Oxidation inhibitors: Oxidation reactions are exothermic and very complex reactions. Oxidation in lubricants that contain paraffinic and naphthenic hydrocarbons may lead to the formation of acidic or soluble products in lubricants, and oxidation in aromatic hydrocarbons may form sludge and varnish that are insoluble in lubricants.
Lubricants frequently contact with air in their area of use. When this happens, a series of complex oxidation reactions occur. Generally, viscosity of the lubricant increases when it is oxidized, acidic wastes and carbon products may be formed in the lubricant. These deposits may lead to an increase in the surface temperature as they create insulation on the surface. This impact may lead to malfunctions and failures as it prevents the elements, like in the piston segments, from functioning properly.
Corrosion inhibitors can be divided into two groups according to their functions:
i. Additives to prevent chain reaction: They prevent the formation of acid and sediments by stopping or retarding the reaction between the hydrocarbons in the lubricant and oxygen in the air. In this way, the corrosion which is a result of oxidation is prevented.
ii. Additives to prevent reaction with the lubricant: They cover the surfaces of metal fragments and other metal parts which are formed as a result of abrasion and act as catalyzers, and therefore prevent the chemical reactions between these metals and the lubricant.
Petroleum based lubricants may contain certain natural inhibitors. Structure and amount of these inhibitors depend on the type of crude oil, refinery method and degree. In addition, oxidation inhibitors are mostly provided with synthetic materials. An effective anti-oxidant hampers the formation of organic peroxide and stops the chain oxidation reaction. Substances such as sulphide, hydroxy sulphide, phenols and organic amines that contain compounds like sulphur, phosphor and nitrogen; and also compounds including zinc, tin or barium are widely used as anti-oxidation compounds. These compounds prevent the formation of acid by decreasing the amount of oxygen that the lubricant can contain, and ensure oxidation in the additives rather than in the lubricant.
Corrosion inhibitors: Oil acids are formed when the fuel is not fully combusted in the engine. Moreover, rust and/or corrosion may be formed on metal surfaces when the water vapor in the air combines with the water vapor and gases which are formed as a result of the combustion in the combustion chamber of the engine. Corrosion inhibitors can protect the bearing alloys, metal surfaces, and the iron-free parts of the engine from the chemical impacts of acidic residues in lubricants by creating a film on metal surfaces. Corrosive impacts of these residues depend on the material of the lubricated part, the operating temperature and time. The most important factor that brings a penetration impact to petroleum and petroleum products is free sulphur and compounds. A product that contain higher than 0.5 percent sulphur is considered high-pressure for steel. Engine fuels contain dissolved sulphur compounds that accelerate corrosion.
Corrosion may lead to leakage in critical engine parts and accelerate abrasion on sliding surfaces. Penetration of combustion products, particularly oxide particles, in the lubricant causes abrasion in every part of the engine. There are two main methods to minimize corrosion: neutralization of acidic products before reaching the metal surface, and preventing corrosive materials from contacting the surface by creating a passivating film that has strong adherence capacity.
The additives that are used for preventing corrosion in engines contain polar groups. These poles cause adsorption of additives in metal surfaces. Adsorbed additive creates a film that will prevent the contact of corrosive materials with metallic surfaces. This film strongly adheres to the surfaces with cleaning and diffusing additives. Corrosion inhibitor additive compounds consist of organic compounds that contain active sulphur, phosphor or nitrogen; organic sulphides, metal salts, phosphoric acid and sulphurized waxes, basic silicates, nitrites, and compounds with low molecular weight such as basic amines and hydroxylamine.
Rust inhibitors: The additives that are added into the lubricant to prevent rusting in machinery parts are polar organic compounds. These compounds create a film before the chemical reaction with the metal surface, and prevent the metal surface’s contact with water and air. In this way, the machine surfaces are protected from the damage of water vapor and acids in the corrosive environment. Petroleum pipelines, pipes in hydraulic systems, storages, and tanks are mostly made from steel, and therefore they will rust in the presence of water. The corrosion that will expand from rusting areas will eventually open holes in the pipes and tanks. Lubricants, fuels and/or other products in pipelines and tanks get soiled with rust particles, lead to clogging in filters and increase abrasion in pumps and injectors. These additives, which create a protective layer on the metal surface, contain amines, lard, some fatty acids and compounds such as sulphonate.