Corrosive Sulfur

Dibenzyl disulfide (DBDS) is the most aggressive corrosive sulfur compound found in mineral insulating oil. It acts as a complexing agent that reacts with copper surfaces at normal operating temperatures (80–150°C).

• Chemical mechanism: DBDS + Cu → Cu-DBDS complex → Cu₂S deposition
• Concentration thresholds: Corrosion can initiate at concentrations as low as 5 mg/kg
• Source origin: Historically introduced through certain refining processes or as an antioxidant additive in specific oil formulations
• Detection method: IEC 62697-1 standardized test using wrapped copper strips aged at 150°C for 72 hours

The reaction product, copper sulfide (Cu₂S) , is a conductive compound that detaches from copper conductors and migrates through the oil into the cellulose paper insulation. This creates a semi-conductive layer that compromises the dielectric integrity of the winding insulation.

• Conductivity: Cu₂S exhibits metallic-like conductivity, effectively shorting the insulation
• Migration pathway: Deposits form preferentially in inter-turn paper layers where electrical stress is highest
• Visual signature: Grayish-black staining on copper surfaces and paper wraps
• Progression: Deposition accelerates with temperature, forming dendritic growth patterns that bridge adjacent turns

Corrosive sulfur does not produce unique dissolved gas signatures, making it a silent threat that evades standard DGA interpretation. However, secondary fault gases may appear once copper sulfide deposits initiate partial discharge or arcing.

• Absence of early warning: No specific gas ratio indicates DBDS corrosion before electrical failure begins
• Secondary gas patterns: Once turn-to-turn shorts develop, expect elevated acetylene (C₂H₂) and hydrogen (H₂)
• Complementary testing required: Must combine DGA with furan analysis and direct copper strip corrosion tests per IEC 62535
• Oil testing frequency: Annual dissolved metal analysis for copper content can indicate active corrosion

The primary chemical mitigation for corrosive sulfur is the addition of metal passivators, typically benzotriazole (BTA) derivatives, which form a protective film on copper surfaces to block DBDS attack.

• Passivator chemistry: Irgamet 39 is the most widely used commercial passivator, dosed at 100–300 mg/kg
• Mechanism: Forms a monomolecular barrier layer on copper that prevents DBDS complexation without affecting dielectric properties
• Limitations: Passivation does not remove existing Cu₂S deposits; it only prevents further corrosion
• Monitoring: Regular re-testing required to confirm passivator depletion has not occurred, especially after oil reclamation

For severely contaminated transformers, full oil reclamation or replacement is the definitive remediation. Reclamation processes using Fuller's earth or activated alumina can remove both DBDS and ionic degradation byproducts.

• Reclamation efficiency: Can reduce DBDS concentration below detectable limits when combined with multiple passes
• Risk of reclamation: Aggressive clay treatment may strip natural antioxidants, requiring re-inhibition
• Oil replacement: Complete drain and flush is the most thorough option but carries risks of disturbing aged insulation
• Post-treatment protocol: Must re-passivate after reclamation and conduct follow-up DGA at 3, 6, and 12-month intervals

Multiple international standards govern the detection and classification of corrosive sulfur in insulating oils. Compliance with these standards is critical for transformer asset management and warranty validation.

• IEC 62535: Standard test method for detection of potentially corrosive sulfur in used and unused insulating oil
• IEC 62697-1: Quantitative test method for DBDS specifically using gas chromatography with electron capture detection
• ASTM D1275: Standard test method for corrosive sulfur in electrical insulating oils using copper strip corrosion
• CIGRE TB 378: Technical brochure providing comprehensive guidance on copper sulfide in transformer insulation, including risk assessment frameworks