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.

 

[btrueblood]

If it were me, I'd go to abrasives - starting with scrungie pads from the supermarket and a strong detergent, and ramping up to scotchbrite abrasives and possibly scouring powder. Hard to believe you can't clean smooth stainless without resorting to those, but maybe the weld shop and/or rolling mill gave you some really grubby material, where the lubricants used in rolling got baked onto the surface.

 

[runcyclexcski]

Thank you btrueblood. The piece has M4 tapped holes and corners that cannot be easily reached by mechanical scrubbing. I also have read that using "subpar-steel" brushes can cause even more damage. Not sure if this is true.

 

[dik]

From my project notes:

STAINLESS STEEL
-IN THE ABSENCE OF FACTORY PASSIVATION, ALL STAINLESS STEEL WELDING SHALL BE PASSIVATED BY:
CLEANED WITH A MILD ABRASIVE SIMILAR TO ‘SCOTCHBRITE’ PADS;
...WASHED WITH A ‘CAUSTIC DEGREASER THAT DOESN’T HAVE CORROSION INHIBITORS; AND
IMMERSION IN A [20% SOLUTION (AISI S304L) | 40% SOLUTION (AISI S316)] OF NITRIC ACID. FOR 4 HOURS. ATTACHMENTS SHALL BE FLUSHED CLEAN AND DRIED.
IF HEAT TINT OR SCALE IS A CONCERN, THE IMMERSION BATH SHOULD INCLUDE HYDROFLUORIC ACID.
...TRACES OF IRON THAT MIGHT CAUSE RUST APPEARANCE CAN BE DETERMINED BY USING WHITE ‘HEAD AND SHOULDERS’ SHAMPOO.
-INSTALLATION OF STAINLESS STEEL FABRICATIONS SHALL USE NON-FERRIC TOOLS.
-WELD MATERIAL CHARACTERISTICS SHALL MATCH THE PROPERTIES OF THE BASE MATERIALS.


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

-Dik

 

[runcyclexcski]

Many thanks, Dik, this was extremely helpful! And the 1976 paper I found is fully consistent with your notes. Sounds like nitric acid it is.

I understand that your notes do not explicitly mention it, but

-- was the acid purchased as 20%, or did you dilute yourself? Do you happen to recall the supplier? "Fuming red" HNO3 is 70% at Sigma-Aldrich-Merck. A more common lab reagent, in my experience, is 30% (not as crazy).
-- when they write "dip", do they generally imply agitation/mixing, or is static dipping OK?
-- how does your shop dispose of HNO3 waste? I could get a molar equivalent of NaOH to neutralize, but it could be insufficient.

I could get a small amount of HNO3 and try it on a small area of the piece, to see if it's worth paying someone to scale it up. The tray is 5Gal. This is a lot of acid, even cost-wise, not mentioning the health, safety and disposal aspects.

 

[dik]

It was from accumulated stuff and information from Eng-tips... the shampoo trick was mentioned on the site, but I had to check it out to see if it works... really well (I was skeptical; even ferrous tools, leave a noticeable iron trace.). The percentages of acid were ordered; it was not hand mixed.

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

-Dik

 

[runcyclexcski]

>>>The percentages of acid were ordered; it was not hand mixed.

Was it difficult to dipose of? I do not run a business yet, and ordering HNO3 and HF is prohibited in the EU by private individuals (in the USA the rules are a bit more relaxed, from what I understand). Thus, I am looking for a business that can do the HNO3/HF dipping for me.

I also found that the manufacturer of the 304 steel that the welding shop ordered for my part specified in their instructions that pickling and passivation were required after welding, and gave specific conditions for the treatments -- which were along the same lines as your notes. I attached the screen shot. I did not expect, as the end-user of the product, too have to look these things up. Lesson learned (ask for these in advance, or inquire explicitly what additional finishing is required for the product to perform by the manufacturer of the steel).

https://www.aperam.com/product/304-1-4301/

 

[dik]

Disposal was by others.

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

-Dik

 

[TugboatEng]

Sounds like the logic used to reduce carbon footprint

Acids are generally easy to neutralize. Most of the troubles stems from acids turning into fertilizer as they are neutralized. Are you coastal or near a river where algae blooms may be a problem?

 

[dik]

I guess every little bit helps, Tug...

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

-Dik

 

[EdStainless]

Let's back up a bit.
Passivation in SS has two different meanings.
In terms of corrosion resistance, the material will naturally form a passive Cr oxide film on exposure to air.
If the material is very oily or has free Fe imbedded in the surface then you will not get uniform corrosion resistance.
The second meaning is to clean the surface, specifically removing free Fe from the surface.
The only reason to need to use nitric/HF or sulfuric/HF is to remove heat tint and other forms of detrimental oxidation.
If there is serious discoloration of welds you can get pickling paste to do localized cleaning (Red One).
It doesn't look needed in your case.
Scrub with a mild abrasive ScotchBrite pad and an alkaline degreaser.
Then rinse.
A final passivation can be in room temp nitric acid, or warm citric acid, or a few other things.
You should have required that the part be cleaned before welding.
You should be able to wipe with a white wipe and alcohol or acetone and see nearly no discoloration.
A slight gray is no real issue.

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

 

[Prometheus21]

Many good points from the others above.

In my company we usually clean parts that are welded and/or brazed by doing the following:

The part is brushed and pre-cleaned.

1. Submerge the unit in a highly alkaline concentrated solution in water (e.g. Ardrox 185) - this removes heavy rust and scale as well as oil and grease. This removes most if not all of the contaminants you may find.

