Q. We have always used nitric acid, but after seeing your site, we’re more interested in using citric acid from a safety point of view. Can you tell me what the exact procedure is, and does it tick all the boxes regarding quality of passivation? A.I.
A. This seems to be a common topic in this column. Yes, the citric acid has been shown to be an effective passivation treatment and is commonly used, especially for 300 series stainless steels. There are numerous references and papers in the literature that document its effectiveness and the associated process parameters. It is included in ASTM A 967 (Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts) as an approved method to passivate stainless steel.
A compilation of some of the references I have on the process parameters associated with citric acid passivation would be summarized as:
- 5 – 15% by weight of citric acid
- 30 – 120 min of immersion time
- Ambient to 140°F (60°C) application temp
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Obviously, these are broad guidelines. As you increase one parameter, you may be able to decrease others. For instance, if you are currently using a nitric acid passivation and do not have heat available at those tanks, you may need to utilize a higher concentration of citric acid (in the range of 10–15%) and possibly more time to achieve your passivation requirements.