Reasons of damaged oil

Here you can recognize the main reasons causes damaging of the oil.

A dominant factor which determines the service life of the oil is degradation through oxidation. Oxidation occurs when oxygen combines with the original hydrocarbon molecules and gives a chain reaction. Traces (even a few parts per million) of certain metals, particularly copper, iron, zinc, lead and water act aE catalysts and markedly increase the rate of oxidation.
Oxidation produces soluble as well as insoluble products and these may form sticky substances and may deposit as gum or sludge in oil passages, various pump parts, valve spools and ports, etc. blocking them and restricting oil flow and making the machine run sluggishly. As a result of oxidation, the oil becomes progressively darker and gets acidic properties which ultimately deteriorates the life of the oil.

Steps to Combat Oxidation

Anti-oxidant additives are used in hydraulic oils to combat oxidation. Apart from this one can use other means to retard oxidation rate by:

  • Installation of heat exchanger when oil temperature exceeds 60°C.
  • Incorporating magnetic plugs and filters to flush out ferrous contaminants.
  • Keep out copper and brass tubings from the system if the pressure rating is above 10 bar.

When magnetic plugs are used, provision should be made to facilitate frequent cleaning of the plugs, otherwise oil surge in the tank may dislodge the particles from the magnet and circulate them back into the system. Incidentally, use of magnetic plug reduces load on filters.

Oxidation inhibitor

Oxidation inhibitors are additives used in hydraulic fluids to prevent oxygen from being quickly absorbed and hence aging and corrosion processes are reduced.

Oxidation resistance

The aging of hydraulic oil depends on the composition of the oil and may be different in oils with the same specification.

Ageing is accelerated as a result of air released in oil at high pressures, temperature, metals which the oil comes into contact with and contamination due to dirt, rust and water.

Oxidation inhibitors prevent oxygen from being absorbed and hence protect the components from corrosion.

Ageing products may make valves stick, filters clog or heat exchangers become dirty.

Change of Oil

When oil changes are effected, the system has to be thoroughly flushed to remove the old oxidised oil; partially oxidised oil is an effective catalyst for the oxidation of fresh oils. Flushing is done with same oil or with flushing oil at 40°-50°C and should be continued till the filter is seen without any trace of contaminants.


The de-emulsifying capacity of an oil can be demonstrated in the time it takes for an oil/water mixture to separate. It is determined by the agitator method to DIN 51 599.

Water which has seeped into hydraulic oil must be swiftly removed, as it affects both the viscosity of the oil and its resistance to corrosion and leads to deposits. It is important to keep the oil still for as long as possible in the tank,as water is separated from still oil faster than when it is flowing.

The de-emulsification characteristic,

anti-wear property and corrosion protection are all reduced due to aging.

Due to the long “rest” periods during which the oil remains in the tank , good filtration, cooling and regular monitoring of the oil quality can all help to act against aging.

Oxygen absorption

Fluids absorbing oxygen results in the corrosion of components. The aging process is also influenced by oxygen.

See: oxidation resistance


If a negative pressure arises in the hydraulic fluid (negative gauge pressure), which is caused by a suction cross section which is too narrow, by fluid flowing too fast round sharp edges or by breakaway of flow, this may result in cavitation (formation of a cavity) in the hydraulic system. The air dissolved in the oil will form bubbles.

Especially in the suction lines of the pumps this can lead to signs of wear, pressure shocks, power losses and loud noises.


As well as HFCs (solutions of polymers in water) and HFDs (water-free synthetic fluids), emulsions in the form of oil-in-water emulsions (HFAs) or water-in-oil emulsions (HFB) are used in hydraulics as fire resistant hydraulic fluids.

Oil-in-water emulsions, with a combustible proportion of max. 20%, would be the ideal hydraulic fluid , if their properties (viscosity,corrosion protection,anti-wear properties,pour point etc.) equaled those of mineral oil.