Contact-based corrosion mechanism leading to copper sulphide deposition on insulating paper used in oil-immersed electrical power equipment

• Effect of [DBDS], time, air and paper-Cu proximity on CuxS deposition was studied.
• XPS and EDX revealed deposition of CuxS in paper increased with time or [DBDS].
• O2 was observed to have a key role in CuxS displacement from the conductor surface.
• A bifurcated pathway for CuxS deposition in oil-immersed equipment is proposed.

XPS and EDX were used to study CuxS deposition in Kraft insulating paper obtained from ageing paper samples in contact with a copper sheet, in mineral oil containing dibenzyl disulphide (DBDS). Qualitative and quantitative evidence of CuxS were obtained from bulk and surface analyses of insulating paper provided by the different depth resolution of the techniques used. The influence of [DBDS], oxygen, time, and Cu-paper contact upon the formation and mobilisation of CuxS were studied. A contact-based mechanism to explain CuxS contamination on paper and its displacement was proposed. The role of O2 in enhancing CuxS displacement was also investigated.