Electrical insulating mineral oils are mainly manufactured from naphthenic crude sources, with a small number being produced from paraffinic crudes and gas-to-liquids technology. The resulting products are most frequently free of additives, except for certain types of phenol-based oxidation inhibitors, and of a very low percentage by weight, only up to 0.4 %. The main specifications for electrical insulating oils, Doble TOPS, ASTM D3487 and IEC 60296, only allow oxidation inhibitors and any other additives must be agreed upon by the entities involved in the selling and purchasing of the product. Over the years, other additives have been used, sometimes unbeknownst to the purchaser, some of which include pour point depressants, gassing tendency additives, other types of oxidation inhibitors, passivators and metal deactivators. This paper discusses the uses of passivators and how they work in transformer systems. Presently, passivators are being used to control issues with corrosive sulfur but they have also been used to control oxidation stability of the oil and streaming (static) electrification. This paper will focus mainly on the use of passivators to control corrosive sulfur
