Formation of Corrosive Sulfur with Dibenzyl Disulfide in Fluid-Filled

Article pubs.acs.org/IECR

Formation of Corrosive Sulfur with Dibenzyl Disulfide in Fluid-Filled Transformers Veresha Dukhi, Ajay Bissessur, and Bice S. Martincigh* School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban, 4000, South Africa S Supporting Information *

ABSTRACT: The formation of corrosive sulfur as a result of the addition of dibenzyl disulfide (DBDS) to transformer oils has been investigated. Corrosive sulfur in a transformer affects all aspects of its insulation and conductive components by deposition of Cu2S. Previous studies have suggested that the formation of Cu2S was initiated by a DBDS−Cu complex. This work shows an alternate pathway via thiolate formation for the deposition of corrosive sulfur on copper conductors. The open circuit potential results showed no reduction of DBDS onto copper surfaces, but the degradation products of DBDS, which include dibenzyl sulfide and benzyl mercaptan, were both found to adsorb on copper surfaces, forming a copper thiolate. The formation of copper thiolates was also demonstrated by a heterogeneous reaction of CuO and benzyl mercaptan. Thermal degradation of the thiolate eventually produced copper sulfide Cu7.2S4 as identified by Xray diffraction analysis.

1. INTRODUCTION Power transformers are generally insulated with paper and mineral oil. However, insulation deterioration and degradation are the fundamental contributors to transformer failure. This occurs through thermal, oxidative, and electrical aging. In addition, the properties of the insulation material are further hindered by corrosive sulfur. Corrosive sulfur according to ASTM Standard D 2864 is defined as “elemental sulfur and thermally unstable sulfur compounds in electrical insulating oils that can cause corrosion of certain transformer metals such as copper and silver”.1 Corrosive sulfur in transformers generally involves the deposition of Cu2S on copper conducting surfaces.2 Although ASTM Standard D 2864 defines corrosive sulfur as corrosion of metal surfaces, its presence is not limited to the impediment of conductive materials; in addition, the deposition of Cu2S leads to the development of faults with the insulation material itself.3 Faults associated with Cu2S deposition can be categorized by conductor failure and insulation failure which inadvertently lead to shortened transformer lifetime, higher maintenance costs, and increased downtime. Cu2S deposition on the conductor surface decreases its metallic copper content and reduces it electrical properties.4 With respect to insulation, Cu2S can dislodge from copper surfaces and insert itself between insulation paper layers, thereby facilitating short-circuits. While the conductive nature of Cu2S is responsible for paper and oil failure, its presence also impedes their insulation properties (dissipation factor and dielectric breakdown voltage).4 The main causative agent accountable for corrosive sulfur has been identified as dibenzyl disulfide (DBDS),5 which is added © XXXX American Chemical Society

to transformer oil to protect against wear, reduce friction, and increase oxidation stability.6 It has been suggested by many that the development of Cu2S is facilitated by the formation of a DBDS−Cu complex (Scheme 1).7−9 However, further studies to isolate and verify the presence of a DBDS−Cu complex in conditions simulating a transformer have not yet been reported. The formation of a DBDS−Cu complex is not the only theory put forward to account for the formation of corrosive Scheme 1. Reaction Scheme for the Formation of Copper Sulfide from Dibenzyl Disulfide7

Received: October 26, 2015 Revised: January 27, 2016 Accepted: February 25, 2016

A

DOI: 10.1021/acs.iecr.5b04032 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

nitrogen blanket and are exposed to air, and oxygen content can also be attributed to hot cellulose35). A few studies have investigated the degradation of these thiolates. In work by Kino and co-workers36 and Turbeville and Yap,37 the decomposition of copper thiolates formed copper(I) sulfide (eq 1). Furthermore, thiolates have been used as precursors to metal sulfide formation.38 Kuzuya and coworkers39 also showed the formation of copper and zinc sulfides from thiolate precursors.

