When mineral oil (gear oil)is run at high temperatures - 210° to 240° it breaks down quick and starts burning up. I have been calling this "coking" but I don't think it is technically correct. Does anyone know the correct term for this process?

Dear Al K.,

"Coking" is a process/phenomenon in which there is undesirable accumulation of carbon (coke) deposits in the internal combustion engine or in a refinery plant. This is part of distillation procedure whereby petroleum product is distilled to dryness as per ASTM D86.

As per me, this should be aptly called "overheating", "smoking" or "cracking" leading to deterioration in properties of lube/gear oil resulting in carbon or soot formation.

I'd like to have comments from more members.

Regards.

sm

Thermal degradation or oxidisng. Technically their not the same but at those temperatures both would be occuring.

Coking is the carburization of the oil subjected to very high temps and results in hard black particles (coke). It is one result of thermal degredation.

I would call that oxidising due to overheating.

I think thermal cracking would be more correct.

Oxidation normally takes place at lower temperatures around 80 -100'C whereas at 200 - 220 ' C and above,break down of the molecular structure of C-H bonds takes place and therefore the correct term would molecular breakdown ,hope I am right,any views please.

I agree with Franky, at those temperatures the moleculair strings of CH will break down in fractions. I'm not sure this can be described by a single word. Anyone else?

I feel just fine to call it coking. This term is used in the industry from the time first refineries started refining crude oil. I agree with the sm’s opinion/explanation. In addition, high temperatures cause evaporation of all lighter fractions thus concentrating such oil into coke. Viscosity (thickness) will depend on the temperature and the time of oil’s exposure to those temperatures. Considering the viscosity of a coke, I think that in this situation some type of polymerization of molecules occurs rather than breakdown of C-H chains, because shorter molecules would cause viscosity to decrease rather than increase.