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Neutralization of passivation dip


We are having problems with passivating tubular parts. The parts are approximately 6-8" long with a .067" diameter hole. Each part is made by welding three pieces of 17-4 together to get the total length (welding is unavoidable). 17-4 filler is used in the welding process. The parts are then debured and welds sanded and buffed out, finally ultrasonically cleaned with an alkaline soap and passivated in a 50% Nitric acid solution at 120F. After the rinse we have the parts are sent to the quality department for inspection. When a gage pin is passed through the .067 diameter hole it turns black! I am suspecting that our rinse is not effective and residual acid is turning the gage pin (hardened tool steel) black.

Because of the length of the parts and the need to clean out the acid it seems the best thing to do is send it through a neutralization rinse. My question is what rinse would be good and at what temperature, concentration and times.

Kevin Dodd
Medical Device manufacturer - Pflugerville, Texas

That is a tiny hole for that length. The surface tension of the liquid is probably strong enoug to keep the tube from draining. My guess is that you are going to have to flush them with flowing water. Unless you have a massive amount of them, you probably will just set up a small flex line with an on off valve on the end and manually flush them a handfull at a time.

James Watts
- Navarre, Florida

Thank you for the response. We are looking at purchasing something to the effect of a dental pick to jet water through the parts in order to thoroughly rinse out the acid. I do believe this will help correct the problem

Kevin Dodd
Medical device manufacture - Texas, Pflugerville, USA

A dental pick sound like a workable idea. You might want to consider a small ultrasonic unit with warm DI water for a final rinse. There are some great lab sized units available, but they might be out of your price range if this is a small operation on a tiny budget.

For bigger holes than you have, we rigged up a 1/4 flex hose with a ball valve shut off, ran that thru a air blow off valve and rigged the end with a pippettor tip for a tiny orifice. We even used tips made from plastic tubing and drwn to a tiny tip opening (heated over a torch).

James Watts
- Navarre, Florida

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