Q. We want to change over to citric acid bath passivation. Can we use the same tanks that we use with nitric acid? A.V.
A. The conversion to a citric acid passivation process is a good choice to minimize the hazards in your shop. The material of construction used to fabricate and build the nitric acid passivation line generally would be a moderately high grade of stainless steel (316L or higher). In this case, the citric acid passivation line would be less aggressive, and the existing tank material would be fine. Even stepping down a grade of stainless in the future to something like a 304L stainless steel would be sufficient for the citric acid passivation line.
Depending on age of and other potential uses or contaminants in the nitric acid passivation line, taking the time to perform a thorough examination of the tanks when they are emptied is recommended. This would involve adequate rinsing of the tanks and an inspection of all the walls and, in particular, all welds. Incorrect weld material at fabrication could lead to attack of the welds. Ignoring the use of a low-carbon-grade stainless steel could also lead to sensitization of the material in the heat-affected zone of the welds. Assuming the tank integrity is still good, you should be able to get a significant amount of life out of the existing equipment.