“Corrosion caused by dissolution of metals (C4)”
is the criticality of the corrosive property of the oil that causes dissolution of metals (e.g. copper) inside transformers and other electrical equipment under normal operating conditions.This criticality gradually degrades the dielectric and chemical properties of materials and insulators (oil and paper) and is unrelated to corrosive sulfur (C1, C2, C3).
Corrosion
Disintegration of a metal due to chemical reactions with sulfur and other chemical species in insulating liquids
[Sea Marconi translation of technical standard IEC 62697-1 of 2012, para.3.1.5 – page 10]
