My lab has been approached by one of our vendors for a coating that when painted onto stainless remains clear and turns a coppery color for other steel grades. They had some, but lost the bottle.
This is the well-known copper sulfate test for passivated stainless steel. The Cu[sup]+2[/sup] ions in acidic solution are reduce by metallic iron, Fe[sup]0[/sup]. Consequently, immersion plating of copper occurs by an exchange reaction:
The solution and procedure and limitations (on which SS grades it can test passsivation & which give false positive rxn., turning coppery red anyway) are given in ASTM A967. A portion:
"17.2 The test solution is prepared by dissolving 4 g of
copper sulfate pentahydrate (CuSO4·5H2O) in 250 mL of
distilled water to which 1 mL of sulfuric acid (H2SO4, sp gr
1.84) has been added."
For the free version, get the old specs. QQ-P-35C PASSIVATION TREATMENTS FOR CORROSION-RESISTANT STEEL and MIL-STD-753C CORROSION-RESISTANT STEEL PARTS: SAMPLING, INSPECTION AND TESTING FOR SURFACE PASSIVATION,
from ASSIST:
That sure sounds like the test except I was told the liquid was clear, not blue. Hopefully tomorrow I will get more feedback. One can only hope they have not been using a bottle of 6 year old solution for this test. Thanks.