Perhaps second only to particle contamination, moisture contamination in in-service lube oils is perhaps one of the most destructive forms of engine corrosion. Three types of water contamination in oil can be distinguished: free, emulsified, and dissolved water. The oil and engine gears are directly affected by all types of water, but there are also indirect effects. Changes in pH and viscosity are examples of direct effects that might be troublesome on their own. Glycol pollution, soot buildup, and particle corrosion are other factors that contribute to engine corrosion. Each of these problems is made worse by the presence of water, which may result in permanent harm to the engine. Therefore, it is essential for in-service lubricating oils to undergo frequent moisture testing.
Dissolved moisture is the lowest level of moisture contamination in lubricant oils. It occurs due to the lubricating oil gradually absorbing moisture over time as a result of ambient air humidity interaction. The more additives an oil has, the more hygroscopic (attractive to water) it will typically become. The oil is not significantly more compressible or viscous in the normal range of acceptable dissolved moisture concentration, but since it is spread throughout the oil, it is the most chemically reactive water species.
Any polar metal surface that dissolved water molecules come across inside the engine will become coated. This small amount of water may be stripped of its oxygen component and release hydrogen ions at normal engine pressure and heat, which will chemically deteriorate the metal's surface (ball bearings and gears). This makes the metal more brittle, and tiny flakes will start to fall off, causing particles in the oil and gear rust.
Polar additives such as detergents, antioxidants (amines or phenols), friction modifiers, and anti-wear additives are actively sought after by dissolved water molecules. These additives lose their effectiveness as a result of being secluded by water molecules, which could harm the engine.
If unchecked, the amount of dissolved moisture inside the oil sample will keep rising until saturation. Any more water will precipitate out at this stage as hazy, emulsified micro-droplets. This type of moisture is produced by continuously spinning, heating, and applying high pressure to water to transform it into oil. Various lubricants have different saturation points. While other hydraulic fluids have a saturation level as high as 5000 ppm (0.5%), mineral oil has a saturation level of 100 ppm (0.01%). Depending on the kind of oil and the additives used in the lubricant, the saturation point for some oils may rise as they age.
Oil's ability to be compressed is directly impacted by emulsified moisture. The compressibility of a liquid is expressed by its bulk modulus. Compression is more difficult the larger the bulk modulus. If oil and water are emulsified, the blend's bulk modulus is now 3 and higher than that of the water! Costly repairs and worn-out components may result from this.
Similar to dissolved water, emulsified moisture is present throughout the oil sample and can also contribute to corrosion along metal parts, which results in oil particulates. These particulates will mix with the emulsified water to form a sludge that can grind gears and produce more particles. If the oil is not replaced or dried, the positive feedback loop will persist, and the harm is irreversible.
Emulsified water droplets will also clog many oil filters due to their size. This hinders the regular flow of oil and makes it difficult to effectively filter real particles.
Emulsified water is one factor that aggravates the problem, despite the fact that several classifications (such as pH or Total Acid Number) are used to explain the acidification of oil over time. Because there is so much water in the oil, there will be more hydrogen ions, which will interact with the additives. The corrosion of the engine increases with the concentration of acids.
Free moisture is water that is present in a distinct and independent aqueous liquid phase but is neither dissolved nor emulsified inside an oil sample. This type of moisture typically results from condensation or leaks and will never mix with the oil. Since free moisture is denser than oil, it will either accumulate at the bottom of most engines or cover all accessible polar metal parts with a thin film. The engine suffers when a lubricant contains any amount of free moisture. Gears will rust, grind, and shatter significantly. The engine will sustain permanent damage, and the cost of labor and replacement will be high.
The two most dangerous phases in a lubrication system are free and emulsified water. For instance, the incompressibility of water in comparison to oil can cause a loss of the hydrodynamic oil film in journal bearings, which in turn causes excessive wear. A journal bearing's life expectancy can be reduced by up to 90% by as little as 1% water in oil.
The situation is significantly worse for bearings with rolling elements. The intense temperatures and pressures created in the load zone of a rolling element bearing will not only cause water to ruin the oil film strength; they will also cause both free and emulsified water to instantly flash vaporize, which will result in erosive wear.
It is evident that any of the three forms of water are destructive to both the lubricant and to the engine. Engineers and machinists go through great lengths to keep their oil/lubricants clean and dry, but over time moisture will eventually absorb into the engine system. Thus moisture in oil systems is a constant hazard that must be carefully monitored. The VAISALA MMP8, MMT162 and MM70 series are capable of detecting moisture in most petroleum products quickly and easily.