Copper Sulfate LR for passivation test

The copper sulfate test is used to detect the presence of metallic iron and iron oxide on the surface of materials.

Question – What solution strength is needed to copper sulfate solution can be used to determine if a metal is stainless steel or just steel ? 
Answer    – Copper sulfate (CuSO4*5H2O) + H2SO4 (anywhere from 1% to 50% is specified in various test methods).