Cleaning Q&A: Citric Acid Passivation

Q. Enlighten me on the use of citric acid in the passivation of our steel products.

A. Passivation will not be effective on mild steel because it dissolves it. There are two generally accepted processes for passivation of stainless steel: nitric acid-based and citric acid-based. Both are effective in establishing the protective chromium oxide surface of the stainless steel, but differ in their base chemistry. Both methods are considered acceptable and described in detail in ASTM A967. Within this specification, there are directions regarding time, temperature and concentration ranges for both types of processes. There are also a number of functional tests to verify the effectiveness of the passivation process, regardless of type.
As a high-level comparison, the citric acid process generally uses 5–15 percent by weight of citric acid in the passivation tank, while the nitric acid process typically uses about 50 percent by volume. Both processes may be used at ambient temperatures, although more often, the nitric process is run unheated while the citric acid process is usually heated to a temperature of about 140°F. Immersion time will be a function of the preceding factors (concentration and temperature), but will range from 15 minutes to 1–2 hours.
Of the two processes, citric acid has gained more interest because it presents a significantly lower safety risk and creates fewer disposal concerns. Both systems will require rinsing to remove the passivation chemical. Rinsing can be accomplished in one or more overflowing rinses, although the more rinse tanks that are counter-flowed, the lower the flow rate needs to be to maintain adequate rinse water cleanliness. The capital investment will be somewhat similar between the two systems, because similar plant layout and equipment will be used. As mentioned, the citric acid may need to be heated, but will require a lower investment in treatment equipment for the spent chemicals. The size of the capital outlay will be in proportion to the size and throughput of the parts intended to passivate.