304 vs 201 Brushed Finish

[mzacc]

I have seen some posts about a concern for brushed finishes on 201 stainless steel. Does anybody have experience with using a brushed finish with 201 vs 304 stainless steel. Our Salt Fog testing shows that 201 is not as good.

 

[EdStainless]

Were both samples passivated?
Do they have the same S content?
At neutral pH they should behave exactly the same, they have the same Cr content.
In more aggressive environments 304 will slightly out perform 201.
It probably relates to surface finish, cleanliness,inclusions, and passivation.

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[mzacc]

The materials had similar Sulfur contents. The were processes with the same mill process which includes a passivation step at the end. On some panels the 201 would develop a line of pits along the brush pattern that was very persistent. On the 304 if I cleaned the panels with citric acid and let them sit for 48 hours before placing them back in the salt spray would do much better than the 201 stainless steel using the same treatment. Typically the 201 would have rust showing in the next 24 hours whereas the 304 would go much longer. For environments which are severe we use this as a cleaning method on cosmetic surfaces to prevent further rusting if rust starts on the panel.

 

[EdStainless]

What if you nitric acid passivate? I still suspect inclusions or surface Fe contamination on the 201. The 201 does not repassivate as well as 304, so once corrosion starts you are in more trouble.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

"mill process which includes a passivation step at the end. On some panels the 201 would develop a line of pits along the brush pattern that was very persistent."

This behavior suggests oxide inclusions [not removed by nitric acid passivation] along the rolling direction, perhaps due to inadequate pickling prior to rolling. Try brushing pickling paste (alumina/nitric/HF) along the row of defects, rinsing & re-passivating.

But, even if successful, the material will have a pitted surface which could cause future problems. Suggest
1) buying from a supplier with better quality,
2) using a finer grained surface (the smoother the better w.r.t. corrosion resistance), e.g., a #4 finish before brushing, and
3) requiring salt spray testing for qualification of material.