The acids are organic and nothing like as corrosive as, say, sulphuric acid, but they can cause corrosion and accelerate the degradation of solid insulation.
There is, however, a great deal which can be done to reduce oxidation without the use of additives. Firstly by reducing the degree of contact between the oil and air. There are good reasons for not wishing to seal off the oil completely from the external air and these will be identified later. However, in all but the smallest distribution transformers it is economic to provide a conservator. Not only does this reduce the area of contact between the oil and air, but it also ensures that the oil which is in contact with air is at a lower temperature than the bulk oil.
Temperature is, of course, an important factor. Each 7°C increase in temperature above normal ambient doubles the rate of oxidation.
Then there are, as has been mentioned, the effects of catalysts. It is unfortunate that copper is a strong catalyst in the oxidation process. Iron is a catalyst also, but not quite so strongly. There is little that can be done about the copper in the windings, although being insulated does restrict the access to the oil thereby reducing the effect. Bare copper, such as is frequently used for lower voltage leads and connections can be tinned, since tin does not have a catalytic action.
The internal surfaces of steel tanks and steel core frames can be painted with oil-resistant paint.
There is also an effect which is sometimes referred to as auto-catalytic action. Some of the products of oxidation themselves have the effect of accelerating further oxidation. This is particularly the case when some aromatic compounds are oxidized. Hence, oils with increased aromatic content over a certain optimum quantity of about 5–10 per cent are more prone to oxidation. By the use of these measures alone there has been a significant reduction in the extent to which oxidation has shown itself to be a problem over the last 30 or so years. Offset against this is the fact that since the 1970s there has been a tendency to increase operating temperatures, and measures to reduce the degree of contact between the oil and catalytic copper and iron have been reduced as a cost saving measure, particularly in many distribution transformers, it is possible that once again users will begin to experience the re-emergence of oxidation as a serious problem in many transformers.