A reduction of hydrocarbons resulting in the formation of carbonaceous residue that will interfere with the movement of the sliding components in a mechanical seal.

In practice we call it “coking”. The coke can build up on the seal face causing the lapped faces to separate.

This coke build up is not dense enough to act as part of the seal face, so the fluid will either leak through this build up, or a piece of the coke will break off, causing a more severe leakage problem.