Black hydraulic oil due to diesel effect

The light-colored hydraulic oil of our excavator became noticeably black. In the OELCHECK laboratory report, the dark coloration was commented as a "diesel effect".
However, how is it possible that a transparent hydraulic oil becomes so black due to a diesel effect?

OELCHECK answers:

The "diesel effect" in hydraulic oils is not uncommon. Soot particles suspended in the oil are the cause of the dark coloration.

A fresh hydraulic oil contains up to 9% air in dissolved form .This "natural" air content has hardly any effect on operation. However, if the oil has already aged or if differently formulated oils have been mixed, the air release properties of oils can deteriorate considerably. Air introduced into the oil from outside can no longer be completely dissolved in the oil, nor can it be separated sufficiently quickly, but remains in the oil in the form of small air bubbles. Furthermore, cavitation can occur in the hydraulic oil, e.g. due to design or damage to hydraulic components, resulting in the sudden presence of undissolved air in the hydraulic oil. If a sharp rise in pressure now occurs, small, explosive reactions occur between the oxygen from the air bubbles and the hydrocarbons in the oil. These proceed in a similar way to combustion in a diesel engine, in which oxygen from the air self-ignites with hydrocarbons from the diesel due to the compression pressure. However, since the air bubbles often do not contain enough oxygen for this "diesel effect", combustion is incomplete. This results in the formation of soot particles, which turn the oil black.

In most cases, the "diesel effect" has no effect on the hydraulic properties of the oil. However, since the diesel effect can also be an indication of cavitation, in addition to the oil condition and wear data in the laboratory report, the hydraulic system should also be checked regularly and the hydraulic oil replaced in accordance with our instructions in the event of excessive exposure to dissolved air.