Localized corrosion behavior of paper-wrapped copper in DBDS-containing transformer oil under accelerated conditions

Corrosion of copper conductors induced by corrosive sulfur species is an important reliability concern in oil-immersed transformers. This study isolates the role of insulating-paper wrapping by combining field observations from a decommissioned 550 kV reactor with controlled comparisons of fully wrapped, half-wrapped, and bare copper specimens in DBDS-containing transformer oil. Under a nitrogen atmosphere, paper-wrapped copper developed pronounced localized attack whereas bare copper showed little visible corrosion; in half-wrapped specimens, attack was concentrated mainly in the paper-covered region. After 72 h at 150 °C, paper-wrapped systems exhibited a lower dissolved-copper concentration in the bulk oil (0.22 versus 0.85 mg/L for bare copper) but a higher oil acidity (0.18 versus 0.05 mg KOH/g). Air exposure intensified the overall attack. Temperature and formic-acid addition accelerated DBDS consumption, and the apparent activation energy decreased from 89.29 to 62.5 kJ/mol after formic-acid addition. The observations are interpreted using the established crevice-corrosion framework: paper wrapping creates a restricted interfacial region in which mass-transfer limitation, retention of reactive/acidic species, and, when oxygen is available, differential aeration can favor localized corrosion. The results clarify why specimen surface configuration strongly affects corrosive-sulfur assessment and support the engineering relevance of paper-wrapped copper specimens in oil-paper insulation testing.