Q. Why is the copper sulfate test a common test to know if the passivation process is in good quality? C.S.
A. Stainless steels are specified for their excellent corrosion resistance in a variety of environments. Following metalworking, particularly welding, it is common to process the parts through a passivation process in order to fully restore the corrosion resistance inherent with the stainless steels. Passivation is accomplished by immersion in nitric or citric acids in order to re-establish the chromium oxide layer on the metal surface, and dissolve and remove any iron from the surface or weld areas.
As mentioned, passivation is commonly performed on welded stainless parts since even a perfectly good weld will have some areas in it that solidify as delta ferrite. This is essentially pure iron (or iron with a very low solubility limit for carbon) that solidifies at elevated temperature in the weld zone. This phase does not have the corrosion resistance that the balance of the surrounding stainless steel does and needs to be removed in order to restore the corrosion resistance of the welded component to its starting condition. Either of the acids is effective at dissolving this freeiron that may remain on the surface.
The copper sulfate test is done following the passivation process in order to detect even very low levels of iron (delta ferrite or similar phase) that may remain. The copper exists in ionic form in an acid solution (Cu+2), and, due to the driving force of electrochemical thermodynamics, whatever iron is present will dissolve, releasing two electrons that are picked up by the cuprous ion that then plate onto the surface. The two half and overall reactions are shown below.
Cu+2(aq) + 2e- -> Cu(s) E = 0.337V
Fe(s) -> Fe+2(aq) + 2e- E = 0.440V
Fe(s) + Cu+2(aq) -> Fe+2(aq) + Cu(s) E = 0.777V
The seemingly complex electrochemical equations result in a simple-to-use and effective method to validate that the passivation process has removed the free iron at the surface using a clear visual indicator. The presence of any iron will result in discrete areas of copper plating. This may not always be a bright, well-adherent copper deposit , but may also be darker and flakier.