Water is the most important and complex pollutant encountered by lubricating oil during use, and its pollution mechanism is still under study. Due to the inherent water absorption of lubricating oil, its usage conditions and environment can have significant impacts that cannot be ignored.
The environment and working conditions in which lubricating oil is used inevitably lead to water ingress, which mostly causes physical or chemical changes in the lubricating oil. In extreme cases, it can have very serious consequences for equipment lubrication. Therefore, in the process of equipment lubrication management, the focus of attention for in use lubricating oil should be on reducing or controlling the entry of moisture, determining whether moisture exists and detecting its content, and separating and removing moisture as soon as possible.
1、 The existence form of water in lubricating oil
There are three forms of water in lubricating oil, namely dissolved water, emulsified water, and free water. Dissolved water refers to the chemical reaction between water molecules and oil molecules, resulting in uniform dispersion in the form of molecules in lubricating oil. This form of water is invisible to the naked eye and does not significantly affect the appearance of the lubricating oil, maintaining a clear and transparent state; Emulsified water exists in emulsified state when the water entering the lubricating oil exceeds a certain amount, which makes the appearance of the oil appear foggy, turbid or even lotion; Free water refers to the water that precipitates from lubricating oil and exists in a free state when the amount of water entering the oil continues to increase or when the oil has strong demulsification ability. It can be observed with the naked eye and is commonly referred to as "clear water".
The amount of water dissolved in lubricating oil depends on the type of base oil, the degree of refinement, the amount and polarity of additives, the degree of degradation of the oil in use, the viscosity of the oil, and the operating temperature. For example, in general, ester based oils have stronger water absorption than hydrocarbon based oils, Class I based oils have stronger water absorption than Class III based oils, internal combustion engine oil and gear oil have stronger water absorption than hydraulic oil and turbine oil, lubricating oils with higher degradation levels have stronger water absorption than newly put into use lubricating oils, lubricating oils with higher usage temperatures have stronger water absorption than lubricating oils with lower usage temperatures, and lubricating oils with higher viscosity have greater water absorption than lubricating oils with lower viscosity.
The relationship between the solubility and viscosity of lubricating oil and temperature
2、 The impact of water pollution
The most basic requirement for lubricating oil during storage, transportation, and use is "cleanliness". If it is dry, there should be no water, and if it is clean, there should be no dust. Studies have shown that a water content of 500ug/g can significantly shorten the service life of equipment and lubricants, indicating the degree of influence of water on lubricant performance and the importance of waterproofing.
The direct impact of water on equipment is to lead to corrosion and corrosion, thus accelerating wear. When water enters the lubricating oil, it will change the viscosity of the oil, resulting in the reduction of oil film strength. Accelerating the oxidation of the oil and the hydrolysis of additives will weaken or even lose the basic properties such as oxidation stability, extreme pressure antiwear, and clean dispersion. As a result, the anti foam performance of the oil will become poor, resulting in a large number of foam in the lubrication system, which will reduce the lubrication effect, and will also lead to the cavitation and hydrogen embrittlement effects of metal materials.
Below is an analysis of the above hazards one by one:
Rust and corrosion: In general, rust is affected by water, and even dissolved water in lubricating oil can have an impact on rust. Water can directly react with metal surfaces to form oxides, and can also work together with acidic compounds in oil to corrode metal surfaces, accelerating wear in the presence of abrasive particles. Rust and corrosion particles themselves are typical wear particles, and free water flashing on hot metal surfaces can lead to pitting corrosion.
Reduced oil film strength: Lubricating oil not only has certain viscosity temperature characteristics, but also has certain viscosity pressure characteristics, that is, when the pressure increases, the viscosity of the lubricating oil will also increase, thus providing a certain protective effect on the friction pair. However, water does not have such characteristics, and its viscosity changes extremely little with pressure changes. Therefore, when water enters the lubricating oil, it weakens the viscosity pressure characteristics of the lubricating oil, causing contact fatigue and fatigue wear of the friction pair (manifested as typical fatigue spalling in ferrography analysis).
Additive Failure: Water is a solvent based compound with strong polarity, and the vast majority of additives in lubricating oil are also polar compounds. According to the principle of similar solubility, once water enters the lubricating oil, it will react with these polar additives, causing them to hydrolyze or accelerate their consumption and lose their effectiveness, especially for functional agents such as antioxidants, extreme pressure anti-wear agents, detergents, emulsifiers, rust inhibitors, etc. It will also produce insoluble substances that block the filter screen or accelerate wear. On the other hand, it will be lost during the water separation process in the lubrication system, accelerating the deterioration of oil oxidation stability and causing environmental pollution.
Oil foaming: water entering the lubricating oil will reduce the surface tension of the oil, which will worsen the air release and defoaming properties of the oil, causing the oil to bubble with air. If the air entrained in the oil cannot be released in time or forms a stable foam on the surface, it will further accelerate the oxidation of the oil, and will significantly reduce the heat transfer performance, oil film strength, liquidity and other properties of the oil.
Lubricating oil mold: Water is an accelerator for the growth of bacteria, fungi, and other microorganisms. If it exists in lubricating oil for a long time, it will produce a large amount of microorganisms and suspend them in the oil, causing the oil to produce odors or clog the filter screen. Moreover, microorganisms themselves or their metabolites also have a certain degree of corrosiveness.
Water vapor cavitation: If water vapor reaches the low-pressure area of the lubrication system, the bubbles of water vapor will expand and disperse in the oil. When it suddenly enters the high-pressure area, it will instantly explode, similar to micro dieselization, hitting the friction pair and causing cavitation on its surface.
Hydrogen embrittlement: Hydrogen embrittlement is currently a hot research field in the field of tribology, and the source of hydrogen may be water, or it may be produced by electrolysis and corrosion under the action of water. Research has shown that when water enters micro fatigue cracks and comes into contact with free radicals in metals through capillary action, under high pressure, water will crack to produce hydrogen, and hydrogen embrittlement will accelerate the further cracking of micro cracks, leading to final wear.
Water washing effect: For lubricating grease, water pollution can cause the grease to soften and lose, resulting in dry grinding or decreased sealing of the lubrication system due to lack of grease.
The presence of moisture can promote the oxidation and deterioration of oil products, damage the oil film formed by lubricating oil, accelerate the corrosion of metals by organic acids, and accelerate the wear of mechanical parts; Moisture can also cause hydrolysis reactions of additives (especially metal salts), leading to their failure, precipitation, blockage of oil circuits, and obstruction of lubricant circulation and supply. At low temperatures, the moisture in lubricating oil can cause a decrease in its fluidity and a deterioration in its viscosity and temperature properties; At high temperatures, water will vaporize, breaking the oil film and creating air resistance, affecting the circulation of lubricating oil. There are many cases of accidents caused by water entering lubricating oil. One of the focuses of equipment lubrication management is to regularly monitor and determine the form and quantity of water, as well as control the entry and separation of water as soon as possible.
