Duration and Mechanism for Suppressive Effect of Triazole-based Passivators on Copper-sulfide Deposition on Insulating Paper

The suppressive effect of triazole-based passivators such as 1,2,3-benzotriazole (BTA) and Irgamet 39 (TM) (CIBA Specialty, Basel, Switzerland) on the deposition of copper sulfide on insulating paper in transformers and reactors was investigated by heating test with bare and paper-wrapped copper plates. Commercially available mineral oil meeting the specification of IEC 60296 was used. This oil contained 136 ppm wt./wt. of dibenzyl disulfide (DBDS). The deposition of copper sulfide occurred after 144 h of heating at 140 C without these passivators. However, little deposition of copper sulfide occurred even after 432 h of heating when either of these passivators was added to the oil. The concentration of dissolved copper significantly decreased when either of these passivators was added to the oil, which reflects the formation of complex layers on the copper surface caused by the reaction between these passivators and copper. The complex layers were confirmed by time-of-flight secondary ion mass spectrometer (TOF-SIMS). The suppressive effect was derived from retarding copper from dissolving into the oil due to the reaction inhibition between DBDS and copper by the complex layers. The duration of these passivators can be determined by the durability of the complex layers.