sulfur on copper conductive surfaces. It is imperative to note that the degradation products of DBDS are also corrosive in nature. The decomposition of organic disulfides has been investigated by many.10−12 Plaza et al.10,11 have investigated the thermal decomposition of dibenzyl disulfide and diphenyl disulfide (DPDS). Decomposition of DPDS showed the formation of diphenyl monosulfide and phenyl thiol. In studies by Plaza et al.11 and Bovington and Dacre,12 the decomposition of DBDS showed the presence of benzyl mercaptan (BM) and dibenzyl sulfide (DBS). As a result, the presence of monosulfides and thiols are possible upon thermal decomposition of dibenzyl disulfide. Several studies have shown that degraded transformer oil containing DBDS also exhibits the presence of BM and DBS.4,6,8,13−15 Lewand and Reed6 considered the formation of Cu2S to be initiated by the degradation product, benzyl mercaptan, in which it reacts with Cu+ to form Cu2S (see Scheme 2). In this reaction, the other product formed would be ethylbenzene, but to date no evidence for its formation has been reported.

2RSCu → Cu 2S + RSR

(1)

From these previous studies, it seems plausible that copper(I) sulfide deposition may be as a consequence of copper thiolate formation. This concept of thiolate formation (self-assembled thiolates by coordination of thiols, disulfides, and sulfides on the copper metal surface) as a precursor to copper sulfide deposition on copper windings in transformers was suggested previously;40−43 however, there were no confirmatory experimental results. Hence, this research sought to investigate this possibility by conducting electrochemical investigations with copper, DBDS, and its degradation products. It also sought to investigate the formation of copper thiolate from heterogeneous pathways involving copper metal and its oxides in conditions similar to those in transformers.

Scheme 2. Mechanism Proposed by Lewand and Reed6 for Cu2S Formation on Transformer Copper Windings

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade reagents (AR) were used unless otherwise stated. Propanol (99.8%, LabScan-Analytical Sciences), methanol (≥99.9%, Sigma-Aldrich), sodium perchlorate (98%, Promark Chemicals), copper(II) chloride dihydrate (97%, SaarChem), potassium chloride (99−100%, SaarChem), copper(II) oxide (laboratory reagent, Hopkin and Williams), benzyl mercaptan (99%, Sigma-Aldrich), dibenzyl disulfide (99%, Aldrich), dibenzyl sulfide (≥95.0%, Aldrich), tetrahydrofuran (≥99.9%, Sigma-Aldrich), chloroform-d (99.8%, Aldrich), tetrabutylammonium hexafluorophosphate (TBAHP) (≥99.9%, Fluka Analytical), and ferrocene (98%, Aldrich) were used as received. Uninhibited naphthenic-based transformer oil was purchased from The Oil Centre, Durban, South Africa. Copper plates and wires were purchased from local hardware stores. 2.2. Equipment. All electrochemical analyses were carried out on a BASi EC Epsilon PC-controlled workstation for voltammetry and polarography (BASi Analytical Instruments). Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III 400 MHz spectrometer. Infrared spectra were recorded on a PerkinElmer Spectrum 100 Fourier transform infrared (FTIR) spectrophotometer fitted with a universal attenuated total reflectance (ATR) attachment. Raman spectra were recorded on a DeltaNu Advantage 532 instrument containing a 532 nm laser source. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy analyses were carried out on a LEO 1450 scanning electron microscope and a Jeol JSM 6100 scanning electron microscope with a Bruker EDX detector, respectively. Powder X-ray diffraction was carried out with a Bruker D8 Advance diffractometer with a Sol-X energy dispersive detector (Cu Kα source). Gas chromatographic analyses with flame ionization detection (GC-FID) were carried out on an Agilent 6320 GC instrument fitted with a Phenomenex ZB-1 column. The injector temperature was 280 °C, and the injection volume was 1 μL in splitless mode. The column temperature program was

