First principle:
Moisture (rain water or water used during installation, for example) that enters the pipes.
Condensation moisture that forms or drips into the tubes
Dry: Make sure there is no moisture inside the tubes
Second principle:
Oxidation inside the tubes during welding
Dirt, dust or foreign substances inside the tubes
Clean: Make sure there is no dirt inside the tubes
Third principle:
Poor soldering
Poor flange
Seal: Make sure the coolant does not leak
