Water can be present in a lubricant in any of three states:

free water
large droplets of water distinctly separate from the oil, easiest to remove from a lubricant
emulsified water
very small water droplets finely dispersed in the oil forming a stable,milky mixture of oil & water that is difficult to separate
dissolved water
water absorbed within the molecular framework of the oil, that cannot be separated easily

The amount of water that a fluid can contain in dissolved form is referred to as its saturation point. Once the saturation point is reached, any additional water will separate out as free water.

The level of fluid contamination in the system is the result of multiple factors that interact such as: the rate of contaminant entry, rate of wear debris production, filtration characteristics, system duty cycle, design of reservoir, rate of fluid loss & replacement, contamination tolerance to system components,etc
Common reasons for inadequate contamination control is lack of appreciation of the tremendous impact of contaminants on safe & economic operations, & lack of understanding of contamination control technology (filters, breathers, seals).
Some Sources & Types of System Contamination are:

The solubility of water in oil is greatly influenced by temperature – the hotter the oil, the more water it can hold in solution. This can only go so far – as the temperature approaches 100° C, at atmospheric pressure, the water will start evaporating out of the oil at increasingly higher rates. However when temperature drops, solubility drops, saturation point drops and part of the water may come out of solution as free water.

While certain lubricants are formulated to separate easily from water, others are designed to form stable emulsions with water. The degree to which an oil is hygroscopic, i.e. will absorb moisture, will depend upon the type of base oils and additives that have gone into its formulation. Water molecules are polar in nature. Whilst most hydrocarbons in base oils are not polar, and hence follow the old adage “Oil and Water don’t mix”, some base oils are polar to various degrees. The lubricant polarity, and hence its ability to dissolve water, can also increase with the addition of polar additives, accumulation of oil degradation by-products etc.
Small amounts of water can be readily absorbed from the surroundings. Highly polar base oils such as mineral oils from Group I & II and Synthetics such as PAG & Phosphate Esters can absorb and hold more water in dissolved state. Heavily additivised oils such as engine oils, and oils containing polar additives such as hydraulic fluids & gear oils will tend to hold more water in dissolved phase and in stable emulsions than lightly additivised oil such as turbine oils. The by-products of oil degradation tend to be more polar than the base oil & hence aged oils will tend to hold more water.
Humidity from the air can condense on to the walls & ceilings of oil sumps & tanks and enter the oil. Frequent cycles of hot & cold conditions, such as might be experienced in stop/start equipment can increase the ingress of humid air into the system & thus water into the lubricant.
Heat Exchangers:
Leaky oil coolers & steam leakage from tank heating coils are major source of water contamination
Internal Combustion engines produce a lot of water simply from their normal combustion cycle process. While it is true that the crankcase temperature is high enough to evaporate virtually all of the water, when an engine is shut down for the day, condensation occurs as the engine’s temperature reverts to ambient. In large slow speed engines operating at part loads in humid conditions, if combustion chamber liner temperatures drop close to dew point of water, then water vapour can condense and mix with the lubricants. Many other chemical reactions that the base oil can undergo such as oxidation, and additive reactions such as neutralization of acidic components by the alkaline additives can also give off water as a by-product.
Gross Water :
In environments where water is used either as part of the manufacturing process, or in the cleaning process ( e.g. hosedown performed in many workplaces), water can easily ingress though filling holes and housing openings, improper air vents, defective shaft seals , etc, unless these are well designed to exclude water. Improper storage, transfer & filling practices can be a major source of water contamination.
While water may not directly react with hydrocarbons, it helps to promote base oil oxidation, particularly in the presence of wear metals like Fe, Cu, & Sn which act as catalysts. In some types of fluids, water can react with the base oil resulting in the formation of sludge, acids and deposits. Control systems using Phosphate Ester fluids are particularly susceptible to such hydrolysis. Sulphur Phosphorus EP additives can release sulphuric & phosphoric acids in the presence of water. Water also attacks, hydrolyzes , agglomerates, consumes orsimply washes away a host of other additives such as AW, Rust Inhibitors, Antioxidants, Dispersants, Detergents, Demulsfying agents. Once such additive depletion has taken place rapid deterioration of lubricant & attack on machinery sets in. By-products of oil degradation can react with emulsified water to form resinious, sticky materials. Often, Sludge and Varnish formation along with the resultant restrictions on oil flow, valve stiction, bearing metal wipeout, etc can be directly attributed to the presence of water in the oil.

