Properly protected              Copper corroded because
     copper surface                    the lubricant does not
                                                  contain a metal deactiv ator

Metal deactivators have two main functions: First, they must suppress any chemical attack on the surface of copper and copper alloys, which is sometimes called corrosion. Blackening of the copper as shown above occurs most often where sulfur containing lubricants are exposed to high temperatures.  

Second, they must passivate the metal surface thus avoiding that it can act as catalyst for the below mentioned radical formation. Freshly formed metal surfaces, which may be formed due to corrosion or wear strongly catalyze effect on the thermo-oxidative degradation of the lubricant. The catalytic effect can be described as follows:

The state of the art metal deactivators are:
Ciba® IRGAMET® 39 for oil based lubricants and
Ciba® IRGAMET® 42 for water containing fluids