This research investigates the possible formation of copper thiolates on copper surfaces as precursors to copper(I) sulfide deposition. The adsorption of disulfides and thiols on a few metal surfaces has been investigated extensively.16−29 The most common metal in the study of disulfide or thiol adsorption is gold. Early studies showed that the adsorption of alkanethiols and dialkane disulfides on gold surfaces formed self-assembled monolayers (SAMs).16,29−32 The formation of metal thiolates is not restricted to gold and has been studied on a range of other metals including Cu, Ag, and Pb, among others.20,27,33,34 Generally, the preparation of SAMs is spontaneous; however, application of a positive potential can increase the rate of formation and the distinct characteristics of the monolayer.17 Bain and co-workers,16 from their contact angle data, showed that the adsorption of thiol was favored compared with that of the disulfide. Furthermore, other studies involving gold working electrodes used X-ray photoelectron spectroscopy data to show that gold(I) thiolate was formed subsequent to adsorption irrespective of whether the adsorbate was the thiol or disulfide.16 It is important to state that the self-assembly of thiols on metal surfaces can occur irrespective of aerobic or wet conditions;17 hence, the formation of copper thiolate on copper surfaces in transformers is viable (transformer conditions include exposure of the insulation materials to moisture; furthermore, many transformers are not covered by a B


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Industrial & Engineering Chemistry Research set as follows: 50 °C held for 1 min, then increased at 10 °C min−1 to 260 °C and held there for 10 min. 2.3. Cyclic Voltammetry. To determine complex formation, DBDS (0.5 mL, 20 mM) and BM (0.5 mL, 20 mM) were added into 20 mL of solvent (either deionized water or THF) containing 0.3 g (1.8 mmol) CuCl2·2H2O. Cyclic voltammetry experiments were performed with a three-way electrode system consisting of a glassy carbon electrode (GCE) (3 mm diameter) as the working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode (all supplied by BASi), under an inert nitrogen atmosphere. Prior to analyses, the glassy carbon electrode was polished with 0.3 μm alumina, rinsed with acetone, and sonicated for 3 min. The platinum wire counter electrode was also polished with alumina and occasionally soaked in dilute nitric acid and rinsed with deionized water several times. The applied potential of the working electrode was varied from +1000 to −1000 mV vs Ag/ AgCl with a switching potential at −1000 mV. 2.4. Open Circuit Potential Measurements. Open circuit measurements were also carried out in a three-way electrode system in two different solvent systems, namely, MeOH (mercaptan) and THF (mercaptan and disulfide), with 0.1 M NaClO4 as the supporting electrolyte, and with metal working electrodes of either Ag, Cu, or Fe; a platinum wire counter electrode; and a Ag/AgCl reference electrode. The open circuit potential (OCP) analyses were carried out under nitrogen and were terminated once a steady potential was reached. In this study, BM was used at concentrations of 4.5 × 101 mM and 4.5 mM and for that of DBDS and DBS 3.1 × 101 mM and 3.2 × 101 mM were used, respectively. Upon the addition of DBDS under acidic conditions, 0.5 M HCl was used as the supporting electrolyte. In the OCP measurements with BM, a ferrocenium/ ferrocene redox couple was employed, which was analogous to the presence of dissolved oxygen.44 Ferrocene (0.0097 g, 0.05 mmol) was dissolved in 20 mL of MeOH with the supporting electrolyte, and the potential was scanned linearly from −1000 to 1000 mV to obtain sufficient ferrocenium ion in solution prior to OCP analysis. 2.5. Preparation of Copper Thiolate (from Cu 0 Starting Material). The surface of a copper plate (1 × 1 × 2.5 × 10−3 cm3) was cleaned with sand paper and dilute nitric acid. The plate was then placed in a vial to which 0.25 g (2.0 mmol) of pure BM was added. The vial was sealed and stored. 2.6. Preparation of Copper Thiolate (Positive DC Potential Application). The thiolate was prepared in two different solvents, namely, MeOH and THF, by using 0.1 M TBAHP as the supporting electrolyte. BM (0.2 g, 1.6 mmol) was added to the solvent and the mixture was stirred, after which a potential of 2 V vs Ag/AgCl was applied. In the case of THF, the potential was held for 30 min, whereas for MeOH it was held for both 30 and 120 min. The working electrode was a clean copper wire, with a diameter of 1 mm and a length of 5 cm. The plate with deposited thiolate on the surface was then analyzed by SEM and EDX. A desorption study was also carried out by using the thiolate deposited on the copper surface as the working electrode in 0.5 M NaOH (Ag/AgCl reference, Pt counter electrode) which was scanned linearly from 800 to −1500 mV. Cleaned copper surfaces were used as the control. 2.7. Preparation of Copper Thiolate (from CuO Starting Material). BM (0.25 g, 0.002 mol) was weighed into a vial together with CuO (0.08 g, 0.001 mol). The solvents

