Do you know of a procedure for using citric acid at ambient temperature for passivation of 316L SS? B. N.
It would be helpful for you to consult ASTM A967, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts. It provides several different options for passivation of austenitic stainless steels. One approved option is citric acid. The standard gives guidance for several different scenarios, the first of which is four to ten weight percent citric acid at 140–160°F for at least four minutes. The options following this generally deal with lower temperature and increased time. One option is again a four to ten weight percent citric acid solution, this time in the temperature range of 70–120°F. In this case, the recommended minimum time has gone up to 20 minutes. So it is likely that for a room temperature citric acid passivation, you could expect to spend 20–30 minutes immersion time with each batch. Depending on your throughput requirements, it may be beneficial to add heat to significantly decrease the time to the range of four to ten minutes.
In order to check the effectiveness of the passivation treatment, the standard also provides several practices so that you can evaluate the effectiveness of the passivation process. They are all straightforward, although a few require some specialized equipment that you may not have available to you such as a salt spray cabinet or humidity cabinet.