Impact of ferritic nitrocarburizing on galvanic corrosion

[WillCole]

I am hunting around for information on the impact of ferritic nitrocarburizing on galvanic corrosion. Does this process tend to move the anodic index up or down in steels? Or is it generally considered nominal in impact? I am looking at this from a fastener perspective. Any insight or pointers would be very welcome!

Cheers!

 

[swall]

The answer will depend on whether you are talking stainless or just low alloy carbon steel.

 

[TVP]

There is not much in the public domain about galvanic corrosion specifically related to one of the members being ferritic nitrocarburized. In general, the galvanic activity should be reduced between joint members of different materials when one of them has been processed using FNC, due to the presence of a continuous, non-conductive white layer (also called compound layer) that is usually ~ 5-20 [µ]m thick. The predominant phase in the compound layer is [ε]-Fe_2-3(C,N), but [γ]'-Fe₄N may also be present. Some FNC processes include an oxidizing step after nitrocarburizing, which then creates a thin, ~ 1-2 [µ]m Fe₃O₄ iron oxide layer.

 

[metengr]

If you really want to evaluate this it might be wise to have a small mock-up of the fastener joint and subject to corrosion testing using an independent materials lab. This should not be that expensive to so and once again it can be defended for design/use purposes.

 

[WillCole]

swall - If you have any values for A2, D3 and M2 steels that would be a great starting point. Honestly, any info would be welcome including for stainless.

TVP - I did notice the dearth of readily available info. The fact the galvanic coupling is impeded is very helpful information. I am just trying to get a basic understanding of the likely impact before I take talk to a supplier and accept anything they say because I don't know any better.

 

[unclesyd]

I have never or seen an FNC treatment used in threaded areas. We went to great pains to mask any threads. The reson for lack of information on galvanic corrosion as you wouldn't expect to
to see an FNC part coupled in a enviroment where a potential could develop.

Here is some information on salt bath FNC especially the QPQ
(quenched, polished, and quenched) finish. Depending on the media FNC does improved the corrosion resistance of some low alloy metals depending on the processing steps. From my experience I would not consider it a general anti corrosion process.
With the salt bath process the metal can be Q (quenched), QP
(quenched and polished), QPQ (quenched, polished, and
quenched).

The following articles show the results of testing in salt spray
with a much better resistance for the QPQ processing.
The Badger Metal site has information on tool steels, but they too have their limitations in respect to FNC.

http://www.finishing.com/kolene/qpq/

http://www.burlingtoneng.com/salt_bath_nitriding.html

http://www.nitromet.com/A55B6F/nitromet/nitromet.nsf/a615da

0219b54b79852571cb006bc9d2/e12cddaa4d9861b1852571cd00
075581/$FILE/Nitromet%20LNC%20with%20a%20Post%20Process
%20Enhancement,%20an%20Effective%20Alternative%20to%20Cr%
20Plating.pdf

http://www.google.com/url?sa=t&source=web&cd=14&ved=0CCcQFjADOAo&url=http://

http://www.industrialheating.com/Ar...wvyFCg&usg=AFQjCNEASa533yz6Cyql6FEYbJzsnm6dzg

http://www.badgermetal.com/extras/extras.htm

 

[WillCole]

unclesyd - Thank you for the additional information. The badger metal link requires a log in to view.

 

[unclesyd]

I would imagine that this is just want you to register. A Lot of
sites are requiring you to register to keep the site strictly
business.
The registration is required for the the other information on the page.
On the linked page there is a "Extra Resources and Information Menu" that carries most of the information about the process.

 

[cloa]

Not all of badger is restricted (except for a few broken links I had no problem) but the restricted part requires joining NADCA ($75 for individual).