Removing smut and passivating a 304 welded part 3

[runcyclexcski]

Hi all,

I've been searching and posting all over the web about this issue. I apologize for the long question in advance.

I had a 304 tray welded at a welding shop specializing in stainless and laser-cutting. The part is a box about 2' x 1' x 1' with 1.2 mm-thick steel walls (~0.05") and a 1/4-thick flange for a gasket. The intended use is air humidification for indoor HVAC.

I have the certificates for the cold-rolled steel. The supplier is a large Belgian/French stainless company, the batch is from Dec 2023. For humification, the piece is supposed to be resistant to "regular" hardness tap water at mild temperatures (<100F), with occasional descale in tap water with 5% vinegar or 4% citric acid. Thus, I did not think at the time that 316 was necessary, and I've never had custom-made parts in 304 or 316 before; I always purchased standard off-the shelf parts.

The tray was welded directly from the raw SS sheets, without any additional processing. It arrived visually clean, with no fingerprints, and I gave it a rinse with water and "Fairy" detergent (similar to Dawn in the US). However, when I wiped it dry with lint-free scientific grade paper (kimwipes), I noticed a grey residue left on the paper. It did not seem a good idea to have this unknown "soot" in the air for breathing, so I spent the next few weeks trying to get rid of it.

A welding forum suggested that this was grease left over from machinning and welding. So I did a 5% vinegar rinse for 2 hrs, no effect. Then I did a 5% NaOH dip to degrease, followed by a 4% citric acid dip for 2 hrs at 170F (at the time I thought it was a passivation issue). This did not help either; in fact, it appeared to have made the residue worse.

It was then suggested to me at a metallurgy forum that I was dealing with smut left over from the steel manufacturing. This 1976 paper appears to describe what I am dealing with:

https://www.tandfonline.com/doi/epdf/10.1080/00218467608075082

The paper above indicates that the only efficient methods to remove smut from 304 are concentrated HNO3, or chromic acid, or HF. The paper also mentioned electropolishing+HNO3. None of these are DIY-friendly methods, and I do not have access to a chemistry lab at the moment.

Should I give up on the piece and re-oder it in another material that is not as hard to clean and make corrosion-resistant? Or, I could try and outsource the desmutting/passivation. If I do, is there a standard protocol (USA or, preferentially, EU) that I should specifically request from the facility? There are shops within 1 hr who offer SS passivation, but HNO3 or electropolishing are not mentioned. I beleive they mostly do NaOH followed by citric acid -- and this has not helped, as I already tried.

Or, should I have the piece coated all over with PTFE or enamel? These options sound expensive, at least with a one-off piece like mine.

If I have to re-order this from scratch, what spec should I request -- HNO3 electropolishing, followed by an IPA smudge test of the finial product? Or use a differnet material, like a Ti Gr5?

Again, I apologize for the long question. This was driving me a bit insane for the last few weeks.

The picture shows the tray after the NaOH + citric acid treatment.

 

[BiPolarMoment]

...TRACES OF IRON THAT MIGHT CAUSE RUST APPEARANCE CAN BE DETERMINED BY USING WHITE ‘HEAD AND SHOULDERS’ SHAMPOO.

This is amazing; before I go buy a bottle to experiment with, what is the visible reaction that I should expect in such a case?

 

[EdStainless]

It will form blue spots.
The Fe forms a complex with the active ingredient.
 You may need to mist it with DI water to keep it damp long enough to react.
There are two plants making H&S.
One has some steel piping in it, and they dye it blue to get a consistent color.
The other plant is all SS and they just leave it white.
I actually used to do this in our mill.
I have also done it at customers with food grade equipment.

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P.E. Metallurgy, consulting work welcomed

 

[runcyclexcski]

Ed -- i.e. if white H&S turns SS blue, it means SS not passivated properly? You got me really curious, so I am getting a bottle next time I go shopping.

I suspect this might be the ingredient:

https://en.wikipedia.org/wiki/Zinc_pyrithione

The wiki lists it as anti-dandruff agent, and H&S conditioner lists it as an ingredient. Zn, Fe, Cu ions will form complexes with it. The Wikipedia will forgive me for stealing their image. I bet it's dirt cheap.

 

[EdStainless]

Yes, that is how it works.
I have often seen cases where there are blue streaks all over the surface.
Just a thin layer and keep it damp.
30-60 min is more than enough time.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[runcyclexcski]

Not much information on the Iron III complex in the literature, but this 2023 paper describes it as a "black solid". Could have a blue color when diluted. Good absorbance at 318 nm, 515 nm, and 570 nm.

https://pubs.acs.org/doi/10.1021/acsomega.3c00412

 

[dik]

I did the same thing... and it works.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik