Kinetic Analysis of the Reaction of Silver with Elemental Sulfur in Mineral Insulating Oil

Elemental sulfur (S₈) reacts readily with silver, forming highly conductive silver sulfide on silver-coated components of on-load tap changers (OLTCs), forming a highly conductive silver sulfide film at the surface of an OLTC, which can lead to the failure of critical components in power transformers. This study investigates the reaction between metallic silver and elemental sulfur dissolved in mineral insulating oil across temperatures from 60 °C to 180 °C. The process involves three stages: the diffusion of sulfur through oil, surface reaction, and product diffusion. For low-viscosity oil, diffusion is not the limiting factor, and sulfur does not react immediately on the silver's surface, suggesting possible adsorption or intermediate formation. A kinetic analysis revealed that the reaction follows first-order kinetics, with a change in mechanism above 150 °C. The reaction follows the Arrhenius equation in two separate regions: 60-150 °C and 150-180 °C. Activation energy was calculated as 23.67 kJ mol^-1, and it can be concluded that the reaction is controlled by the diffusion of sulfur through mineral oil, and at higher temperatures (150 °C to 180 °C), the calculated activation energy is 160.69 kJ mol^-1, which leads to the conclusion that the combined chemisorption and diffusion through a silver sulfide-oil interface becomes the new limiting factor.