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Passivation of investment cast stainless parts

I'm a mechanical engineer specifying the finish steps required for an investment cast stainless part (CF-8M, most likely) and I'm a little unsure of whether passivation is truly necessary for my application. I understand the basics of iron contamination and the general need for passivation in machined parts, but I'm new to investment cast parts.

Could someone please elaborate on the need (or lack thereof) to passivate solution-annealed CF-8M for a non-medical, non-food, low-stress part that would be used in a piece of winter sports equipment? The part is about the size of a car key and has an expected lifetime of about four years, most of that time spent inside at room temperature and up to approximately 200 hours per year outside in the snow at 20 degrees F. Very rarely will it be under stress. It needs to be generally corrosion resistant, so that it doesn't exhibit significant discoloration, but it doesn't have severe service requirements or a long life, so I don't know whether it can get by without it. Thanks in advance.

Jeff Cox
Mechanical Engineer - Golden, Colorado, USA
September 8, 2009

If there are any mechanical operations done on this part or if it connects to dissimilar metal, it would be a reasonable idea to passivate it. I would take a very wild guess and say that you probably could get away without it, but no one will know for sure until it is put into actual operation.

James Watts
- Navarre, Florida
September 10, 2009

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As far as SS castings versus machined parts, cast surfaces generally need passivation more. On parts that are castings with further machining done to say half the part, the half with the original untouched casted surface will rust first when you stress it. There also tends to be better results if the cast surface has been bead blasted, though it's usually done more for appearance's sake.

That said, given the application you described I'd say there's a decent chance you could get by well enough without passivating.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

September 17, 2009

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October 6, 2009

Thanks, guys. I think we'll be giving it a try without passivation first and see how it goes.

--Jeff

Jeff Cox
- Golden, Colorado, USA