Adequate SS grade for use inside Incubator

[agervais]

I am designing a fixturing component to be mounted inside a CO2 incubator (5% CO2 and 5% O2 concentrations, at 37 deg C). The incubators are also highly humidified, generally 60-90%. The incubators are used to culture live cells so any rust/oxidation is a non-starter for the customer.

We began seeing localized oxidation on prototype parts (304 SS) after ~2 months of use. I am concerned that 316 SS will not be sufficient, but am unsure how to determine which grade will work. I see that 4439/917L has a Critical Pitting Temperature (CPT) greater than 37 deg C, as well as 904. Is this a fair way to determine what to use?

I also don't want to over-specify as cost is an important factor as well.

Thanks,
Newbie MechEng

 

[EdStainless]

I would suggest that you look at using a duplex stainless steel.
You could go to 317L or 904L, but you are paying for a lot of alloy that does not improve pitting resistance.

Yes, PRE or CPT are good tools for comparing alloys.
I believe that you will find that 2205 offers the most corrosion resistance for the money.
There are many other duplex alloys that are lower in corrosion resistance and less expensive, such as 2101. But they are not much better than 316.

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Plymouth Tube

 

[agervais]

Thanks EdStainless for the reply.

2205 was one of the other alloys I was considering. I am new to spec'ing SS so I don't have a very good grasp on how various grades compare on price, so that is good info

Also, this part will be machined, are there industry standards for surface finish call outs on machined SS? I would imagine that surface finish would play a large part in localized (grain boundary) corrosion/pitting no?

Should I be considering having it passivated?

 

[EdStainless]

Yes, it must be passivated after all machining/polishing to clean the surface.
All machining and parts should be arranged to minimize crevices and to not have real flat surfaces.
You want things to drain if there is condensation.
Typically people want surfaces smoother than a 180 grit finish, smoother is better, but you don't want anything buffed.

2205 is a lot stronger than 304/316, so in many cases you could use thinner material.
It should be very similar to 316 in price, and it has significantly better corrosion resistance.

There are specs for equipment designed for hygienic service, but that might be overkill for your application.
Search for ASME BPE.

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Plymouth Tube

 

[MikeHalloran]

Also, if there are any welds, you need to pickle or use abrasives to remove any 'heat tint' before passivation.



Mike Halloran
Pembroke Pines, FL, USA

 

[agervais]

Thanks Guys! I think I will get quotes for a duplex 2205 alloy

One last question:

This part will have a small spring plunger threaded into it to retain a disposable sensor. The highest grade alloy I can find for a spring plunger is A4 (passivated). I understand this is the equivalent of 316? I know dissimilar metals promotes corrosion, but what about dissimilar alloys? Will the assembly be more prone to corrosion if an A4(316?) part mates with a duplex 2205 part?

 

[EdStainless]

Unless they are in contact and they are wet it isn't a big deal.
There are springs made in various Ni alloys that would be a lot more corrosion resistant, and a lot more expensive.
Give the 316 a try, it may be good enough.

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Plymouth Tube