Corrosion of INCONEL 625 in SOFC testing

[Dglock18]

Hi,
My company uses a lot of INC 625 superalloy for our fixture equipment in testing SOFC (Solid Oxide Fuel Cells), because of it's well know high temperature corrosion resistance (SOFC's typically operate at around 800C). This metal has been great for the most part, until recently...

Upon disassembling cell stacks, I notice many times that we have heavy pitting and greenish-type corrosion occurring in air at about 800C, and sometimes not for very long at that temp (a few days even). This is especially severe when we use silver mesh (99.9%, 80 mesh) as a means to increase electrical conductivity between the top of a fuel cell stack and the INC 625 plate with a current collecting rod attached.

I have looked all over the internet to learn more about why this is occurring, and to try and mitigate this effect from happening, as this material is VERY expensive to both purchase and then machine into usable fixtures for use in fuel cell testing.

Does anyone have any idea what I could do to reduce or eliminate this type of corrosion?

Thanks, Dglock18

 

[EdStainless]

Use a more corrosion resistant alloy.
I am not sure what electrolyte you are exposing these to but the green could be a Ni hydroxide.
Normally these form in stages, but with high temp and moisture present you can form this.
Having Ag in contact with the 625 will greatly increase the corrosion rates. Since the Ag is more noble it will drive galvanic dissolution of the 625.
What testing lead you to 625?
I would think that a high Cr (maybe with Mo) alloy with very low (or no) Ni might be a better option.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Dglock18]

@EdStainless -
Sorry for the late response. I have still been investigating this. The research engineer before me had used this material before I started working here; I think that he experimented with Haynes 230 and Crofer 22 as well. My guess is that the INC 625 looked the best after hundreds of hours at 800 C. Not sure why we ended up choosing INC 625 in the end though...

We have been looking at other options, but haven't been very successful.

INC 625 has between 20% and 23% Cr in it, so you are thinking even higher Cr then?

Thank you for your previous response!

 

[EdStainless]

Look at alloys like 622 (C-22), 59 and 33. I especially like 33.
Are these fabricated and machined or is there welding?
No welding, then a superferritic would even be better (AL29-4C).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Dglock18]

Awesome Thank you so much for the info! There has been welding on previous parts. They are usually milled, so yes, they are machined.

I'm not familiar with any of those alloys. They aren't all Inconel are they?

 

[EdStainless]

Inconel is a trademark that Special Metals (formerly Inco) uses for Ni based alloys. (they use Incolly for high alloy Fe based grades)
Try to use ally designation, either common names or UNS numbers and avoid tradenames.
Saying Inconel 625 infers that you want material only sourced from Special Metals.
The "C" alloys are usually trademarks of Haynes.
Alloy 59 and 33 are VDM grades.
Alloys 59, 622, and 686 are similar to each other, but different.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Dglock18]

Great thanks for the clarification.

These all look great, but from looking at each alloy's spec sheet, most look like they have better corrosion resistance at lower temps. The 622, like you said, is pretty similar to the 625, so that's a possibility.

I was also told that stainless steel 441 was gaining popularity in the SOFC industry, though I can't find anyone that carries it, at least not in the US...

Thanks again for your responses!

 

[EdStainless]

622 is far superior to 625 in acidic and chloride environments.
33 is much better.
AL29-4C is a ferritic (like 441) with significantly higher corrosion resistance.
No one would use 441 at your temperatures (I hope).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube