Mineral Insulating Oil Passivation Effectiveness of Passivation to Stop Copper Deposition Prof. Bruce Pahlavanpour and Mr. Kjell Sundkvist ynas aphthenics Ltd, Wallis House, 76 orth Street, Guildford Surrey, GU1 4AW, UK Abstract Recent international findings are indicating that presence of potentially corrosive sulphur compounds in the mineral insulating oil, in special circumstances, may lead to copper deposition in the transformer insulation and consequently transformer failure. On the other hand sulphur compounds also greatly contribute to oxidation stability of the oil. In any case to avoid copper dissolution and subsequent deposition, the use of a metal passivator is recommended. This paper reports investigations on the effectiveness and stability of a passivator in mineral insulating oil. Introduction The failures of several newly installed transformers and reactors during the last few years have been linked to formation of copper containing deposits in the solid insulation. Although the known cases seem to be limited to systems operating at the upper end of loading and operating temperatures the problem is obviously of concern. At ynas we have undertaken extensive investigations into the chemistry behind these phenomena to understand the mechanisms behind the failures. 1 From the international experience a few conclusions can be drawn. The main reason for failure seems to be a reduction in the insulation performance of the solid insulation caused by the formation and the deposition of copper sulfides on the cellulose in the windings. A combination of conditions including high loading and operating temperatures must be considered to be risk factors. There is at present no clear understanding of the underlying mechanisms, but to us it is clear that the root cause is chemical interaction between copper, paper and oil. To minimise these interaction a copper passivator can be added to the transformer oil. Such a compound adheres to the metal surfaces and forms a layer which protects the copper from the oil and the oil from the copper. In this paper we summarise our research results and present our conclusions about the efficacy of one such passivator, namely Irgamet 39 (Ciba Specialty Chemicals). Copper Corrosion Most corrosion tests for insulating oil are based on the degree of staining of copper plate immersed in heated oil. Herein lies the first problem; staining may in fact show formation of a stable surface layer which would protect against real corrosion. By our understanding corrosion is a process where metal is transported away from the surface. A shiny copper surface may indicate a continuous transport away form the surface. Therefore we have 1
mostly used the term copper dissolution to describe what we study. We have concluded from experiments that copper sulfides (CuS and Cu 2 S) immersed in hot oil do not contribute to higher copper content of the oil. 2 This indicates that the transport mechanism must be more complex. Fore these reasons we believe that for understanding of the mechanisms it is more relevant to measure copper content of the oil itself. In most of our own investigations this is the approach taken. Possibly corrosion tests where paper is wrapped around the copper could be more relevant for batch testing of new oil as it seems very likely that the paper would capture leached copper. The rate of all chemical reactions increase dramatically with temperature and therefore it is quite possible that transformer oils which are non-corrosive according to older standards will in fact be corrosive under more drastic conditions. Worldwide there is an ongoing effort to revise tests for corrosion to take into account higher operating temperatures. However, it must be stressed that at a certain point all uninhibited oils will stain copper. We have found that there is a strong link between oil oxidation and corrosion. 3 Both early oxidation products such as hydroperoxides, and the later formed carboxylic acids play an important role in copper corrosion. Oxidation stability of oil is therefore of paramount importance. Theory and Mechanism of Passivation Passivation is a term used in corrosion science to describe the formation of nonpermeable protective layers on metal surfaces. The formation of such layers prevents corrosion and dissolution by decreasing the exchange currents between the electrolyte and the metal. 4 Passivators thus create a chemical layer that protects the copper from the oil, and the oil from the copper. This prevents formation of oil soluble copper complexes that can catalyse oil oxidation. The protective layer thus fills two functions; to prevent direct copper transport and to prevent formation of corrosive oil oxidation products. TTA H Irgamet 39 TM R R Figure 1. TTA-type passivators In the electrical industry the organic passivators most commonly used are derivatives of benzotriazole (BTA). The methyl-substituted variant known as tolytriazole (TTA) is slightly more oil soluble than BTA. But only when it is made significantly less hydrophilic by -aminomethylation does it becomes fully miscible with oils. One such example is Irgamet 39 (CIBA Specialty Chemicals) (Figure 1), which we have been studying. The general understanding of the function of benzotriazole type passivators is that the heterocyclic part of the molecule forms complexes with copper or copper ions. 5 From 2
