Study of degradation of sulphur compounds and depletion of metal passivators during thermal ageing of mineral oil

Presence of sulphur compounds in mineral transformer oil poses serious problems of corrosion of copper conductors. Thermal degradation of sulphur compounds like DBDS, 2- mercaptobenzothiozole and depletion of metal passivators like Irgamet 39 and BTA in mineral oil are complex processes. This paper presents results of investigations on degradation of mineral oil with these additives under thermal ageing in air and nitrogen. Efforts are made to explain independently the observed degradation of DBDS, 2-mercaptobenzothiozole and depletion of Irgamet 39 and BTA over a range of temperatures. The effect of sulphur compounds on the performance and depletion of BTA and Irgamet 39 are also explained. Irgamet 39 and BTA deplete in oil under thermal ageing to lower levels, but continue to protect copper from corrosion due to their adsorption by paper which acts as reservoir of metal passivators. The advantages and limitations in the use of Irgamet 39 and BTA are also discussed. To further substantiate the results of the study, effect of these additives on acid formation, interfacial tension and copper ion concentration are discussed.