface–molecule interactions including the adsorption geometry conformations. Based on this information a physical model of the investigated system was proposed. A characteristic feature of it is the presence of interfacial phases between the bulk metal and the oxide and between the bulk oxide and the electrolyte. The derived model was relayed to an equivalent circuit which included the whole metal–oxide electrolyte system and the impedance data were fitted to it. The obtained estimates of the total dc path resistance showed that the studied thiadiazole derivatives were excellent copper corrosion inhibitors.

Introduction

The search for new corrosion inhibitors has been a subject of longterm and active efforts [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. While they have led to a development of successful inhibiting systems for a variety of environments, some problems still remain unsolved. One of them is connected with the necessity to introduce low-toxic inhibitors as some of the most effective ones currently used are toxic substances [25], [30]. Another motive to continue the studies of the copper corrosion in the presence of inhibitors originates from the still existing ambiguities concerning the nature of the protective action. One major problem that prevents the elucidation of the inhibitive mechanism is connected with the uncertainties about the chemical composition, structural properties and reactivity of the copper corrosion products. Their knowledge is important as the adsorption of inhibitors takes place on the bare copper surface only if special measures are taken to prevent the formation of corrosion product layers either by cathodic polarization or by conducting experiments in highly aggressive acidic environments. Even then, the presence of intermediate adsorbed species and the formation of porous layers are possible under certain conditions [1], [10], [31], [32], [33], [34], [35], [36]. The modification of the surface by the presence of corrosion products should be even more taken into account in milder environments as in this case the presence of various compact or porous corrosion layers is always detected on the surface [1], [2], [37], [38], [39], [40], [41]. As a result, the main prerequisite for a successful understanding of the inhibiting mechanism is the realistic description of the surface initial condition and of the changes that are inflicted by the introduction of the inhibiting compounds. The recognition of the importance of surface characterization resulted in a large number of studies utilizing electrochemical, spectroscopic, ellipsometry, and STM methods [3], [5], [6], [7], [11], [14], [16], [42], [43], [44], [45], [46], [47], [48], [49], [50]. The majority of data reflecting the general behaviour of the inhibited systems was gained by electrochemical methods. Often, however, problems arise when those methods are used to establish the mechanism at atomic level as the signal from them is averaged all over the surface while the interaction of the inhibitors with it has a local nature. It is feasible to assume that a variety of structural and compositional forms are created and they have different influences on the atomic scale processes, and hence, have different contributions to the total response of the surface. Because of that, it is always necessary to have structural and chemical information on an atomic level in order to support the process of the interpretation of the electrochemical experiments. This information can be obtained from both theoretical and experimental approaches as the most often used experimental methods sensitive to the local atomic arrangement are STM, X-ray absorption [51], [52], and vibrational spectroscopies. The systematic use of STM, for example, allowed to collect a large information base about the structural reconstruction of the copper surface in different environments. Now it becomes more and more possible to complement this experimental information with one which is derived from various theoretical approaches. Some recent and major advancements in the quantum-chemical theory and practice, for example, led to the development of implementations that allow to treat at ab initio level systems without a symmetry and consisting of thousands of atoms [53]. This status of the computational quantum-chemistry opens the possibility for a realistic simulation of the surface–molecule interaction in the framework of a full geometry optimization, and hence, a derivation of properties relevant to the investigation of the inhibiting action.

The general aim of this work is to investigate the inhibiting efficiency of 2-amino-5-mercapto-1,3,4-thiadiazole (AMTD) and 2-methyl-5-mercapto-1,3,4-thiadiazole (MMTD) and to compare it with that of the widely applied for copper corrosion protection benzotriazol (BTAH). The electrochemical behavior of copper in 0.5 M NaCl solutions with and without the presence of the studied compounds is characterized by impedance spectroscopy. While other robust methods exist for the estimation of the corrosion resistance alone, we use the impedance spectroscopy in order to increase our knowledge about the investigated system by fitting the spectra into an appropriate equivalent circuit. The derivation of an equivalent circuit which in a best possible manner represents the physical structure of the system was augmented by an ab initio investigation of the structure relaxation and adsorption interactions on the surface of the oxide. All ab initio calculations of the adsorption process are curried out on a substrate of Cu₂O as according to some preliminary investigations and to the available literature data [1] this is the oxide that forms at the steady-potential in neutral NaCl solutions.