MicroWeld Stainless Tubing

[Guest102023]

Ed, I figure you may know something about this:

I have a sample of duplex 2205SS HX tube that was made by welding and cold drawing. While there has been uniform corrosion, there has also been some preferential corrosion of the weld, which is darker, indicative of an etching effect. The base metal also has an faint etched appearance.

I have questions about what welding process was used and what subsequent heat treatment was done (if any; the online literature is somewhat vague, except to say the long seam weld is autogenous).

BTW this is an application where 316 was previously resistant.

 

[Guest102023]

I've since discovered the tubes are bright solution annealed after welding.

 

[EdStainless]

What was the service environment?

They would have been in-line bright annealed probably.
So no hydrogen on the ID, Ar only. And the distinct risk of light ID surface oxidation.

The welds are autogenous, probably TIG. There may (or may not) have been N in the weld gas.
If not then the welds are low in N, and low in pitting resistance, and may have too much ferrite.

There is also the issue of possible microstructure issues in the welds.
Besides phase imbalance there is a risk of detrimental secondary phases (mostly intermetallics).
There is also a good chance of this material being underannealed and there simply being a lot of residual segregation in the welds.

Even if none of these is the case you may have either intergranular corrosion of a preferential attack of one phase or the other (or the attack of the interface between phases). In any case you would have more pronounced attack in the welds just because of the coarser grain size.

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Plymouth Tube

 

[Guest102023]

Tube: ¾"OD x 0.047" (actual), weld width = 0.06" (eyeball)

Product: Mostly organics (biodiesel process), but with a small amount of sulfuric acid. Copious amounts of sludgy deposits covering 100% of the tube-side surfaces.

Sulfuric acid is reducing and aggressive to ferritic SSs; is it sensible to think it might preferentially attack ferrite phase in a duplex SS?

The manufacturer's literature recognizes the issue of the higher ferrite of the autogenous weld but focuses more on the dimensional tolerances and good surface finish of this product.