After degreasing, pickling, and passivating 316L sanitary pipes, there is still black residue left when wiped with cotton swabs! ! !
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The newly manufactured 316L sanitary-grade polished pipes were treated by circulating a 5% sodium hydroxide solution at a temperature of over 60°C for 1 hour, followed by rinsing with RO water. After that, they were treated by circulating a 40% food-grade phosphoric acid solution at around 70°C for 1 hour, and then rinsed again with RO water. Then passivate it for another hour using a 10% citric acid solution, at a temperature of around 60-70°C as well. After that, it was rinsed thoroughly with RO water, but when wiped with a cotton swab, black residues as shown in the attached image remained. I would like to ask experts: what is the reason? What methods can be used to remove it? I would be extremely grateful for some advice from those who are more experienced!
The project is urgent; I’m seeking advice from experts!