Thermal Degradation

Thermal Degradation

If the system temperature exceeds 200oC, the typical thermal stability point of lubricant, thermal cracking will occur. The excessive heat and cracking involved causes:

Molecule shearing: they volatize; leaving no deposits.

Polymerization: they condense, causing dehydrogenation and producing lacquer and coke deposits.

Decreasing the viscosity: it is the primary indicator of thermal degradation, compared to the increased viscosity occurring during oxidation.

Overheating Sources:

1. Lubricant comes in contact with hot surfaces.

2. Insufficient circulation of the oil.

3. Micro-dieseling.

It is a sudden increase in temperature associated with the adiabatic compression of entrained air bubbles in pressurized lubrication environments.

Mechanism: If the gas bubbles pass through a higher pressure zone, they will violently collapse. The compression of these bubble in the pressurized side of the pump is adiabatic.

For Example: consider a hydraulic system with a suction-side air leak that lets the bubbles at a atmospheric pressure and 37 oC , and then pressurizes the fluid to 125 bar. The temperature in this example, which is typical of a hydraulic system with an air leak, would be just more than 1,100oC.

Chemistry of Thermal Cracking:

Thermal cracking is taking place in three steps:

1. Chain Initiation Step: Homolytic bond cleavage

Homolytic bond cleavage

R-R R + R

2. Chain Propagation Step:

At High Pressure: Hydrogen abstraction

R + R1-H R-H + R1

At Low Pressure: Scission

3. Chain Termination Step:

Radical Combination:

R2 + R3 R – R


2 R2 R – H + R-CH=CH-R

Thermal Failure Indicators:

Lubricant Evaporation: Decreasing in Viscosity and Flash Point.

Insoluble long-chain molecules: Coke Forming.


Abdelrhman Sabry

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