aqueous solution it is known that BTA forms thick multilayers. 6 We have undertaken the first ever surface science study of -methylamino substituted triazoles in hydrophobic media (oil). 7 In that study we utilized a combination of two techniques, ellipsometry and TOF-SIMS. From the ellipsometry (an optical technique which depend on the phase shift of reflected polarized light) data we could conclude that layers of at least monomolecular thickness were built up in matters of hours. Also it was found that virtually no material left the surface after re-emersion in passivator free oil indicating irreversible binding. From the mass spectroscopic TOF-SIMS technique we could draw conclusions about what type of chemical species were present on the copper surfaces. TTA and Irgamet 39 left very similar surface species, which indicates that the -methylamino part of Irgamet 39 is split from the rest of the molecule on binding. In these experiments the metal surfaces were washed with toluene and methanol before measurement, so also these results indicate irreversible binding of the passivators on the copper surfaces. This is important because it means that once passivators have had a brief chance to adhere to copper the concentration in the oil no longer matters. Stability of the Passivator It has been observed by several parties that the concentration of Irgamet 39 goes down when the oil, in which it is dissolved, is severely oxidised. With aid of Katritzky s extensive work on benzotriazoles 8 we set out to investigate the mechanism and kinetics of this phenomenon. Benzotriazoles are known in the literature to be thermally very stable. 9 As expected we could not provoke degradation of the passivator in oil by heat alone. Complete gas-phase pyrolysis of several benzotriazole derivatives have been achieved in the temperature range 210-450 C. 10 Benzotriazole itself has very recently been shown to decompose exothermically in the temperature range 306-410 C, 11 which is unrealistically high for insulating oils in service. For these reasons it seems unlikely that the passivator should decompose thermally under the conditions in a transformer. 1,2,3-Triazoles are also generally relatively resistant to both oxidation and reduction. Since it was found that the passivator concentration did not diminish as a consequence of reaction with secondary oil oxidation products like aldehydes, ketones, esters or carboxylic acids, 12 it was suspected that radical reaction of hydroperoxides were involved. Hydroperoxides are the direct products of hydrocarbon (oil) oxidation and they readily give peroxy radicals, which take part in further chain reactions to form other oxidation products. We found that decomposition of hydroperoxides had a profound effect on Irgamet 39 and TTA. The rate of disappearance was found to be proportional to the square of the hydroperoxide concentration. 12 So yet again, oxidation stability of the oil is important. The more oxidation stable an oil is, the lower the hydroperoxide concentration will be at any given time. It should be pointed out that the very high concentration of hydroperoxides necessary for rapid degradation can rarely, if ever, be found in transformer oil in service. Our experiments are, as ever in short experimental tests of transformer materials, gross exaggerations of real conditions. So we know that oxidation of transformer oil will eventually lead to degradation of passivator in solution. However, we also know from the surface science studies that 3
passivator binding to the copper is essentially irreversible in hydrophobic media such as oil. This suggests that once copper surfaces have been covered with passivator, there should be a hindrance to corrosion even if the oil then oxidises heavily and hydroperoxides break down the passivator left in the oil. In fact, that is exactly what the experimental results show. 800 700 600 Cu (ppm) 500 400 300 Untreated Treated 200 100 0 0 2 4 6 8 10 12 14 16 18 Figure 2. Simulated oil aging and copper dissolution Hydroperoxides (mm) The graph in Figure 2 shows the difference in copper dissolved in the oil as function of initial hydroperoxide concentration for pieces of copper not treated and treated with Irgamet 39 prior to the start of the experiment. The oil in the experiment did