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Dark Areas of Stainless Steel Welds
2

Dark Areas of Stainless Steel Welds

RE: Dark Areas of Stainless Steel Welds

The dark color is heat tint (oxidation) from poor gas shielding. I have seen it before on fillet and groove welds.

RE: Dark Areas of Stainless Steel Welds

(OP)
That's what I was afraid of.

Short of removing the weld, is there any way to correct the issue?


RE: Dark Areas of Stainless Steel Welds

Try buffing the welds with new Scotch brite pads. Don't use used ones.

RE: Dark Areas of Stainless Steel Welds

(OP)
some said it could be Silica?

RE: Dark Areas of Stainless Steel Welds

I don't believe it would be silicon.

RE: Dark Areas of Stainless Steel Welds

Silica is white... ferrous oxide is black...

Dik

RE: Dark Areas of Stainless Steel Welds

Buy some RedOne pickling paste. You will need to rinse and neutralize after use but it will remove oxides.
Grinding the surface will only embed this material into the weld metal, and buffing will not remove the Cr depleted metal under these oxides.
Since these are in the weld the could be pools of oxide that formed on the weld puddle and then got trapped.

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P.E. Metallurgy, Plymouth Tube

RE: Dark Areas of Stainless Steel Welds

A mild nitric acid solution may also help.

Dik

RE: Dark Areas of Stainless Steel Welds

(OP)
The welds shown in the original picture were passivated.

RE: Dark Areas of Stainless Steel Welds

Passivation is a cleaning operation, it will not touch oxides.
Nitric acid is fine after the material has been cleaned, but you need HF to strip the oxides.

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P.E. Metallurgy, Plymouth Tube

RE: Dark Areas of Stainless Steel Welds

Be very careful when handling HF. Does the environment of the welds on the OD surface require optimum corrosion protection or is this for appearance?

RE: Dark Areas of Stainless Steel Welds

That has a strange appearance, almost as though somebody used Solar Flux, then did not get it all off.
B.E.

You are judged not by what you know, but by what you can do.

RE: Dark Areas of Stainless Steel Welds

Ed and metengr... thanks for the added info.

Dik

RE: Dark Areas of Stainless Steel Welds

The silicon used as a deoxidizer in many filler metals tend to leave deposits of silicon oxide on the surface of welds, but they are glass-like deposits that "shatter" when scraped or tapped with a welder's slag pick. I would tap a couple of the blackened areas to see if the deposits "shatter" in which case, it is likely the silicon oxides I've mentioned.

I suspect simple oxidation from insufficient shielding would be more consistent over the entire surface of the weld. Stainless filler metals can contain increased amounts of silicon (ER308Si, ER316Si for example)) that can produce more silicon oxide on the surface of the weld if a CO2 base shielding gas is used. The silicon acts as a deoxidizer, but it also changes the fluidity of the weld pool and helps reduce the amount of undercut along the toes of the weld.

The attached photograph depicts some porosity, but it also depicts some of the silicon islands I mentioned. Granted, the base metal is carbon steel, but the silicon islands can appear on stainless welds as well. Once the islands are chipped and removed, there is a slight difference in color under the silicon oxide islands.

Best regards - Al

RE: Dark Areas of Stainless Steel Welds

I concur with GTAW 100%. I have witnessed this as a common occurrence with GMAW-P on carbon, low alloy, lean duplex, and stainless steels with the Si (or Si+Mn for carbon/LAS) additions from multiple manufacturers (Lincoln, ESAB, Sandvik). My only concern is that the "islands" which form do on occasion show up as inclusions along the weld toe/tie ins when not removed via interpass cleaning. The Si additions help considerably with puddle fluidity and tie ins.

RE: Dark Areas of Stainless Steel Welds

Don't mess with HF. It is dangerous.

RE: Dark Areas of Stainless Steel Welds

Pickling paste containing HF is the only effective way to remove serious weld oxides.
The problem with grinding or blasting is that you are re-embedding the surface material and leaving the problem.
Nitric acid (or citric) will remove free Fe, but it will not break down oxides.

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P.E. Metallurgy, Plymouth Tube

RE: Dark Areas of Stainless Steel Welds

I've seen passivating done on stainless tanks made at a former employer, and it was not much more than a glorified car wash.
As previous posters have made clear, diagnose what it is before attempting treatment.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

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