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Does post processing remove passivation layer on 400 series stainless?\

We have 400 series ss that is investment cast and gone passivation process. we then put several steps to process this part to finish good. We heat treat this part and followed with bead blasting process for the finish look we want and applying polishing in some area for finish look. we also do parts washer between processes. My question is that would glass bead blast process ruin the passivation layer? if so how do we control the thickness of passivation layer to make sure that the surface is tough enough to withstand bead blast process.

Andro Purnomohadi
designer - Portland, OR, USA
August 15, 2009

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Anything that you do after passivation, will damage the passivation! You can not help it by having a thicker layer, because it very nearly has only one thickness. It is a simple thing to have it repassivated IF you do not have any other metals in or on it. James Watts - Navarre, Florida August 18, 2009 Passivation produces a surface which has no free iron, and it is very, very thin. Bead blasting, polishing and any other process which disrupts the surface on a micro level will require re-passivation. Passivate as the very last step in fabrication. Jeffrey Holmes, CEF Spartanburg, South Carolina August 18, 2009 August 18, 2009 Yes it will ruin the passivation. Passivation should always be the last step. Sincerely, Micheline Forth - Georgia, USA

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Andro,
Passivation is always the LAST thing that is done to a stainless steel part, after all machining, blasting, polishing, etc. steps are completed. Let us know if we can be of assistance.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

August 21, 2009

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Thank you for your respond. Passivation at the end of the process is causing to remove grease, oil tha twe put in during assembly process. This affect the performance and function of the finished good.

The finished good is a precission scissor.

Andro Purnomohadi
- Vancouver, Washington
September 25, 2009

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well, you have a choice to make. Proper passivation or an oily part.
There is always the possibility that passivation before assembly will be adequate. Try to use stainless contacts with the parts to hold down the smearing from steel contacts.

James Watts
- Navarre, Florida
September 25, 2009

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October 5, 2009

OK, We're going to try to wash the finished good in the parts washer that uses detergent and some rust inhibitor in it. Hopefully we'll remove the un-alloyed iron/Free Iron this way before packaging it. This method doesn't remove the grease in the joint.

Let's suppose I have good passivation process that yields good Chromium Oxide layer on the SS surface. Let's suppose that I blow iron dust on the surface. Would I get rust? Would this be rust that you can you can remove by wiping it (above passive layer) or is it the one that attacking the parent material (breaching the passive layer)? what is the mechanism that the iron particle would breach the passive layer of SS. How does the passive layer play role.

Thanks

Andro Purnomohadi
- Vancouver, Washington, USA

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So what you're saying is you assemble these stainless steel parts, including the addition of lubricants for the moving bits, then you bead blast it?

Probably the best thing to do is use a topical passivation, like a gel or spray bottle type, rather than immerse it in a bath.

Blowing iron dust at it probably wouldn't hurt anything. Most iron contamination occurs during tooling or if the stainless piece is scraped across a piece of carbon steel. Of course bead blasting exposes new surface iron from the alloy content of the steel itself.

How worried you need to be about sufficient passivation depends on what kind of environment your product is going to be placed in.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

October 7, 2009