used were either propanol or transformer oil at a volume of 2 mL. The slurry was left to stir for 8 h, during which time the color of the slurry began to change, and a white precipitate was visible on the sides of the vial. After this time, the solid was filtered and washed several times with acetone and hexane, after which the precipitate was set aside to dry in a desiccator (containing self-indicating silica gel as the drying agent). FTIR, Raman, and SEM-EDX analyses were carried out on these samples.

3. RESULTS AND DISCUSSION Previous contributions have suggested that the formation of a DBDS−Cu complex facilitates the deposition of Cu2S.7,45 In this work, the synthesis of a DBDS−Cu complex was not carried out. However, a cyclic voltammetry study of Cu2+/Cu+/ Cu showed a possible copper dibenzyl disulfide complex to exist. 3.1. Formation of Possible Coordinated Complexes ([Cu(DBDS)2]2+ and [Cu(BM)]). This section describes the investigation of copper coordinated complex ions that could possibly exist. A cyclic voltammetry study was undertaken to qualitatively decipher complex formation. Figure 1 shows the

Figure 1. Cyclic voltammograms of cupric chloride in 0.1 M NaClO4 prepared in H2O, showing the addition of DBDS. Conditions: GCE working electrode vs Ag/AgCl, Pt auxiliary electrode, scan rate 100 mV s−1.

voltammogram of cupric chloride in 0.1 M NaClO4 compared with that which has been spiked with a 0.025 M DBDS solution (0.5 mL). For the spiked voltammogram, there is a shift in potential for the anodic oxidation of Cu to Cu+ and Cu+ to Cu2+, in which both have split toward more positive potentials. After successive scans, the peak at 0.09 mV slowly diminishes while the peak at 0.13 mV slowly increases. This shift to more positive potentials is indicative of complex formation. Habib et al.46 similarly showed complex formation by the shift to more positive values of the Cu/Cu+ and Cu+/Cu2+ anodic peaks for the formation of a copper−leucine complex. The cathodic peaks, however, did not shift to more negative potentials (first reductive peak remains the same), indicating that the complex easily decomposes. Considering that DBDS is readily soluble in THF, further voltammetry experiments were carried out in THF. In Figure 2, a similar shift is seen (a−b), where a (0.5 mV) is the Cu+/Cu2+ oxidative peak, which shifts to a more positive potential (0.585 mV). However, the cathodic peak shows no shift to a more negative potential. C


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Figure 2. Cyclic voltammograms of cupric chloride in 0.1 M NaClO4 prepared in THF upon the addition of DBDS. Conditions: GCE working electrode vs Ag/AgCl, Pt auxiliary electrode, scan rate 100 mV s−1. Peak a is the second oxidation peak which shifts to b after addition of DBDS (1−3 show successive scans).

Figure 4. Cyclic voltammograms of cupric chloride in 0.1 M NaClO4 prepared in THF upon addition of DBS. Conditions: GCE working electrode vs Ag/AgCl, Pt auxiliary electrode, scan rate 100 mV s−1. Peaks a−d are oxidation peaks and e is the reduction peak (1−3 show successive scans).

