Stability and Reactivity of Sulfur Compounds against Copper in Insulating Mineral Oil: Definition of a Corrosiveness Ranking

Corrosive sulfur is a well-known critical issue for mineral insulating oils used in power transformers. The objective of this work is to study the reactivity of different groups of sulfur containing compounds (aliphatic and aromatic mercaptans, sulfides, disulfides, thiophenes) that may be present in insulating oils. The stability of nine compounds, pertaining to the aforementioned groups, was investigated with regards to temperature and aging time, under oxidative and inert atmosphere, in order to simulate the natural aging of both hermetically sealed or free breathing type transformers. Their reactivity against copper was also investigated. A gas chromatography atomic emission detection method was optimized to quantify the consumption of each compound. The tendency of 22 sulfur compounds to react with copper and form copper sulfide was quantitatively assessed at different temperatures. The resulting corrosiveness ranking for sulfur compounds families differs from what is usually reported in the literature and was proven to be temperature dependent