Another Citric Acid Passivation Question
Q: We are having a problem with citric acid passivation of 303 stainless steel. The parts continue to fail a 24-hour salt spray test. Currently we passivate for 30 min. Should we double the amount of time in the citric acid? Do you have any suggestions for us? W. S.
A: I am not sure that doubling the time in the citric acid passivation bath will solve your problem. 303 stainless steel contains more sulfur than some of the other stainless steels and this excess sulfur can lead to possible failure in the salt spray test. Pre-cleaning of your 303 stainless steel with a good alkaline cleaner should help remove the excess sulfur from the surface. After good alkaline cleaning rinse thoroughly and then proceed to the passivation step.
You did not mention in your e-mail whether you are using a “home brew” or a commercial product. My preference is to use a commercial product. They are more stable and give more consistent results. I know of at least one vendor that manufactures a complete line of citric acid based passivating materials. A bonus is that if you use a commercial product should be able to call the vendor and get some technical support.