Passivation layer

[Kinsrow]

What happened when 410SS investment cast surfaces were sand blasted after being chemically passivated? Would the passivation layer (the chromium surfaces) grow back? how long would it take? and would it grow back as good as the original layer that was promoted by chemical passivation. Should the passivation process be the last step before shipping?

I'm trying to undertand what mechanism does the passivation layer become non-effective anymore.

Thanks

 

[EdStainless]

The passivation should be the last step.
Your blasting will remove it.
It will naturally reform quickly.
However, you will have an inferior passive layer because you will have surface contamination from blasting. Either stray particles of metal, fragments of blast media, or even small pieces of the stainless itself. Any of these can lead to rapid local corrosion.

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Plymouth Tube

 

[MartinSr00]

When you say "sand blasted", you should mean something like garnet sand.

Don't use steel grit, or you're screwed.

 

[Kinsrow]

We use aluminum oxide to sand blasted the parts.

In the utopia world, I would think that the a good & controlled sandblasting process yield a good clean & rust free surface. I think the problem is maintaining the Free Iron contamination to the sand blasting system. Therefore passivation should be after sandblasting. There is enough debate over this that the only way to proof it by continues comparison of destructive test (humidity test).

I wish I have a faster way to test and to quantify how much free iron left in the suface after both proceseses. Copper sulfate and salt spray don't work with 400 SS. The only thing that works is Humidity test. Do you guys know others?

 

[EdStainless]

With any blasting you re-embed enough of the material itself that passivation is needed.

410 has such low corrosion resistance that humidity is about your only option.

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Plymouth Tube

 

[Kinsrow]

Ok..

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.

 

[EdStainless]

The dust will rust and blush the surface. It will probably not lead to further corrosion. However if there was any mechanical damage to the passive layer or the dust layer was thick enough to trap moisture under it then you could have problems. The moisture under the dust will become acidic as it rusts, this could be aggressive enough to break down the passive layer in a low Cr material like 410.

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Plymouth Tube

 

[Kinsrow]

EdStainless,

Thank you for your great insight. I scraped off rust dust on top of polyetheline tank.

I don't think I'm only dealing with Iron Oxide type of rust, I saw some weird looking film that is rust color.

I don't know, maybe I have chloride that is contaminating the surface and lead to rust development.

Feel like I'm beating a dead horse trying to get rid of rust without chemical passivation process and rely only the ability for the parent material so self passivate.

Thank again though