This information was obtained from: Textar, the manufacturers of Cera Tec a ceramic based anti-squeal lubricant
This article will hopefully clarify why copper grease should not be used on the brakes and road wheels of modern motor vehicles.
What is Copper Grease?
Copper grease is an anti-seize compound consisting of fine copper particles in a non-melting Bentone (a type of clay) base to which are added highly effective oxidation and corrosion inhibitors. Copper grease is used for mating surfaces which work under high temperatures.
When working under these high temperatures, the lighter fractions evaporate and leave copper and other contents behind, giving an anti-seize property.
There is a problem however, aluminium is very susceptible to an electrochemical reaction known as a galvanic corrosion when it comes into contact with copper in the presence of an electrolyte, even salty water will do!
So what is a Galvanic reaction?
Metallic materials in electrochemical contact can form a galvanic cell, or battery. Different metals have different electrode potentials and when two dissimilar metals come into contact with an electrolyte (salt water for instance), one metal will act as an anode, or negative (–) electrode and the other the cathode or positive (+) electrode. The reaction will cause the anode metal to dissolve into the electrolyte with some of this dissolved metal depositing on the cathode. This is most commonly seen as the white chalky powder seen on the back of alloy wheels.
The anodic index compares the electrochemical voltage that will develop between any one metal and gold (the most noble metal). To find the relative voltage of a pair of metals, you just subtract one from the other.
For harsh environments, as found around the brakes or road wheels of a motor vehicle, there should be no more than 0.15V difference between any two metals. In the case of cast iron and cast aluminium (materials often used for modern brake discs and calipers) you will see a voltage difference of just 0.10V therefore minimal corrosion will take place. However, if you compare cast aluminium and copper as found in copper grease, the voltage difference is now 0.60V. (0.95 – 0.35)
This voltage difference will cause a large galvanic reaction between the two metals causing the cast aluminium to dissolve to the copper, thus causing seized and damaged components. It is also worth noting that using copper grease even on cast iron calipers will still show a voltage difference of 0.50V so a significant reaction will still occur.
What should we use then?
The simplest way to prevent a galvanic reaction on brakes is to electrically insulate the two metals from each other. If they are not in electrical contact, no galvanic reaction can occur. This can be achieved by using a non-conductive material between the two metals.