It is important to note that these results do not show if a counterion, such as chloride, neutralizes the charge on the complex. However, to counteract the positive charge of the complex in either its cuprous or cupric state without cleavage of the disulfide bond a counterion of charge −1 or −2 is required. The synthesis of the [Cu−(DBDS)2]2+ complex has been reported in which the charge on the metal was maintained by chloride anions.47 A similar study with palladium also showed chloride counterions on the [Pd−(DBDS)2]2+ complexes.48 Transformer oil is void of free ionic species, and as a result it would be very difficult for the complex to form without the presence of a counterion to maintain a neutral charge on the complex. The formation of copper complexes with the breakdown products of DBDS, namely BM and DBS, was then similarly investigated. Figure 3 shows the formation of a Cu−BM

It is imperative to note that possible complex formation shown by the cyclic voltammetry experiments does not denote the exclusive structure of these complexes, nor if these complexes can exist without the need of counterions. As a result, further experiments were carried out incorporating OCP analyses. 3.2. Adsorption of DBDS, BM, and DBS onto Metal Surfaces. Investigations on complex formation between DBDS and Fe were performed by Ohno and co-workers.33 Their findings showed that in acidic media, DBDS adsorbs onto the iron surface to form an iron thiolate. This work also investigated the adsorption of DBDS on silver, iron, and copper by means of OCP analyses (Figure 5). From the OCP data it can be observed that no adsorption of DBDS on any of the three metals takes place in THF (Figure 5a). However, under acidic conditions, upon the addition of DBDS, there is a sharp increase in the potential of the working electrode, immediately followed by a decrease and then a plateau (Figure 5b). This is indicative of reductive adsorption, implying that DBDS adsorbs onto Ag and Fe. eq 2 describes the formation of thiolate via addition of disulfide on metal surfaces as outlined by Hager and Brolo.49 However, the OCP data show no adsorption of DBDS on Cu in either media (Figure 5). RSSR + M + e−(M) → RS−M + RS−

(2)

Figure 6 shows thiol adsorption on Cu metal surfaces with the degradation product of DBDS, namely, BM. Figure 6a represents the OCP measurements of a concentrated BM solution. Once the solution is spiked with BM, there is an immediate drop in potential. This potential drop (anodic current) is indicative of thiolate formation by oxidative adsorption (eq 3).20,49

Figure 3. Cyclic voltammograms of cupric chloride in 0.1 M NaClO4 prepared in THF upon addition of BM. Conditions: GCE working electrode vs Ag/AgCl, Pt auxiliary electrode, scan rate 100 mV s−1. Peaks a−c are copper oxidation peaks; d and e are copper reduction peaks (1−3 show successive scans).

RSH + M → RS−M + H+ + e−(M)

(3)

A discharging process (shift to a more positive potential) is not observed. A similar result was noted by Cohen-Atiya and Mandler20 in which a discharging process was not observed for their Hg electrode, which they attributed to the slower kinetics of proton reduction on the Hg electrode. Chon and Paik44 used the ferrocenium/ferrocene redox couple (analogous to the presence of dissolved oxygen) which was also carried out here (see Figure 6b). In the presence of the ferrocenium/ferrocene

complex. Peak a develops once BM is spiked into the voltammetric cell. Peak b (0.18 mV) splits and eventually develops into the larger peak c (0.455 mV). Both d and e shift to more negative potentials. These shifts are indicative of possible copper(I) BM and copper(II) BM complex formation. In Figure 4, peaks a and e diminish and peak d develops upon the addition of DBS. D


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noted; however, within a week the copper plate began to blacken slightly (possibly due to CuO formation), and after a month a yellow precipitate (possible thiolate formation) formed in the solution. The solution in which the copper plate was immersed was dissolved in CDCl3 and filtered through a cotton wool plug. The presence of DBS (sideproduct) with BM (starting material) was confirmed by means of NMR spectroscopy (see Figure S1). This is in accordance with the work of Whitley and coauthors27 who suggested the reactions shown in eqs 4 and 5. With regards to eq 5, Peach50 also showed the formation of a thiolate with a ligand (thiophenol) similar to BM. O2 + 2Cu → 2CuO

