Formation of Corrosive Sulfur with Dibenzyl Disulfide in Fluid-Filled Transformers

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 X-ray diffraction analysis.