SS cracking - presence of high amounts of chromium and oxygen in the cracked regions

[AnsAvi]

Hi,

This is a question I have had for a while. I look at cracked 316L SS parts fairly frequently. Sometimes I notice that the cracked regions show elevated amounts of Oxygen and Chromium when conducting EDS analysis. This is when the analysis is done on metallurgical polished mounts.

Just recently, I looked at a 316L part that has all the makings of Chloride SCC (branched cracks, presence of chlorine on the crack surface etc). However I noticed that once again I was noticing elevated levels of chromium (~26%) and oxygen (~40%) in some regions on the crack. I could think of the following possibilities:

1. I was looking at Chrome oxide particles. However, my understanding is that Chrome oxide only appears as a protective film that is very thin (~nm) and so an EDS should not be able to pick it up. Is formation of Chrome oxide possible as an inclusion as well?
2. Perhaps I am looking at a combination of rust (Iron Oxide)trapped in the crack and Chrome carbides, in which case, the steel might have been sensitized. But the bulk material passes testing for Intergranular corrosion.
3. Perhaps I am looking at precipitates of sigma phase in the bulk material. Is that likely on 316L grade?

Can someone help me figure out what might be going on?

Thanks

 

[EdStainless]

Sigma is unlikely unless these parts have extended elevated temperature exposure.
The high Cr in the cracks could be because of low Fe, the Fe will be selectively leached first in corrosion and this would leave a strongly Cr enriched surface.
Are you sure that the grain boundaries are clean of carbides? Look more carefully there.

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P.E. Metallurgy, Plymouth Tube

 

[AnsAvi]

Hello EdStainless,

I say the material appears free of carbides because it passes ASTM A262 practice A. Is there a better tes

 

[EdStainless]

Have you looked at them in the SEM?
Perhaps probing some of the GB phases would help you out.

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P.E. Metallurgy, Plymouth Tube