color of nitrided stainless steel surface

[ras85]

what is the color of nitrided stainless steel surface? it depends on FeN or CrN formation on the surface?

 

[EdStainless]

When I have ion nitrided 300 stainless it came out a dull grey, not too dark but very flat (dull).

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Plymouth Tube

 

[ras85]

thanks.

did corrosion resistance of nitrided 300 stainless decrease?
what is temperature of nitriding?

 

[swall]

Nitriding of stainless destroys the passivity, so, yes, the corrosion resistance decreases. Typical nitriding temperature is around 1000F.

 

[EdStainless]

The reason that nitriding at standard temps reduces corrosion resistance is that some of the Cr is tied up in nitrides and therefore reduces the Cr in the alloy available for corrosion resistance. You have to watch for intergranular attack in these conditions.
These materials even after nitriding will repassivate just fine.
There are lower temperature nitrogen implantation methods that just stuff nitrogen into the structure below temperatures where nitrides will form. This would not decrease the corrosion resistance.

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Plymouth Tube

 

[swall]

Ed, my comments come from my years at Bendix Aerospace. It was our practice to NOT passivate nitrided stainless (mainly 410)parts.We would flash chrome to restore the passivity.

 

[EdStainless]

410 hardly has enough Cr to be stainless in the first place, after some of it being tied up in nitrides your surface Cr is prob down around 8% or so. Definitely not stainless any more.

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Plymouth Tube

 

[swall]

Yes, that is a good hypothesis and probably why we didn't passivate nitrided 410 (I never thought to ask the question at the time).