M1.6 carbon steel hardened thin head black zinc plated screw

Industry knowledge：

How to distinguish whether stainless steel screws are qualified?

In the material inspection, we usually use special stainless steel material testing agent, drop it on the screw, and wait for a while to see the result. If conditions permit, you can also find a third-party testing agency and ask them to help carry out salt spray test on the screw to get accurate results.