Should my oil be this colour? Why hydraulic, turbine and compressor oils turn dark?

This article will answer the following questions.

  • Why has my oil colour and turned brown, black, dark etc?

  • Why do lubricating oils change in appearance?

  • Does an oil colour change mean the oil needs replacing?

  • Why is used lubricating oil darker than new oil?

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  • Have you ever wondered why your new lubricating oil starts off much lighter in colour than the old oil you are replacing? Well you might be forgiven for assuming it’s entirely down to dirt and contamination. In the case of a diesel engine oil you would for the most part be right as the black colour can be attributed to suspended soot particles. However in a typical steam turbine or hydraulic oil, the used oil can actually be much cleaner than the new oil but still be darker. This may seem strange to grasp at first but it’s totally normal.
  • If you take an everyday reaction in making your toast of a morning you are browning the bread by the Maillard reaction where the heat breaks apart and rejoins the proteins and sugars in the bread to give it it’s ‘toasty’ colour. For those of you reading this over a cold beer of an evening (which some may have noticed my title image is taken from 3 pints) the same process makes the beer darker in the production process.
  • The same is very much true of a mineral oil in that the hydrocarbon molecules break apart and reform to give the colour change. The following article will discuss how an oil changes colour, and whether it is something to worry about.
  • Should I change my oil if it’s changed colour?

    The answer is not necessarily. The first thing to establish is why it has changed colour. Lubricating oil is very suspectible to colour changes through oxidation and thermal degradation and it doesn’t take that much to cause a visual change. In fact colour changes are often some of the earliest signs of oil deterioration or localised overheating, but just because it’s darker doesn’t mean the oil is totally spent and it may still be in its prime of life. However there are other causes of colour changes such as contaminants and also evidence of air entrainment causing microdieseling that are cause for concern. This turns the oil as black as diesel engine oil. Furthermore, an oil can be perfectly crystal clear and light yet still not be serviceable so a colour change is important information, but should not be something to take a knee jerk reaction to.

    The first action you should take is to take a lube oil sample, which compared to a costly oil change or an even costlier machine failure is well worth doing and has considerable cost savings.

    The test suite should include ASTM D1500 colour or a close equivalent method to monitor the darkness of the sample. If this is not a one off and a regular analysis programme you can trend this data. Equally if your lab photographs samples then you can visually see the colour changes across samples over time.

    What colour should my oil be?

    The first thing to establish is there is no good or bad colour of an oil. Most oils start off light in colour, but naturally get darker over time with some doing so quicker than others. What is important to note is any rapid changes or changes that do not occur normally with this lubricant.

    Causes of colour change.

    The most common causes of a colour change include:

    • Contamination – contaminants such as high concentrations of dirt, soot, other oils and water can all cause a change in colour.
    • Air entrainment and microdieseling – this is the process where air bubbles in a hydraulic system when collapsing cause the lubricant around it to burn and form soot particles. You can find out more about this phenomenon in our article on the subject.
    • Oxidation – this is the general degradation of the oil over time that leads to darkening. This is usually linked with high oxidation and acid formation.
    • Thermal degradation – this is very localised oxidation where a hotspot in the system causes the oil to burn, which can cause a rapid darkening of the oil without a significantly noticeable increase in oxidation, viscosity or acidity when measured across the entire system.

    Remember: colour changes where the oil gets lighter are also significant too so it is not just darker. This can happen when the darker oxidised oil is being mixed with a lighter oil through e.g. seal leaks.

    Measurement System for colour

    To allow a standardised way of referring to the darkness of an oil your laboratory will implement something close to the ASTM D1500 Colour scale from 0 which is water white to very dark almost black 8. The actually colour needs to be ignored in this scenario as it could be a dyed lubricant and it’s the darkness that is important. So calling it a colour scale is really a misnomer and it should be called a darkness scale.

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    How to prevent and manage the causes of colour changes?

    There are several ways to address causes of darkening oils related to the causes highlighted above. An oil analysis sample is a great way to begin this process of identifying the cause. Once identified you have the following options.

    • Filtration – this addresses contaminants such as dirt and water that can lead to changes in colour. If using ultra-fine filtration it also removes wear particles such as iron that catalyse the oxidation process and speed up degradation of the oil. If you also have a varnish problem that leads to oils darkening as well as deposition of oxidised sludges and varnishes in cool areas of the system (e.g. cooling parts), this can rapidly insulate the cooling system and lead to reduced heat loss. I have come across many success stories in using long term offline filtration to remove varnish material including from deposits on component surfaces as le chatelier’s principle makes the deposits start to be redissolved and come back into solution as the varnish is removed. One word of caution is to always accompany long term offline filtration – especially ultra-fine filtration – with regular lube oil and filter analysis as it can mask longer term problems. It should be remembered that offline filtration is a treatment rather than a cure.
    • Contamination prevention – filtration is good for maintaining system cleanliness and removing contamination, but equally is important is identification and prevention of entry by ensuring good maintenance of seals, breathers etc
    • Temperature control – Ensure there is sufficient cooling of the system and monitor system temperatures to ensure adequate cooling. If suspect a hotspot use a thermal imaging camera to identify the source.
    • Lubricant selection – this may seem obvious but ensuring the lubricant provides the right film thickness is vital too. Too thin and you don’t prevent sliding wear particles that only reduce component life but catalyses oil oxidation. Equally too thick an oil increases system temperatures and reduces cooling efficiency. In addition to this ensure you select a lubricant designed for your needs such as high temperature handling and good oxidation resistance etc.
    • Full instead of partial oil changes – An oil change earlier than planned should be a last resort to correct problems on large fluid volume system and generally means the other strategies have failed. If doing a change because of sludges and deposits in the system it is wise to ensure it is done properly rather than through a top-up or partial changes. It is common with very large oil volume systems because of budgets to change oil on an accounting basis rather than on a condition basis. I.e. we can’t afford a full oil change so we will “freshen” the oil with 25% or 50% change. This does give a temporary reprieve but the problem is oxidation causes further oxidation so all that happens is the new oil rapidly becomes like the old used oil as if no change had been done at all. Hence if doing an oil change it is better to do regular lube oil monitoring to highlight well in advance of a large system oil change so that it can be planned for. The following article explains more about the perils of partial oil changes.

    If you want to find out more about what to do about colour changes in your lubricants then click the contact us button at the bottom right of this screen to get in touch.