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Staining of CF3M (316L Cast) Stainless Steel

Staining of CF3M (316L Cast) Stainless Steel

Staining of CF3M (316L Cast) Stainless Steel

After autoclave sterilizing a mechanically polished cast CF3M (316L) stainless steel part with steam at 121C for 25 minutes and drying under vacuum for 30 minutes, I observed a discoloration of the stainless steel. Click on the link to see the discoloration...


I was able to remove the discoloration using a Surfox 204 weld passivation machine and on subsequent autoclave cycles, the staining did not return to the spots that were passivated.

Any idea what caused the discoloration on the first sterilization cycle? Also, is there a name for this phenomenon?

Thanks in advance!

RE: Staining of CF3M (316L Cast) Stainless Steel

I cannot be sure without having the piece in my hand, but that looks suspiciously like iron contamination.
Were the parts polished on an abrasive belt?
It may be that mild steel had been polished before or a part was contaminated, and that stuff found its way onto the belt.


The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

RE: Staining of CF3M (316L Cast) Stainless Steel

The part was polished using a combination of sanding belts, flap wheels, and buffing wheels with abrasive compound. Iron contamination from the polishing process is a possibility, but it seems unlikely as the above tools are dedicated to only stainless steel polishing. Is it possible that the surface of the stainless steel casting is iron enriched? It is an investment casting.

RE: Staining of CF3M (316L Cast) Stainless Steel

Even small re-embedded particles of stainless will rust easily under autoclave conditions.
Buffing may produce pretty looking surfaces but the smearing and lapping of the surface can trap a lot of debris (buffing compound) and re-embed particles of metal.
From the standpoint of corrosion resistance I would never allow buffing, mechanically polish and EP would be the desired process.
You might avoid this with a good nitric acid passivation after mechanical polishing and again after buffing.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Staining of CF3M (316L Cast) Stainless Steel

Thanks EdStainless - So, if I understand correctly, you suspect that the discoloration is due to re-embedded particles rusting during the autoclave process - yes? Using a buffing compound greatly increases the problem as it traps a lot of debris into the surface of the stainless steel?

I was contemplating the same solution that you proposed, namely, a 220 grit mechancial polish followed by EP. So, we are in good agreement on that point.

It sound like you like you have some first hand experience with autoclaving stainless steel parts. Have you seen this type discoloration after an autoclave process in the past?

We have in-house citric passivation (ie. CitrusSurf 2050 diluted 14:1, working pH = 1.6-2.0). Nitric acid passivation done out of house only when a client specifically requests it. Any idea how nitric compares with citric?

Thanks in advance!

RE: Staining of CF3M (316L Cast) Stainless Steel

Is your Citric warm? It does OK, but takes longer than Nitric.
I have seen the evils of buffing many times.
The need in service is a microscopically smooth surface, it doesn't matter if it is wavy.
Go to 220 and EP. And don't sweat the occasional irregularities in surface appearance.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

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