316SS used in DI water... 5

[quizzical1]

Can 3i6SS be used in DI water? I've always assumed it would cause pitting on less corrosion resistant SS's (303, 304, etc) but am not totally sure.

We seem to have had success in the past when would EP the 316SS but am now wondering if there are better alternatives to 316SS or the process (passivate / EP).

TIA

 

[TugboatEng]

SS is fine for clean water provided there is no stagnation.

 

[EdStainless]

Certainly EP 316L is the standard for high purity water systems.
The most critical thing is how the welding is done, and how the welds are cleaned.
Any heat tint can be a problem and passivation cannot remove that.
Another thing that gets overlooked is the use of SS castings, commonly in valves and pumps.
Castings are fine, as long as they have been solution annealed and certified as low residual delta ferrite.
Of course that goes for the tubing also, the long seam welds should be >0.5% ferrite.
I have seen testing over the years that points to the micro surface finish being more important than the macro.
People fixate on Ra because they can measure it, but it is not very significant. I can EP a rougher surface and remove the microscopic features and get much better pitting resistance.
EP works because;
1. smooths the micro-surface
2. selectively removes less corrosion resistant material
3. leaves a very clean surface

There are not a lot of other good alloy choices. The only other thing available in the marketplace is AL-6XN, and it is way overkill for HP water.
In cold systems I have seen FRP used, but you have to be very careful there also.


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[quizzical1]

Thanks Guys!

for the CF8M casting - what treatment is best? EP also?

 

[EdStainless]

EP would be optimal, but if it isn't low ferrite then you will have issues anyway.

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P.E. Metallurgy, consulting work welcomed

 

'Rouging' or reddening is a phenomenon observed on very smooth SS surfaces in deionized, deoxygenated water; i.e. ultrapure H2O. It stumped corrosion engineers for a long time, and I'm still not certain they fully understand the mechanism.


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

 

[EdStainless]

Rouging is the transport of dissolved iron, and it deposits when it gets to locations in the system where the concentration exceeds the local solubility.
One of the most common culprits is low pH water from dissolved CO2 and pumps with un-annealed (high ferrite) impellers. That can put a lot of Fe into solution in high purity water.
Some Parma operations chase removing all rouging while others monitor the iron at the point of use and as long as it remains below some control limits they leave the rouging in place. The latter is a much better approach.

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EdStainless is never stumped...

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

 

[EdStainless]

I have been making SS and Ni tube for about 30 years, eventually you get there.

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P.E. Metallurgy, consulting work welcomed

 

So if I understand correctly, ferrite is a rebel coming in from a bad neighbourhood, spreading graffiti...

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

 

[quizzical1]

EdStainless - how do I go about confirming low ferrite for the CF8M SS castings? We contacted the foundry and all they sent us was the chemical composition with the Fe% circled.

 

[EdStainless]

Well that confirms that they are amateurs.
In castings it nearly always measured magnetically.
And to get it low nearly always requires a solution anneal.
Are you using a casting spec that requires and anneal? If not you should.
Ask again, 'residual delta ferrite" is the phrase.
If they don't get it look for another supplier.

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Thanks so much ES!!!

No, we don’t currently list a solution anneal on our print. I feel so stupid having to ask all these questions and I apologize if it’s irritating anyone

 

[dik]

Don't feel silly. It was a great question... it's educational; some of us were completely unaware of these issues.

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

-Dik

 

[EdStainless]

It is a learning process. This is what I do for a living.

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P.E. Metallurgy, consulting work welcomed

 

[dik]

...and you're great at it. It's not what I do, but I find it interesting and informative.

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

-Dik

 

[quizzical1]

This site is a godsend. I am so thankful for all of you taking the time to help.

~Q

 

[bimr]

For ultrapure DI water, Polyvinylidene Fluoride (PVDF) piping is commonly specified.

 

[quizzical1]

thanks bimr, that one I knew but we need valving that could handle more than 125 psig @ 140F