(4)

4RSH + 2CuO → RSSR + 2RSCu + 2H 2O

(5)

Our findings are also in accordance with the mechanism set out by Toyama and co-workers7 in which they also described the reformation of DBDS in the synthetic route to Cu2S formation. 3.4. Copper Thiolate Formation through Application of Positive Potential Route. To produce the thiolate complex, a 2 V potential was applied to the copper working electrode immersed in methanol containing BM. After 8 min, a film developed on the copper electrode, and after a period of 120 min, a yellow solid had formed on the electrode surface (Figure 8). The solid was suspended in aqueous ammonia which, after a short period of time, turned the solution light blue, indicative of the presence of copper. The preparation of the thiolate was also carried out in THF. The product was adsorbed onto the copper surface as shown by Figure 9. Only the section of copper that was immersed into the solution was coated with a yellow precipitate. Figure 10 shows the SEM image and the EDX data after deposition of the yellow precipitate on the copper working electrode (see Figure 9). The EDX analysis shows regions of copper with sulfur, and it is clear that the deposition is not uniform from the large areas of green (copper). However, there are regions in which thiolate adsorption is pronounced. Desorption studies carried out by Widrig and coauthors51 focused on the reductive desorption of thiolate from a gold substrate. The desorption of the thiolate from a gold surface was given by eq 6.51

Figure 5. Open circuit potential of the metals Ag, Fe, and Cu with the addition of DBDS carried out in (a) 0.1 M NaClO4 in THF and (b) 0.5 M HCl in THF. Conditions: metal working electrode vs Ag/AgCl, Pt auxiliary electrode.

redox couple, the discharging process is observed; however, a negative drift still occurs. When a more dilute BM solution is used (Figure 6c), a drop in potential is followed by an instantaneous increase. This latter process is termed the discharging process. However, the positive shift is soon followed by a slow negative drift of the working electrode. If oxygen is bubbled into the cell, there is an increase in the potential of the working electrode (Figure 6d). The results shown in Figure 6a−d were conducted in the protic solvent methanol. The experiments were repeated in the aprotic solvent THF. As can been seen in Figure 6e, the results are similar to those in Figure 6a−d, namely, a negative shift in potential then an increase to more positive potentials followed by a slow negative drift. From Figure 6, it is clear that the oxidative adsorption of BM occurs in both protic and aprotic media. It is thus likely that adsorption of BM on copper surfaces can occur in transformer oil. Figure 7 shows the adsorption of DBS onto a copper metal surface in both protic and aprotic media. Upon addition of DBS, there is a sudden negative shift in the potential (anodic current) followed by a positive shift and then a steady potential. The positive shift is indicative of the formation of the thiolate by oxidative adsorption (similar to eq 3). Thus, DBS is also able to adsorb on a copper metal surface via thiolate formation. The formation of copper thiolate complexes in transformer oil is thus possible because no counterion would be required to maintain the charge on copper as these will be neutral complexes. 3.3. Copper Thiolate Formation through Cu0 Route. To determine if the thiolate can form without the application of a potential (spontaneously), a copper plate was left to react with pure BM. After 4 days little or no evidence of reaction was

RSAu + e− → RS− + Au

(6)

Similarly, in this study, the copper surface onto which the thiolate was adsorbed (Figure 9) was used as a working electrode in 0.5 M NaOH. The results are shown in Figure 11. The scan of a cleaned copper surface shows two cathodic peaks at −0.59 and −0.85 V (scan 1); however, desorption of the thiolate shows one reductive peak at −0.69 V (scan 2). The subsequent scan shows two reductive peaks (scan 2 repeat). This is possibly due to stripping of the complex layer from the electrode surface. From the above analyses, it is apparent that the formation of the copper thiolate complex can easily occur on the surface of copper windings of fluid-filled transformers. 3.5. Copper Thiolate Formation through CuO Route. Apart from the reaction of the copper plate and thiol, copper oxide was reacted with BM in both propanol and transformer oil. After a period, a white solid formed on the sides of the tube, which was confirmed to be DBDS by NMR spectroscopy and GC-FID (see Figures S2 and S3). Eventually the black mass E


