Metal finishing Q&As since 1989

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Acid weeping after passivation of assemblies

Hi,

I have a question about passivating assemblies, in particular parts with close & light press fits. The application is for Stainless surgical instruments. I have received some conflicting information about acid weeping from close fits even after neutralization.

Anyone have any experience with this? Is it different for citric vs. nitric?

Thanks!

Keith
- San Diego, California
2005

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Keith,

Recently we had made some gages made of 420SS with a press fit plug. The hole was intorpolated on a CNC mill because of its size and the plug was gound. Because the hole was not perfectly symetric to the plug there was a gap that allowed acid to seep into the void behind the press fit parts. After rinsing and a heated drying cycle acid would still be seen seeping out from the void behind the press. To midigate the problem we placed the parts in our ultrasonic washer and rinsed several times. I monitored the heated rinse water pH in order to verify that the acid had been flushed out and dryed the parts. The time to correct this mistake was not efficiant at all. The next time we manufactured the parts they were passivated then pressed together.

Kevin
- Pflugerville, Texas
2005

Ed. note: Keith, Kevin, sorry for any misunderstanding but this is not an anonymous service. We must post your full real name. Thanks.

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There is a HUGE difference between nitric acid and citric acid formulations as it might relate to trapping of the acid in gaps with later weeping. Nitric acid will not only cause damage to the parts and surrounding area, but can cause extreme pit corrosion inside the gap. Citric will not cause this.

However, you should not allow ANY chemicals to be trapped in the gap of a surgical or orthopaedic instrument that could later cause contamination. Even the relatively safe citric acid should not be left in the gaps.

Best procedure is to passivate prior to assembly. Let us know if we can help further.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2005