So why would the compressor have copper plating on the inside? They certainly aren’t manufactured that way.
The short answer is that acids inside the system can eat away at the copper and brass components in the system. The copper is then deposited in the high pressure/temperature environment of the compressor.
Why does this happen?
The presence of any acids in the system can cause this to occur but the most likely causes are the combination of air and moisture reacting with the refrigerant oil (most prevalently POE) to create an environment in which the copper is dissolved internally and redeposited on the steel in the compressor.
The result inside the compressor is reduced clearances and ultimately locking, overheating and even short circuits if the mechanical failure results in winding damage as is fairly common.
So for a technician, what we can do in ensure we are properly evacuating the system and installing appropriate filter driers to reduce or eliminate the presence of air and moisture.
P.S. – For an in-depth analysis of a study on copper plating in compressors you can read here