|Laboratory tests have indicated that citric acid passivation procedures were more prone to "flash attack" than nitric acid procedures. Factors causing this attack included excessive bath temperature, excessive immersion time and bath contamination. Citric acid products containing corrosion inhibitors and other additives (e.g., wetting agents) that reportedly reduce sensitivity to "flash attack" are commercially available. |
Special procedures have been found necessary for the passivation of stainless steel beverage fittings. ASTM A967 criteria may not be applicable to free-machining stainless steels for such applications. A shop needing free-machining stainless bar for beverage fittings may seek technical assistance from his stainless steel supplier.
The ultimate choice of passivation will depend on the acceptance criteria imposed by the manufacturer for whom the parts or components are to be made. For more information, refer to ASTM A967 "Standard Specification of Chemical Passivation Treatments for Stainless Steel Parts." The specification can be accessed at www.astm.org.
Testing Passivated Parts
Tests are often performed to evaluate the surface of passivated parts. The question to be answered is, "Did passivation remove free iron and optimize the corrosion resistance of the free-machining grades?"
It is important that the test method be matched to the grade under evaluation. A test that is too severe will fail perfectly good material, while one that is too lenient will allow unsatisfactory parts to pass.
The 400 series precipitation-hardening and free-machining stainless steels are best evaluated in a cabinet capable of maintaining 100% humidity (samples wet) at 95°F (35°C) for 24 hours. The cross section is usually the most critical surface, particularly for free-machining grades. One reason for this is that the sulfides, elongated in the direction of working, intersect this surface.
Critical surfaces should be positioned upward, but at 15 to 20 degrees from the vertical to allow any moisture to run off. Material that has been properly passivated will be virtually free of rust, although it may show some light staining.
Austenitic non-free-machining stainless grades also may be evaluated by means of a humidity test.
When so tested, liquid droplets of water should be present on the surface of samples, revealing free iron by the presence of rust formation.
A faster method is available using a solution from ASTM A380, "Standard Recommended Practice for Cleaning and Descaling Stainless Steel Parts, Equipment and Systems." This test consists of swabbing the part with a copper sulfate/sulfuric acid solution, maintaining wetness for six minutes and observing whether there is any plating of copper. As an alternative, the part may be immersed in the solution for six minutes. Copper plating occurs if iron is dissolved. This test should not be applied to surfaces of parts for use in food processing. Also, it should not be used for the martensitic or lower-chromium ferritic stainless steels of the 400 series because false-positive results are likely.
Historically, the 5% salt spray test at 95°F (35°C) also has been used to evaluate passivated samples. This test, too severe for some grades, generally is not necessary to confirm that passivation has been effective.