Tech Briefs

Procedures for commonly used stainless steels can use citric or nitric acid solutions.

The importance of cleaning should not be underestimated. Sometimes a machine operator might skip the process, assuming that by immersing the grease-laden part in an acid bath, it will be both cleaned and passivated. Instead, however, the contaminating grease reacts with the acid to form gas bubbles that collect on the surface of the workpiece and interfere with passivation. Even worse, contamination of the passivating solution, sometimes with high levels of chlorides, can cause “flash attack,” producing a heavily etched or darkened surface.

{MOSIMAGE}After cleaning, the part is ready for immersion in a passivating acid bath (see Figure 1). Any one of three media can be used: nitric acid, nitric acid with sodium dichromate, or citric acid. Which bath is used depends on the grade of stainless steel and prescribed acceptance criteria.

Citric acid passivation has become increasingly popular with fabricators who want to avoid the use of mineral acids or solutions containing sodium dichromate, along with the disposal problems and safety concerns associated with their use (see Figure 2). Citric acid is on the GRAS (Generally Regarded as Safe) list compiled by the U.S. Food and Drug Administration (FDA) as a material that is safe for humans to handle.

{MOSIMAGE}Tests are often performed to evaluate the surface of passivated parts. The 400 series, martensitic precipitationhardening 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. Austenitic non-free-machining stainless grades also may be evaluated by means of a humidity test. A faster method is available using a solution from ASTM A380, “Standard Practice for Cleaning, Descaling, and Passivation of 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.

It is important that the test method be matched to the grade under evaluation. A test that is too severe will reject perfectly good material, while one that is too lenient will allow unsatisfactory parts to be accepted.

This article was written by Terry A. DeBold and Ted Kosa, staff specialists in stainless alloy R&D at Carpenter Technology Corp. For further information, contact Terry DeBold at This email address is being protected from spambots. You need JavaScript enabled to view it. or Ted Kosa at This email address is being protected from spambots. You need JavaScript enabled to view it.; write to them at P.O. Box 14662, Reading, PA 19612-4662, or call (800) 527-6900.

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