not contain any passivator, thus the experiment shows that the protective layer of passivator does not need to be continuously renewed with fresh material from solution to exercise its protective function. It is also worth noting the tremendous effect on copper dissolution that the hydroperoxides exert. Interaction Paper-Passivator In one of the oldest forms of chromatography paper (cellulose) is used to retain organic compounds from a solution. It is therefore natural to suspect that the paper insulation around the copper should be able to absorb the relatively polar passivator molecule. Such an effect could at least partly explain diminishing passivator concentration in the oil of transformers in service. We devised a series of experiments to show this absorption under various conditions. The study will be published in its entirety elsewhere, but it is clear to us that the absorption is very significant at temperatures ranging from ambient to 120 C. 4
100 95 90 Passivator (ppm) 85 80 75 70 65 60 55 50 0 2 4 6 8 10 12 14 16 18 20 Time (weeks) Figure 3. Absorption of Irgamet 39 by insulating paper at 80 C. The kinetic behaviour (Figure 3) is typical of an absorption as it levels off with time. These results indicate that a significant fraction of the recommended 100 ppm added concentration of the passivator will be absorbed by the paper insulation. 2 For reasons easy to understand the passivator must pass the paper on its way from the oil to the copper. A reservoir of passivator close to the metal surface should therefore not be unwelcome. Conclusions We have found that the copper passivator Irgamet 39, an -aminomethylated benzotriazole, exerts a very significant protection against copper dissolution in insulating oil. In our view it is this dissolution of copper which can lead to subsequent deposition of copper containing material in the solid cellulosic insulation of a transformer. It appears that hydrocarbon (oil) oxidation products are chiefly responsible for the dissolution. The notion is supported by the finding that oils exposed to solid copper sulfides do not show high copper contents after aging experiments. Thus oxidation stability of the transformer oil is of importance also to prevent copper corrosion/deposition. Furthermore, it was found that the passivator itself degrades under the influence of hydroperoxides. Therefore also the longevity of the passivator in oil is prolonged when the oxidation stability of the oil is good. The surface chemistry studies undertaken have shown that binding of the passivator to the copper surfaces is essentially irreversible. Therefore, passivator concentration does not need to be replenished when it drops. This was also confirmed by practical corrosion experiments where copper once exposed to passivator retained its passivation even on severe oil oxidation. 5
It was also seen that a significant amount of the passivator will be absorbed by the paper insulation, a likely reservoir of passivator over time. References 1. Controlling copper corrosion; ynas apthenics, 2007. 2. Wiklund, P.; Levin, M.; Pahlavanpour, B., Copper dissolution and metal passivators in insulating oil. IEEE Electrical Insulation Magazine 2007, in press. 3. Kalantar, A.; Levin, M., Factors affecting the dissolution of copper in transformer oils. Submitted to Lubr. Sci. 2007. 4. Atkins, P. W., In Physical Chemistry, 5th ed.; Oxford University Press: Oxford, Melbourne, Tokyo, 1994; pp 1026-1029. 5. Xu, Z.; Lau, S.; Bohn, P. W., The role of itrogen and Sulfur heterocycles in corrosion inhibition. 1. Initial steps in the adsorption of benzotrizoles at Copper(I) and Copper (II) oxides. Langmuir 1993, 9, 993-1000. 6. Cotton, J. B.; Scholes, I. R., Benzotriazole and related compounds as corrosion inhibitors for copper. Brit. Corros. J. 1967, 2, 1-5. 7. Levin, M.; Wiklund, P.; Arwin, H., Adsorption and film growth of - methylamino substituted triazoles on copper surfaces in hydrocarbon solvents. Appl. Surf. Sci. 2007, in press. 8. Katritzky, A. R.; Lan, X.; Yang, J. Z.; Denisko, O. V., Properties and Synthetic Utility of -Substituted Benzotriazoles. Chem. Rev. 1998, 98, 409-548. 9. Joule, J. A.; Mills, K.; Smith, G. F., Heterocyclic Chemistry. Stanley Thorns (Publishers) Ltd: Cheltenham, 1998. 10. Dib, H. H.; Al-Awadi,.; Ibrahim, Y. A.; El-Dusouqui, O. M. E., Gas-phase thermolysis of benzotriazole derivatives: part 1- synthesis of alfa-(1)- and (2)- benzotriazolyl ketones and kinetics and mechanism of their gas-phase pyrolysis. J. Phys. Chem. 2004, 17, 267-272. 11. Katritzky, A. R.; Wang, Z.; Tsikolia, M.; Hall, D. C.; Carman, M., Benzotriazole is thermally more stable than 1,2,3-triazole. Tetrahedron Lett. 2006, 47, 7653-7654. 12. Wiklund, P., The Chemical Stability of Benzotriazole Copper Surface Passivators in Insulating Oils. Submitted to Ind. Eng. Chem. Res. 2007. 6