The insulating oils are continually in contact with metals subject to corrosion, such as silver cobreo . So the presence of corrosive sulfur compounds would prejudice the result of deterioration of the materials. The presence of this undesirable compound in the oil can be caused by poor refining .
In recent years , numerous found failures attributed to the presence of corrosive sulfur in oil transformers. This is because the sulfur reacts with copper conductors and silver contacts to form copper sulfide and silver sulfide on the surface of metal. Failures resulting arc source in the coils, insulation loss due to the precipitation of crystals of metal sulfides in the cellulose of the windings. Mineral oils used for insulation purposes , originally containing different amounts of sulfur compounds depending on the degree of refining , reliable methods to detect new oil corrosive sulfur and service are ASTM – 1275b and ASTM -130 .