The presence of some metals such as iron and copper are known to increase the rate of the oxidation reaction, that is, they can act as a catalyst. For this reason it is important to remember that equipment used in contact with oil should never be made of copper or alloys of copper. The presence of trace amounts of iron in the oil is inevitable since a lot of the handling and storage equipment are made of iron and machinery wear and tear will contribute to some trace level of iron in the oil. However, the use of rusty equipment or tanks that can result in a high level of iron being resent in the oil should be absolutely avoided