2. The unit is then rinsed with water before being submerged in 62% nitric acid bath for a very limited time to ensure cleanliness, followed by some further acid baths containing sulfuric acid and hydrogen peroxide combinations. (Like EdStainless states; to remove heat tint and other forms of detrimental oxidation)

Now this is typically used when cleaning for oxygen service where there is a limit of 10-100 mg/m^2 maximum allowable quantities of foreign matter.

We have also had success using ultrasonic baths to remove scaling, oil, grease and other residue by using multiple cleaning steps.

1. Ultrasonic cleaning with an acidic aqueous cleaner.
2. Rinse.
3. Ultrasonic cleaning with a heavy alkaline aqueous cleaner.
4. Rinse.

Then again there are many, many ways to clean a part. And it all depends on how clean it needs to be.

 

[runcyclexcski]

Thank you for your response. I have already done the 10% NaOH myself, this had no effect (actually, the opposite -- it made the part shed more grey smutt). HNO3 you mentioned seems consistent with Dik's notes and the instructions from the manufacturer.

In the mean time, I have received a quotation from a local shop to enamel- coat of the entire inner surface of the unit for about $200 + shipping. I am tempted to take them on the offer; although this would defeat the purpose of the stainless steel, this saga might be over. Another shop has offerred to PTFE-coat the whole piece, which is fine with me, too (this option might be more costly).

Next time I will explicitly ask for professional treatment and passivation from the welding shop. They may not be able to do it, but they at least should be familiar that this might be necessary, and needs to be outsourced. The welding shop that made the part in the OP did not know about cleaning, pickling and passivation, and never responded to my (polite) inquiries on how to bring the part to the steel manufacturer's specs. I should have done my homework at asking the right questions before going with them, but I did not know enough at the time.

 

[runcyclexcski]

>>>You should have required that the part be cleaned before welding.
You should be able to wipe with a white wipe and alcohol or acetone and see nearly no discoloration.
A slight gray is no real issue.

EdStainless: I did not expect to have to give specific instructions. I naiively assumed that 304 steel used in an HVAC application was enough information. Now I am learning that there are several specs within 304, the treatments etc.

After extensive 10% NaOH treatment and extensive rinsing, the grey discoloration with a white wipe and ethanol got worse, not better. Other stainless steel finishing shops do not offer further insight, and I am afraid that I will get back the piece in the same state as it started with. I do not know what test to ask from them; "no discoloration of a kimwipe" does not sound like a standardized test. So I think I am going for PTFE or enamel coating, just to be done with it. This has gone on for nearly 2 months, and the next step of the project cannot go ahead. I will be more careful next time.

 

[Prometheus21]

"Thank you for your response. I have already done the 10% NaOH myself, this had no effect (actually, the opposite -- it made the part shed more grey smutt). HNO3 you mentioned seems consistent with Dik's notes and the instructions from the manufacturer." - Interesting. We had a similar problem back in the day so we upped the concentration to 50% and introduced a pneumatic raise&lower platform, making the solution "scrub" the unit, improving the speed and effectiveness of the cleaning process. It works very well and we have yet to discover any discoloration as of today.

Alternatively we also used a heated solution consisting of sulfuric acid, hydrogen peroxide and methylated spirit as another way to remove remaining residues after the (weaker) alkaline solution.

But like you say, lesson learned for next time.

 

[runcyclexcski]

Prometheus21 -- wow, hydrogen peroxide+H2SO4 (piranha) and 50% HNO3 are way beyond any DIY I can do at home. I am familiar with piranha from my biochemistry/biophysics work, any volume above 1 cup is bad news. Ditto on any ultrasonic/vibration devices. I did not expect SS 304 to be so tricky, and I did not expect the shop to leave such (potential) treatments to the end consumer of the product. So I will look for a more professional/informed company next time.

 

[TugboatEng]

There are commercially available products that don't require much more than gloves and goggles. See Wonder Gel as an example. Pickling paste will get you more options. This follows a degreasing of course. You can degrease chemically or consider baking in an oven at 500°F.

 

[EdStainless]

If you have serious heat tint on SS cleaning it requires acid mixtures with HF in them, or an electrochemical process.
We used to passivate in 40%HNO3 at about 85F.
The easiest is to buy CitriSurf.
They say that it would at RT, but I never could get good results below about 120F.
It always pays to tell people up front how clean you expect things to be.
Using an alkaline may help remove organics, but it does nothing for the surface of the SS.

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

 

[Prometheus21]

Fair enough; we use immersion baths of about 200L of that solution, scary stuff but works like a charm.

I did not expect SS 304 to be so tricky, and I did not expect the shop to leave such (potential) treatments to the end consumer of the product. So I will look for a more professional/informed company next time." - TBH the few times we have outsourced products like this we never had to specify such treatments - it was an automatic part of the offer they gave us to do the job. But like EdStainless says; it pays to tell people up front how clean you expect things to be, or in the very least it can't hurt.

 

[runcyclexcski]

>>> it was an automatic part of the offer they gave us to do the job.

Exactly. When I buy, say, a custom-made sink in 304 to use in my kitchen, I expect it to work as is more or less out of the box, and not to have to dip it in a 200L tank of HF+HNO3. I did not ask the shop to pick the cost of the additional finish, I merely asked them for a recommendation on where the finishing can be done. I got no response. A welding shop that has never dealt with finishing, maybe, is not very professional, after all.

All SS pickling companies quoted me around $500 for the surface finish of the piece (which would be in addition to $1000 I paid for the piece to be welded), but never specified what exactly they would do. They did not seem keen to follow the steel manufacturer's recommended protocol that I provided and said they will do "whatever they normally do". An enamel company asked $300 (that went up from $200 when I asked for a food-safe enamel). They even sent me a hand-signed certificate of food safety, with the manufacturer's contacts. So, this time I will do the enamel coating and move on. Next time I deal with 304 steel I will be more careful.