The ferroxyl test (available from Digestion.eu) is performed to determine whether austenitic stainless steel should be pickled or to assess whether the pickling of stainless steel has been done correctly (test in accordance with ASTM A380 or ASTM A967).
If the surface structure of austenitic stainless steel was not evenly renewed and the presence of metallic iron (called foreign iron) is found, then the properties of stainless steel will be affected. The presence of free iron in the structure of stainless steel damages the thin layer of chromium oxide and, as a result, causes its local reduction in corrosion resistance.
If the foreign iron is not removed, severe pitting corrosion can occur.
The test is based on the reaction of potassium cyanide with iron in a strong acid medium. The foreign iron dissolves in the strong acid medium, passing as a ferrous ion into a form of a compound called ferrous blue. If the blue color appears during the test, it means that the surface of the stainless steel does not yet have the correct structure - that is, the chemical etching process has been poorly performed. However, if the blue color does not appear during the test (the surface of the paper should remain clean), it means that the etching process is satisfactory.
The kit consists of properly prepared reagents, which are easily combined. Using a pipette, we wet the filter paper with the solution and place it on the surface to be inspected. We wait for about 1 minute. If a blue color appears (even a few small dots!), the tested surface has been improperly etched and there is foreign iron on it. The absence of blue color means correct etching. Finally, after removing the paper, thoroughly wash out the traces of the test solution with demineralized water.