Spray the surface with a solution of copper sulfate in water. If free iron is present, a copper film will form. In this test, the chemical reaction is:
Fe + CuSO4 = FeSO4 + Cu
The copper film is immediately visible.
Copper sulphate test: a galvanic type test in which the more noble copper plays upon less noble iron particles but not on SS. Patches of copper sulphate shows a poorly passivated surface. Passivated parts are immersed in a Copper Sulfate solution for five minutes, rinsed and visually examined. Any copper (pink) color indicates the presence of free iron and the test is considered unacceptable.
Ferroxyl test: causes coloured iron compounds to appear as specks.