Water in oil effects machinery in a variety of ways. Water simply does not lubricate as well as oil. Dissolved water can increase metal corrosion. In Transformer oils, dissolved water can greatly reduce dielectric strength. When water comes out of solution and forms either an emulsion or free water, the reliability of the machinery can be seriously compromised.
Loss of Film Strength:
Hydrodynamic bearings depend upon oil viscosity to provide critical clearance under load. Wa-ter viscosity is low ( 1 cSt at 20 C) and water globules pulled into a bearing load zone reduce surface clearance and result in contact of opposing surfaces. In rolling element bearings and other EHL contacts, where local pressures can exceed 10000 bar, lubricant viscosity increases exponentially (according to their pressure-viscosity coefficient α) and the lubricant momentrarily forms a solid, that provides the separating film. Water viscosity remains virtually unchanged regardless of load. This results in collapsed film followed by fatigue failure. Under pressure, water can flash or explode into superheated steam in bearing load zones, and this can sharply disrupt oil films and potentially fracture surfaces.
Vapourous Cavitation
Where a lubricant is subjected to sudden change in pressure vapourisation followed by rapid condensation of water can occur. The rapidly condensing and collapsing vapour bubble can form a microjet that implodes on the machine surface.

Water can enter microscopic fatigue cracks by capilliary action. Within these cracks, under extreme pressures, water disassociates in contact with unoxidised metal to release hydrogen ions which then permeate the interstitial spaces of the metal lattice causing embrittlement of the metal.

Loss of Fatigue strength of Steel
It has been shown that as little as 10 ppm water reduces fatigue strength of steels by 10%. The increase in water concentration in oil progressively reduces the fatigue life. 100 ppm of water reduces life by about 32-48%, while with 6% water of water concentration fatigue life is reduced by about 70%.
Rusting & Corrosion
Rusting requires water, and water dramatically increases the corrosive potential of acids. Rust & corrosion cause pitting and etching on bearing surfaces, resulting in loss of surface profile thus disrupting the formation of HD and EHL oil films.

