Rust-looking on 316L material

[czhong]

Parts are made of SS 316L. We found about 1% of parts with brown marks, look like rust.
The OD surface go through grinding and cleaned with Alconox and Oakite 31, rinse with DI water.

What could be the cause? Is it from the material? A lab told us they found Manganese Sulfur inclusion in these areas. Does it make sense?

When parts come off our product line, we could not find anything like this. We can see some surface imperfections, rounded dots look rough and not as shiny as surrounding area. Our customer clean parts with Alconox and rinse with "soft water", then they found them. Does that mean our parts have something which could cause this?

Thank you for your help.

 

[EdStainless]

Yes, excessive MnS inclusions do cause this. They serve as crevices where corrosion can start easily.
These also could be from Fe in the surface. If the parts contacted anything made of steel you would leave little pieces of Fe in the surface which will corrode easily.
Even very small fragments of SS that get re-embedded in the surface will cause rust spotting (smearing from poor machining or grinding will do the same).
Take some of your parts and drape clean cloth over them, then wet the cloth with DI water.
Check in the morning and see if you have rust spots.

It could be inclusions that they cut into when machining.
Lower the S and clean up your metal.

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Plymouth Tube

 

[enmax]

Statistics might help - were the 1% affected parts all from the same cast? Were they from the same production run (the grinding and cleaning phases)?

http://uk.linkedin.com/pub/terry-roberts/28/a9a/a10

 

[czhong]

Thank you for your inputs.

Our parts are very tiny and we have been using material from same heat lot with same grinding supplier for over two years. We start having this issue 6 months ago.

If it is the MnS inclusion, how does it lead to pitting or rusting? Time sitting in inventory? Specific contents in post cleaning (washing), like Chloride?
Is it common to have MnS inclusion in SS316L? Will Nitric passivation per ASTM A380 help by eliminating them?

Thanks,

 

[EdStainless]

First, I hope that these parts get nitric acid passivated, ALL SS should be passivated to clean the surfaces before it goes into service.
The S levels in SS can be highly variable. But if this is from the same heat then I would not suspect that.
I am more concerned about changes in the grinding. Any change in coolant, grinding media, cleaning methods, or grinding process could change the surface texture and result in this showing up.

Nitric passivation will only help a little. If this is really driven by MnS inclusions then you would need to pickle or electropolish to remove them.

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Plymouth Tube

 

[metengr]

Have a metallurgical analysis done to evaluate the material condition/surface before you do anything else or continue to speculate.