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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).
How does copper sulfate test for passivation work?
ANSWER : Chromium and nickel will not be replaced with the Copper sulfate test solution (which also contains sulfuric acid), only the Fe or other less noble metals. If there is free iron on the surface it will be replaced by the copper, forming the distinctive copper color on the surface in this 6 minute test. Of course, this test should only be used on austenitic stainless steel. Some people use it on other grades, but you can get false failures.
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.