Passivation of 300 Series SS 2

quizzical1 · Jan 8, 2022

Hi All,

Need a debate solved:

Should all 300 series SS parts be passivated after machining or can they be used as-is?

TIA

~Q

TugboatEng · Jan 8, 2022

That depends on application. Also, machining is a form of passivation.

dik · Jan 8, 2022

with non-ferrous tools?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

TugboatEng · Jan 8, 2022

Tools are generally coated nowadays. I didn't say machining is the best passivation but it does remove surface contaminants.

EdStainless · Jan 8, 2022

If you want them clean then chemically passivate.
Odds are that the parts touched chucks, benches, and other items that are steel.
Nitric acid is fast and easy, but more difficult to handle.
There are Citric acid mixtures sold (such as CitriSurf) that work, but they must be heated and allow a longer soak time.

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dik · Jan 8, 2022

Sorry tug... I understood that contamimation of caused by tools and equipment was one of the major problems with working on stainless...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Compositepro · Jan 8, 2022

Should not a distinction be made between etching and passivating? Otherwise there is much room for confusion.

EdStainless · Jan 8, 2022

Pickling uses very aggressive acids, typically Nitric +HF for stainless. This will etch the surface and will remove oxidation as well as impurities that are on or embedded in the surface.
Passivation of SS is a cleaning operation that only remove iron contamination on the surface.
If the material is clean then passivation is all that is needed.

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quizzical1 · Jan 9, 2022

Thanks all!

We’ve been making valves for water and water glycol and oil applications for years and haven’t noticed any corrosion issues in 19 years. Obviously, for DI water and sea water we EP the 316 SS parts.

Some say why start now for the water, glycol and oil applications where others say we have springs that always come in passivated to A967.

Are we ok staying the course?

EdStainless · Jan 9, 2022

Using 316 in seawater is a ticking time bomb. I know that people do it, but pitting is inevitable.
I don't see a reason to add a final passivation for the glycol mix applications.

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P.E. Metallurgy, consulting work welcomed

quizzical1 · Jan 9, 2022

Thanks Ed,

Any issues with using passivated springs with non passivated parts (302SS springs with 303SS and 316SS springs with 316SS)?

EdStainless · Jan 10, 2022

The usually isn't an issue with such mixing. The non-passivated parts may have rust spots on them, but it usually doesn't matter.
There is no what to keep the 303 from pitting in the long run so don't worry too much about it.

I do find it interesting that everyone in the US is still using 316 when for the same money you can get higher strength and better corrosion resistance from a lean duplex alloy. Again, Europe leads the way.

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P.E. Metallurgy, consulting work welcomed

dik · Jan 10, 2022

Which duplex? Thanks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Jan 10, 2022

dik,

But then you have to weld the d**n stuff (although lean DSS is less trouble than 22/5).

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

EdStainless · Jan 10, 2022

Both machining and welding lean duplex is a piece of cake.
In 316 the welds have less corrosion resistance than 304, in duplex they solidify in the desired structure.
2205 isn't much trouble at all, unless you expect it to weld like mild steel. It is no more effort than should be put into welding 316.

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P.E. Metallurgy, consulting work welcomed

dik · Jan 10, 2022

Thanks, Iron and Ed...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

quizzical1 · Jan 10, 2022

thanks guys - as usual I am VERY appreciative of your help!!

Jan 10, 2022

EdStainless said:
Both machining and welding lean duplex is a piece of cake.

Unless you have corrosion testing requirements or consumables from reputable makers that have trouble just meeting the F-A spec. Easy on paper but not so straightforward implementing it in the shop.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

EdStainless · Jan 10, 2022

There are certainly two critical issues with lean duplex alloys.
- Lack of availability. Some product forms are nearly impossible to find. It will take a few more years before they are ubiquitous enough.
- Application temperature range. The lean grades are a bit better than higher alloy duplex, but to be honest I still stick to -40 to 600F as a max range.

I have been very pleasantly surprised at how well they machine. This is a big plus for them.

If Ni or Mo prices explode lean duplex will replace 95% of the 316 in use.

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P.E. Metallurgy, consulting work welcomed

dik · Jan 10, 2022

F-A ?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

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