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Hello,
I work for a small machine shop and run the chemical processing department. We have a small set screw made from 416 SS that calls for Citric passivation. Our process is a 1-hour alkaline bath —>Di rinse —> 5 minute citric passivation —> Di rinse —> 15 minute hot Alkaline neutralization. About 50% of the time when we are a few minutes into the passivation bath the parts will start to fizz and the passivation solution turns yellow (smells like sulfur). We pull the screws out and they are now dark grey or blue. Once inspected under a scope the parts look to be etched. The passivation solution is at 4% citric acid and is heated to 145F. I believe I am having this problem because the parts aren’t heat treated (hardened) and they can not be according to the customer. Does anyone have any thoughts on things to try so this reaction doesn’t occur?
Thanks For any input,
Noah U
1. First, in a beaker on a separate panel, let’s see if that alkaline cleaner will rmove the tenacious cutting fluod from the threads.
2. Let’s insert an acid activation betweew the cleaner and the passivatio0n. About 20 % Sulfuric acid
3. Then let’s look for chloride contamination of the passivstion.
Thread 575/23 starts out being about staining in 416 SS during citric acid processing, but offers many comments about proper pretreatment, concentration, pH, and rinsing. I think it’s worth a couple of minutes of quick review to see the many suggestions and perspectives. Good luck.
Hello, I work for a small machine shop and run the chemical processing department. We have a small set screw made from 416 SS that calls for Citric passivation. Our process is a 1-hour alkaline bath —>Di rinse —> 5 minute citric passivation —> Di rinse —> 15 minute hot Alkaline neutralization. About 50% of the time when we are a few minutes into the passivation bath the parts will start to fizz and the passivation solution turns yellow (smells like sulfur). We pull the screws out and they are now dark grey or blue. Once inspected under a scope the parts look to be etched. The passivation solution is at 4% citric acid and is heated to 145F. I believe I am having this problem because the parts aren’t heat treated (hardened) and they can not be according to the customer. Does anyone have any thoughts on things to try so this reaction doesn’t occur? Thanks For any input, Noah U
As a free machining grade, 416 can definitely give you sulfur issues. What are you using for that alkaline pretreat? If it’s not hydroxide, I recommend switching to something that is. The goal is to have the sulfur off the surface before the parts go into the passivation acid bath, so that the acid bath doesn’t gain a sulfur load.
As you suspected, the description leads me to believe the parts have not been fully heat treated. 400 series parts without heat treat to fully hardness will be readily ruined in the passivation acid bath. I would tell the customer that if the parts “cannot be heat treated” then they “cannot be passivated”.
5 minutes is not likely to be an adequate acid bath passivation time for 416, even at high temperature. I generally start at 20 minutes at 150°F and often have to increase that to 30 or 40 minutes to get the desired results.
Let’s insert an acid activation betweew the cleaner and the passivatio0n. About 20 % Sulfuric acid
Activation of stainless is a step taken prior to plating. Doing such a thing for a passivation line is counter-productive. Activation and passivation are opposite processes.
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