This paper presents an exhaustive study regarding the behaviour of copper in various insulating fluids when exposed to 1100C for 1000 hours in closed systems, with no access to atmospheric oxygen. Following the analysis carried out, it was found that the variation of oxygen concentration in the investigated oil samples, during the thermal treatment, may be best described using an exponential decrease versus time. Moreover, from the gas chromatography determinations with respect to the evolution of the CO and CO2 content in the oils during the thermal treatment one found out that these fit very well the Richards model, which corroborated with all the other analysis confirms the fact that the copper films undergo consistent morphological changes only when exposed to mineral oils, so that it is of paramount importance that, taking into account this aggressive character, the equipment placed in direct contact with these oils should be carefully inspected on a periodical basis. This behaviour is thought to be a direct consequence of the sulphur compounds contents for these oils, the secondary oxidation products of these compounds increasing their corrosive aggressiveness.

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