welding stainless steel 1

[jbm27350]

We have welded type 316 stainless steel pipe (18 gage) using a mig welder with 316 wire. We are now getting corrosion at the weld joints. How do we prevent this?. Can we use a stainless wire brush to remove the rust? If not, how to remove the rust and prevent it from returning. Will this rust cause a problem?

 

[JAE]

Carbide Precipitation perhaps.....the welding may have taken the stainless steel - austenitic (316) through a temperature range of 800 to 1400 deg F and this sucks out the chromium from the adjacent steel into the weld....reducing the corrosion resistance of the adjoining steel...

The Lincoln Procedures book suggests a heat treatment solution where you take your assembly through 1850 deg - 2100 deg F and then rapidly cool it through the 1400 to 800 deg range. They warn that this may cause distortion or warping though.

 

[metengr]

Yes, the rust is caused by surface oxidation in areas of the stainless base metal that have experienced local sensitization from welding. Sensitization of the base metal heat affected zone is a phenomenon that occurs during welding, as described above by JAE. Within a certain temperature range, the chromium that is dissolved in the stainless base metal forms carbides. The formation of carbides robs the stainless base metal of the minimum amount of chromium necessary to assure corrosion protection. Most likely, during welding, the interpass temperature exceeded the recommended value of 350 deg F. In addition, you did not use a low carbon grade of stainless (AISI Type 316L), which would reduce the severity of sensitization during welding.

Unfortunately, you will experience local corrosion problems due to sensitization. Re-solution heat treatment is your only bet to restore corrosion resistance.

 

[EdStainless]

Unless this is very old pipe, or imported, the chance that it has high C is pretty slim.
There are two other reasons that you may be getting this local corrosion.
1. You didn't over alloy the weld filler. As you move into alloys with higher Mo content weld segrigation is a big issue. The low Mo areas of the weld will pit rapidly compared to the rest of the alloy.
2. These welds may have been "cleaned" wrong to start with. If any Fe was imbeded in the surface, or even if metal was just smeared it could lead to accelerated local corrosion. I have see wire burshed welds that looked shiney, but the oxide hadn't been removed, only buffed.
3. Of course, if you have higher C material sensitization is your most likely problem.

You need to take a new grinding wheel (not too coarse) and clean up a few places. If the pits are shallow you might try leaving them ground and see if they re-rust.
If the pitting is deep, then you don't have much choice other than cutting the welds out and re-doint the system.

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Corrosion never sleeps, but it can be managed.

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[btrueblood]

Ed,

Would you recommend a 308 or 309 alloy for welds on 316 material, or is there a high-chrome alloy with moly wire that he should use?

 

[EdStainless]

It all depends on your goals for the weld.
Since you are looking for corrosion resistance you need an alloy filler.
I don't know what the actual composition of 316L filler is these days. If it has less than 2.5% Mo then I would be tempted to use 317L filler.
You could use a high Ni-Cr-Mo filler, but you don't need it.

Are you getting any inside corrosion? I still wonder if these welds were 'cleaned' with steel tools.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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