Why does Hydraulic Oil turn Dark?

Why does Hydraulic Oil turn Dark?

Q. Why does Hydraulic Oil turn Dark?  I understand, (I think), how Engine oil is contaminated by carbon from combustion, but what turns my hydraulic fluid dark over time?

A. It is accepted that the darkening of oil is an indication of ‘ageing’. There are Chemical and Mechanical factors that contribute to this ageing.

Temperature, pressure, friction loads (shearing the oil molecules) and the presence of oxygen will eventually break down or age the oil. Some of these elements may have been designed and built into your equipment, but they can often be reduced (if not eliminated) by careful investigation and, if required, modification. (More about this later).

Contamination is a Key cause of deterioration of the oil.

Water, Chemicals and Particles are all sources of contamination. These should be well in your control to manage.

Particles   Pure Oil filtration services

Contaminants tend to multiply themselves. Each dirt or metal particle will have multiple opportunities to score surfaces and produce more particles each time it passes through the system. These particles must be removed by High Efficiency filtering , either within the system or as a by-pass process. Dirt and dust entry needs to be controlled by filters on vents and cleanliness at fill and dip points. New Oil can be dirty, A good target for cleanliness ISO 16/14/11
Dirty New Oil at ISO 25/22/19


Chemical contamination can enter the system from by- pass from a production process, poor product handling, equipt maintenance or interactions between contaminant particles, water, microbes, oil additives and the mechanics of the system. Chemical analysis can be used to identify the unwanted chemicals in the oil. It is then necessary to locate the actual sources of these contaminants and put measures in place to address them.





Water not only affects the mechanical components but changes the oil chemically and modifies its physical properties.

• Thermo-Oxy stability (resistance to oxidation);
• Hydrolysis (resulting in alcohols and acids);
• Deposition and varnishing
• Additive depletion and precipitation.

• Viscosity;
• Lubricity and load capacity;
• Compressibility in hydraulic systems

Water can be present in several forms:

Free water tends to settle in the bottom of the reservoir tank. It can be detected by using water detecting paste on the end of a dip stick, (electronic detectors are also available). Removal by a suction pump. If your lucky, your tank might have a test and drain point on the bottom. (If so, use it, regularly.)
Failure to attend to free water can also lead to contamination multiplication, in this case by Bacteria, Yeasts & Moulds and particularly by Hormoconis Resinae aka Cladosporium fungus. This “bug” is responsible for huge expense & system failures, world-wide. It lives at the interface of the oil (it’s source of food and warmth) and the water (it’s oxygen). A proficient breeder, its by-product is a dark sticky resins and solids (the resinae) that clog filters and fine metering equipment and promotes corrosion.

Emulsified water is very small particles of free water blended into the oil. Emulsions tend not to readily settle out of a working system. It reduces the load capacity of the oil, it can be turned to steam on hot working and bearing surfaces resulting in cavitation erosion and can also transport water and dissolved chemicals to other parts of the system. Filters that coalesce, separate and polish can address water emulsion in the hydraulic system.
Dissolved water is fully dissolved into the oil. Undetectable by eye and many test processes, it blends Oxygen, dissolved corrosives and other chemical contaminants into the oil. Vacuum Dehydration is the most effective technology, in this situation.

Plant & Equipment Design.
Parts of the plant that might be aging the oil could be found where there are very high pressures and loads, excessively hot areas, (thermal imaging equipment might find these), very sharp changes of direction (shearing) and aeration of returning fluids (introducing oxygen).

Pure Oil Services use and recommend HY-PRO Filtration  

Daryl B Jones – 2018

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