Rust particles flake of from surfaces and fall into the lubricant. Not only is the surface finish destroyed but the rust particles then circulate throughout the system causing abrasion and fatigue.
Aeration and Foaming
Water lowers oil’s interfacial tension, which can destroy its air-handling ability, leading to aeration and foam. Air weakens oil films, increases heat, induces oxida¬tion, causes cavitation, and interferes with oil flow. Oil delivery to bearing surfaces is disrupted.
Microbial Growth
Many micro-organisms have the potential to use hydrocarbons as a source of carbon and energy for growth. However, as this growth can only take place where free water is present, microbiological growth will usually be found at an oil/water interface. The micro-organisms often develop as a thin mat of fungal threads floating at the interface and as the biomass accumulates, bacteria and yeasts also grow in the mat, trapping particles such as dust, grit, metal oxides and swarf. The main bulk of the biomass is increased by production of polymers due to microbiological activity, leading to an emulsified mass between the bulk oil and the water layer. Organic matter from the interface will eventually sink to the bottom of the tank, building up a layer of sludge and debris. Under the sludge blanket, oxygen is scarce and so here anaerobic conditions will predominate, producing a favourable environment for sulphate reducing bacteria. These micro-organisms do not use hydrocarbons as growth substrates, but reduce sulphates to produce sulphides (usually manifested as iron sulphides or H2S) during growth on other carbon sources. Bacteria favour neutral pH values (but may tolerate up to 9 or 10). They usually produce low molecular weight carboxylic acids, which tend to reduce the pH. Once the pH has decreased, ie as the acidity increases, fungi, moulds and yeasts take over as the dominant micro –organisms. Products of microbial growth can cause formation of stable emulsions, considerably reducing centrifuging and filtration efficiency. Further consequences are local corrosion, reduced load carrying and EP properties. Sulphate reducing bacteria, produce sulphides, precipitating as iron sulphides, which are implicated in severe local pitting corrosion.
Water washing
Water sprays can directly wash out lubricating oil or grease from the lubricated zone. Lu¬bricant density is lower than that of water, and too much free water can displace lubricant if allowed to accumulate in a bearing. When grease is contaminated with water it can soften and flow out of the load area.
With the exception of particulates, water is the most harmful of all contaminants Although the presence of water is often overlooked as the primary root cause of machine problems, excess moisture contamination can lead to premature oil degradation, increased corrosion, and increased wear.
Proper sampling to determine the quantities of water present requires evaluation of the lubrication system for each machine. To determine the moisture levels that can potentially affect the formation of the lubricating film and the corrosion of bearing surfaces, the oil in the live-zone should be observed. In circulating systems, samples taken from the supply lines indicate the quality of the oil supplied for the lubricated components. Samples taken on the return lines include the same information, plus any additional water being introduced to the system at the lubricated components. The moisture content thus detected is different, however, than for the maximum concentration of moisture in the system. This would typically be found in the bottom drain of the reservoir, where moisture can accumulate by design.
That sample is also important, however, to indicate excessive accumulation of moisture in the sump, the need to investigate sources, and possibly to clean the tank.
Visual Crackle Test
The simplest way to determine the presence of water in oil is to use the Visual Crackle test. Although this is an effective test for identifying free and emulsified water, down to approximately 500 ppm, its biggest limitation is that the test is non-quantitative and fairly subjective. It is useful as a screening tool in the lab and the field when a quick yes or no answer is required for free and emulsified water.
Karl Fischer Moisture Test
The Karl Fischer Moisture test is the method of choice when accuracy and precision are required in determining the amount of free, dissolved, and emulsified water in an oil sample. All Karl Fischer procedures work in essentially the same way. The oil sample is titrated with a standard Karl Fischer reagent until an end-point is reached. The results can be reported as parts per million, or as a percent of water in the sample.
Calcium Hydride Test Kits
One of the simplest and most convenient ways to determine water concentrations in the field is by using a Calcium Hydride test kit. This method employs the known reaction of water with solid calcium hydride to produce hydrogen gas. The amount of hydrogen gas liberated is directly proportional to the amount of water present in the sample. The water content of the sample is determined by measuring the rise in pressure in a sealed container due to the liberation of hydrogen gas. Used correctly, these test kits are reported to be accurate down to 50 ppm free or emulsified water.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR can be an effective method for screening samples containing in excess of 1,000 ppm of water, provided a correct new oil baseline is available for spectral subtraction. However, due to its limited precision and comparatively high detection limits, FTIR is not adequate in many situations where precise water concentrations below 1,000 ppm, or 0.1%, are required.
Dean and Stark Method
The classic method for determining the presence of water in oil is the Dean and Stark distillation method (ASTM D95). This test method is fairly cumbersome and requires a comparatively large sample to ensure accuracy, which is why it is rarely used in production-style oil analysis labs today.
Saturation Meters
When the amount of water present in an oil sample is below the saturation point, Saturation (dew-point) meters can be used to indirectly quantify water content. Most saturation meters use a thin film capacitive device, whose capacitance changes depending on the relative humidity of the fluid in which it is submerged. Monitoring and controlling water levels in any lubricating system is important. Whether it is a large diesel engine, a steam turbine, a hydraulic system, or an electrical transformer, water can have a significant impact on equipment reliability and longevity.
The key to controlling water contamination is to restrict its ingress. Water is present in virtually all lubrication environments, hence excluding it completely can be very challenging.