The Corrosion and Activation Mechanism of Thiophenic Sulfides in the Oil-Paper Insulation

The corrosive sulfides in the insulating oil have been confirmed to induce insulation faults; thereby, the corrosive sulfides can be removed during the refining process of oil to overcome the corrosion threat. Some inactive sulfides are retained to improve the oxidation stability of oil, mainly including thiophenic sulfides. However, the activation of thiophenic sulfides under the operating conditions and whether it causes corrosion of copper winding remain unclear. Therefore, this article focuses on the reaction mechanism of copper winding corrosion induced by the pyrolysis of thiophenic sulfides in oil from the perspective of microscopic reaction mechanism, mainly including the analysis of thermal decomposition products of thiophenic sulfides, the activation energy of thiophenic sulfides on varied heating rates, the molecular dynamics simulation of thiophenic sulfides degradation, and the thermal aging verification test. The results indicate that the pyrolysis products of thiophenic sulfides consist of sulfur-containing micromolecules and inorganic products, such as hydrogen sulfide and its ion (H2S, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {HS}}^{-}$ </tex-math></inline-formula>), elemental sulfur (S, S2), and hydrogen and its ion (H2, H+). Thiophene has the lowest activation energy; thereby, it is the most prone to pyrolysis, followed by benzothiophene and dibenzothiophene. The continuous energy accumulation results in the decomposition of thiophenic sulfides due to long-term thermal action and generates low molecular corrosive sulfides, further intensifying the oil corrosivity and leading to the corrosion of oil-paper insulation.