The authors investigated how changes in the molecular structure of sulfur compounds in mineral insulating oils affect their corrosivity. A method for the quantitative evaluation of corrosivity of insulating oils was developed and used in this study. Several types of sulfur compounds were found to cause sulfur corrosion. However, hydrotreated oil was noncorrosive because the sulfur compounds which cause sulfur corrosion were easily removed through hydrotreatment. The results of thermal aging tests using model sulfide compounds showed that the corrosivity of sulfide-added oils increased with longer heating times due to generation of sulfoxide, caused by the oxidation of sulfide. The corrosivity of aged oils depends on the concentration of sulfoxide in the oils. Meanwhile, the corrosivity of a commercial insulating oil did not change drastically in the thermal aging test, despite the fact that this oil contained a certain amount of sulfide.
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