The usage conditions of coatings are divided into the following three categories according to the degree of climate environment.
Category 1: severely corrosive working environment.
Category 2: moderately corrosive working environment.
Category 3: slightly corrosive working environment.

From the perspective of the requirements for protecting the base metal from corrosion, the following can generally be considered:
A. Precious metals, stainless steel containing more than 18% chromium, rolled magnetic alloy materials, and nickel-copper alloys generally do not require additional protective layers.
B. Parts made of carbon steel, low alloy steel and cast iron are prone to corrosion in the atmosphere and should be protected with a protective layer.
C. Parts made of copper and copper alloys are protected by bright pickling, passivation, electroplating or painting according to different usage conditions. Precision parts made of phosphor bronze or beryllium bronze do not need surface treatment.
D. Parts made of aluminum and aluminum alloys can be anodized and sealed.
E. Parts made of zinc alloy can be protected by phosphating, passivation, electroplating or painting.