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Figure 6. OCP measurements of a copper electrode after the addition of (a) 4.5 × 101 mM BM, (b) 4.5 × 101 mM BM containing the ferrocenium/ ferrocene redox couple, (c) 4.5 mM BM (Inset, magnified region upon addition of BM), (d) 4.5 mM BM with oxygen bubbled through the solution, all carried out in 0.1 M NaClO4 in MeOH, and (e) 4.5 mM BM carried out in 0.1 M NaClO4 in THF (Inset, magnified region upon addition of BM). Conditions: copper working electrode vs Ag/AgCl, Pt auxiliary electrode.

used to investigate thiolate formation. This study utilized Raman spectroscopy to monitor cleavage of the S−H bond of BM (see Figure S6 for spectra). The Raman modes in Table 1 are a compilation of the spectra obtained by the two methods of copper thiolate preparation (deposition at 2 V and synthesis from CuO). This data was compared with that obtained by Ohno and co-workers33 from their work on silver and iron thiolates with DBDS in acidic media. It is imperative to note that in both methods of copper thiolate preparation (deposition at 2 V and synthesis from CuO), there is an absence of the thiol proton (2548 cm−1), indicating cleavage of the S−H bond. 3.6. Formation of Copper Sulfide. The question still arises as to whether the thiolate is responsible for the formation of metal sulfides on copper conductors. The thiolates that were synthesized were then subjected to heating in an oil bath at 150

became green. The products were isolated, and the results of the SEM and EDX analyses are shown in Figure 12. No NMR data was obtained for these products. From the SEM results (Figure 12), it is seen that the morphology of the copper thiolates prepared in propanol and transformer oil are very different. The thiolates formed in transformer oil are more globular, whereas those synthesized in propanol were stemlike and tubular. The EDX analysis shows regions with an accumulation of sulfur, which can be inferred to be thiolate formation. From the data obtained it is evident that the thiolate is able to form in transformer oil. The ATIR−FTIR spectra of the solids obtained were found to be similar to that of BM (see Figures S4 and S5). In studies by Ohno et al.33 and Uehara and Aramaki,52 surface-enhanced Raman scattering (SERS) spectroscopy was F


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Figure 10. SEM image (A) and EDX data (B) indicating thiolate formation on the copper surface of the copper working electrode.

Figure 7. OCP measurements of a copper electrode after the addition of 3.2 × 101 mM DBS carried out in (a) 0.1 M NaClO4 in MeOH (inset, magnified region upon addition of DBS) and (b) 0.1 M NaClO4 in THF. Conditions: copper working electrode vs Ag/AgCl, Pt auxiliary electrode.

Figure 11. Desorption of copper thiolate in 0.5 M NaOH solution. Only linear cathodic scans were performed. Conditions: copper working electrode vs Ag/AgCl, Pt auxiliary electrode, scan rate 50 mV s−1.

Figure 8. Photograph of the electrochemical cells showing the formation of the copper thiolate complex prepared in MeOH upon application of a potential of 2 V to the copper working electrode following the addition of BM at (A) 8 min and (B) 120 min.

Figure 9. Photograph of copper thiolate formation on a bare copper wire prepared in THF 30 min after the addition of BM upon application of a potential of 2 V vs Ag/AgCl to the copper working electrode.

Figure 12. SEM images of (A) copper oxide, (B) copper thiolate complex prepared in propanol, and (D) copper thiolate complex prepared in transformer oil. EDX analysis of (C) copper thiolate complex prepared in propanol and (E) copper thiolate complex prepared in transformer oil.

°C (as per ASTM Standard D 1275B); within an hour, the green thiolate had changed to a black powder. The black powder was washed several times with acetone, air-dried, and examined by PXRD (Figure 13). The acetone filtrate was preconcentrated and examined by GC-FID.

