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The ability of lubricating oil to resist oxidative degradation is called the oxidation stability of the lubricating oil. Lubricant stability consists of antioxidant stability and thermal oxidation stability. The first is antioxidant stability, which refers to the ability of thick-layer lubricants (with an oil layer thickness greater than 200 μm) to resist oxidation under the effects of high temperatures and oxygen in the air. Lubricants are stable at normal temperatures, but at high temperatures their color turns black after a short period of time, their viscosity changes, their acid value increases, and precipitates form. Generally, lubricating oils do not oxidize easily below 30°C, but oxidation occurs rapidly above 150°C. Oxidation of lubricating oil in things such as internal combustion engine oils, gearboxes, transformers, or oil pipelines is an example of this. Therefore, the lubricant is required to have good antioxidant properties. Antioxidants are added to some lubricants to improve their oxidation resistance and stability. The second is thermal oxidation stability, which refers to the ability of thin-layer lubricants (with an oil layer thickness of less than 200 μm) to resist oxidation under the action of oxygen in air at high temperatures. Lubricating oils that operate between the piston and cylinder in internal combustion engines, as well as those that work between the shaft bearings in steam turbines, fall into this category. At this time, the operating temperature of the lubricating oil is high (200–300°C), and the catalytic oxidation of the lubricating oil by metals is also strong. Oxidation results in the formation of a highly adhesive film that covers the metal components, leading to increased wear, reduced power output, and difficulties in heat conduction; in severe cases, it can even cause the components to burn out. During storage and use, lubricants undergo oxidation and deterioration due to exposure to light or heat, as well as the action of oxygen in the air and metals; this causes their color to darken, their viscosity to increase, and the formation of acidic compounds, gums, and sediments. The oxidation process of lubricating oils, which are composed of a mixture of hydrocarbons with various structures, is highly complex. Because the components of the lubricating oil are different. Acidic oxidation products include hydroxyc acids, phenols, etc.; further oxidation leads to the formation of low-molecular-weight acids. These products increase the acid value, and the resulting acidic substances can corrode metal components. But sometimes oxidation can produce only a small amount of acidic substances, with the majority forming neutral products.