Why is corrosive sulfur in transformer oil a serious risk?

Corrosive sulfur can react with copper and silver conductors to form conductive sulfide deposits on paper and winding surfaces. These deposits can reduce dielectric margins and contribute to partial discharge and catastrophic failures, especially in high voltage windings. The risk is significant because it may develop over time without obvious external symptoms until insulation strength is compromised. Utilities manage this by specifying non corrosive oils, screening oils for corrosive sulfur, and using passivators where appropriate under controlled guidance.