【Q&A Question 345】December 29, 2017

Viscosity, viscosity index, flash point, ignition point, acid value, moisture, corrosion, freezing point, residue, specific gravity, color.

Viscosity, viscosity index, flash point, ignition point, acid value, moisture, corrosion, freezing point, residue, specific gravity, etc

Viscosity, acid value, water-soluble acids and bases, flash point, mechanical impurities, moisture, liquid-phase corrosion

Kinematic viscosity, moisture, mechanical impurities, flash point, density

Viscosity, flash point, freezing point, moisture content, acid value, antioxidant stability, anti-emulsification properties, etc.

Viscosity, viscosity index, flash point, ignition point, acid value, moisture, corrosion, freezing point, residue, specific gravity, etc

There are viscosity, acid value, water-soluble acids and bases, flash point, mechanical impurities, moisture, and liquid-phase corrosion. Among them, viscosity has always been the most important parameter for lubricants.

The main quality indicators of lubricating oils and their significance in use 1. Water-soluble acids and bases: Water-soluble acids and bases in lubricating oils refer to inorganic acids and bases that can dissolve in water, as well as low-molecular-weight organic acids and basic oxides. It arises due to poor quality of the oil during refining, insufficient neutralization, or contamination and oxidation during use and storage. The determination method follows the national standard GB/T259-88 for the testing of water-soluble acids and bases in petroleum products. The significance of water-soluble acids and bases in use: 1) If oil contains water-soluble acids or bases during use, it is mainly due to oxidation and degradation; this also indicates that the oil has poor antioxidant properties. 2) The presence of water-soluble acids or bases in the oil can severely corrode metal components, especially when moisture is also present, as the corrosion proceeds more rapidly in such conditions. Therefore, it is also an indicator that ensures components are not corroded and controls the degree of oil refinement. 3) The presence of water-soluble acids and bases can reduce the emulsification resistance of turbine oil and decrease the insulation properties of transformer oil. Therefore, the presence of water-soluble acids and bases is generally not allowed in oils. When water-soluble acids and bases are present in the oil, it should be treated immediately or replaced with new oil. 4) For certain engine lubricants containing floating multi-effect additives (such as diesel engine oils and gasoline engine oils), since the additives are alkaline, an alkaline reaction is permitted during testing; this should be distinguished from those containing water-soluble acids and bases. 2. Corrosion test: The corrosion test is used to determine the corrosive effect of lubricants on metals under certain temperatures and specified conditions, in the absence of oxygen. Used to test the corrosion of metals by acids or sulfides in lubricating oil. And determine the changes that occur at the metal surface and in the lubricating oil under high-temperature conditions. The determination method is carried out in accordance with SH/T 0195-92, Lubricant Corrosion Test Method. The significance of corrosion testing in practical applications: 1) Low-molecular-weight organic acids and inorganic acids present in lubricants are highly corrosive to metals such as copper, lead, and tin, as well as their alloys; they increase wear and sludge formation, and can even damage moving parts in equipment. 2) Corrosive sulfides accelerate the deterioration of engine lubricant, resulting in the formation of large amounts of sludge and carbon deposits. 3) Active sulfur, free sulfur, and acidic compounds in oils are highly corrosive to copper and lead metals. However, the polar molecules of neutral sulfides can improve the lubricity and corrosion resistance of oils. For example, in certain oils, to improve their corrosion resistance and lubricating properties, a certain neutral sulfide can be added; its molecular polarity enables it to adhere firmly to the metal surface, thereby forming an oil film with high strength. Sulfur-containing compounds such as zinc dialkyldithiophosphate and dibenzyl disulfide are added to hypoid gear oils and guide rail oils. 4) During use, the acid value of lubricating oil gradually increases; once it reaches a certain level, it will cause corrosion to the machine components. Therefore, corrosion testing is also used to determine the degree of deterioration of oil. If the oil fails this corrosion test, it should be replaced with fresh oil immediately. 5) The corrosion test is also an indicator for controlling the degree of oil refining and the quality of used oil regeneration. 3. Mechanical impurities: These are insoluble substances that are suspended or settled in lubricating oil (such as dust, sediment, metal powder, grinding wheel powder, etc.), and they are collectively referred to as mechanical impurities. Generally, the content is determined by diluting the oil sample with a solvent. It is expressed as a mass fraction. The determination method is in accordance with GB511-88, Method for Determining Mechanical Impurities in Petroleum Products and Additives. The significance of mechanical impurities in use: 1) Mechanical impurities are one of the important indicators of lubricant quality; their presence can destroy the oil film, accelerate the wear of moving parts, and even directly damage the components, leading to shaft seizure. Additionally, they can block oil passages and filters, causing lubrication problems in the equipment. 2) Mechanical impurities in transformer oil can reduce insulation performance. 3) When the mechanical impurities in the oil exceed a certain level (with a mass fraction greater than 0.2%), the oil should be replaced immediately with fresh oil. 4. Thermal oxidation stability: Thermal oxidation stability refers to the degree to which a thin layer of lubricating oil is oxidized on a metal surface under the action of heat and oxygen in the air. The measurement of this parameter is primarily aimed at simulating the working conditions of engine lubricant in the engine piston ring area, in order to study the tendency of the lubricant to oxidize and deteriorate under conditions of a thin oil layer, high temperature, and metal contact. It is mainly used to evaluate the quality parameters of diesel engine lubricants. The determination method follows SH/T 0259-92 for the determination of thermal oxidative stability of lubricating oils. The significance of thermal oxidation stability in practical use: 1) Thermal oxidation stability indicates the tendency of lubricants to form gum-like deposits or carbon deposits on engine pistons and rings. Oils with good thermal oxidation stability produce less gum and sediment, whereas those without such stability generate more gum and sediment. 2) For diesel engine lubricants, high thermal oxidative stability is required. This is because diesel engine lubricants are exposed to harsh conditions such as high temperatures in the areas where pistons and piston rings operate; their poor thermal oxidative stability can accelerate the wear of the cylinder walls.

What are the quality indicators for lubricating oil? Answer: Viscosity, acid value, water-soluble acids and bases, flash point, mechanical impurities, moisture, and liquid-phase corrosion. Among them, viscosity has always been the most important parameter for lubricants.