[M4-18 thread [USN-4031] For tapping type 2 B type and type 4 AB type]
■Material: Iron
"Iron" is "steel" and is an alloy of iron containing carbon.
Although it is commonly used in many circulations, it is cheap materials, but due to the downside of it is easy to rust, many surface treatments will reduce corrosion.
■Surface treatment: Trivalent white
It has the same corrosion resistance as "chromate" and has the same possibility of imitation self-healing.
Unlike hexavalent chrome, it is non-toxic and compliant with RoHS directive. So it is used as an alternative to unichromate/chromate.
Color is "Silver Color".