The PXRD of the black powder showed the presence of Cu7.2S4 digenite (JCPDS entry 24-0061) (2θ values of Cu7.2S4 G


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the copper thiolate in transformer mineral oil, whose maximum temperature range spans 80−110 °C53 and the maximum hotspot temperature that can be tolerated is 150 °C.54

Table 1. Raman Modes of Thiolates Prepared by Deposition and the CuO Synthetic Route wavenumber (cm−1) modes

BM

Cu thiolate (deposition)

Cu thiolate (CuO route)

C−S stretching out-of-plane C−H deformation in-plane ring deformation in-plane C−H deformation in-plane ring deformation

730 838

666 760

644 736

656/648 766/758

1024

1001

987

1005/1004

1053

1029

1016

1033/1033

1212

1226

1218

1222/1233

1258 1589

1605

1604

1604/1602

2548 2928 3057

− 2929 2038

− 2940 3059

n/a n/a n/a

in-plane ring deformation υ (S−H) υ (C−H) υ (=(C−H))

Ag thiolate/ Fe thiolate33

4. CONCLUSIONS We have successfully shown the formation of copper thiolate as a consequence of DBDS degradation in transformer oils and a possible pathway to copper sulfide formation. We have also highlighted through experimental investigations that the formation of a DBDS−Cu complex formation may not be possible, thereby clarifying the controversy between the two pathways previously suggested. Thiolate formation was shown to occur via oxidative adsorption on copper surfaces upon exposure to the DBDS degradation products, BM and DBS. Thiolates are formed by self-assembly and are easily deposited on copper surfaces by applying a potential to the copper electrode. They are also easily formed by the reaction of CuO and BM. At temperatures specified in ASTM Standard D 1275B (which denotes accelerated aging conditions for detection of copper sulfide formation), the copper thiolate decomposes to copper sulfide (Cu7.2S4) as identified by PXRD. The possible reaction scheme is outlined in Scheme 3, in which benzyl mercaptan (a) is the precursor to copper thiolate formation (b) and dibenzyl disulfide (c) is a formed as a sideproduct. The thiolate then decomposes to form cuprous sulfide and dibenzyl sulfide (d). This work will enlighten the transformer-based industry on the corrosive action of DBDS through an alternate pathway involving thiolate formation, with substantial experimental evidence to validate the mechanism, which has not been previously shown.

are represented in red on the diffractogram in Figure 13). These results show that copper thiolates are fairly rapidly thermally decomposed to copper sulfide. The GC-FID chromatograms of the degradation side-products revealed the formation of a sulfur-containing component, namely, DBS (see Figure S7). However, over extended periods, in addition to DBS, DBDS is also formed (see Figure S8). The formation of DBDS could possibly have occurred from the oxidation of DBS. The findings from our results are in accordance with those of Turbeville and Yap,37 in which they found the presence of Cu31S16 and C7S4 from the decomposition of copper(I) hexanethiol. Thus, this shows that Cu2S can be formed from

Figure 13. Powder X-ray diffractogram of copper thiolate after heating for an hour. The peaks marked in red denote the 2θ values of Cu7.2S4. H


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Scheme 3. Postulated Reaction Scheme for the Formation of Copper Sulfide

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04032. 13 C NMR spectra of DBDS, BM, and reaction mixtures; ATR-FTIR spectra of BM and copper thiolate; Raman spectra of BM and copper thiolate; and GC chromatograms of degradation products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +27 31 2601394. Fax: +27 31 2603091. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank ESKOM for research funding and the National Research Foundation of South Africa for a Ph.D. bursary to V.D., as well as Dr. Remy Bucher from iThemba Laboratories for use of their XRD equipment and Vishal Bharuth from the Microscopy and Microanalysis Unit (UKZN).

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REFERENCES

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Formation of Corrosive Sulfur with Dibenzyl Disulfide in Fluid-Filled

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