(1) Based on the acid value, the amount of acidic substances contained in the oil can be determined. Generally speaking, the higher the acid value, the more acidic substances are contained in the oil. The amount of acidic substances in oil varies with the degree of raw material and oil refining.
(2) The acids in lubricating oil can be divided into two types: organic acids and inorganic acids. There are two sources of organic acids in fresh lubricating oil: one is that the original oil is not completely removed during refining; The second is the intentional addition of acidic antioxidant and anti-corrosion additives. The main source of lubricating oil in use is organic acid, which is produced by self oxidation. When the content of organic acid is low, it does not have much corrosive effect on metals, but instead can increase the oiliness of the lubricating oil to maintain good boundary lubrication performance. When its content is high, it will cause corrosion to some bearing materials (non-ferrous metals and their alloys, especially lead). Inorganic acids refer to sulfuric acid, which has a strong corrosive effect on metals. Sulfuric acid is generally not allowed in lubricating oil. The sulfuric acid that may be present in fresh lubricating oil is residual after acid washing and neutralization during the refining process; The lubricating oil in use may form sulfuric acid due to the leakage of combustion products from sulfur-containing fuel into the crankcase.
(3) The corrosion properties of oil products on metals can be roughly determined based on their acid value. There is a low content of organic acid in the oil, and it will not have a corrosive effect on metals when there is no water and the temperature is low. But when the content is high and there is moisture, it can corrode the metal. The smaller the molecular weight of organic acids, the greater their corrosion resistance. When water is present, even trace amounts of low molecular acids have a strong corrosive effect. Although naphthenic acids in petroleum fractions are weak acids, they can also corrode certain non-ferrous metals in the presence of moisture, especially lead and zinc, resulting in the formation of metal soaps. Such soaps can cause accelerated oxidation of lubricating oil. At the same time, soap gradually accumulates in the oil and becomes sediment, disrupting the normal operation of the machine. The acidic substances generated by the oxidation of gasoline during storage are more corrosive than naphthenic acids, and some of them can be dissolved in water. When water falls into the oil tank, it increases its ability to corrode metal containers.
(4) The acid value of diesel has a significant impact on the working condition of diesel engines. Diesel fuel with high acid value can increase the buildup of carbon in the engine, which is the cause of piston wear and nozzle coking.
(5) The degree of deterioration of lubricating oil in use can be determined by its acid value. After a period of use, lubricating oil gradually deteriorates due to oxidation, manifested by an increase in acid value. When the acid value exceeds the limit, the oil should be replaced with a new one.