-
Welcome back Guest! Did you know you can mentor other members here at H-M? If not, please check out our Relaunch of Hobby Machinist Mentoring Program!
Pickling Stainless Steel
- Thread starter dbb-the-bruce
- Start date
- Joined
- Feb 28, 2019
- Messages
- 499
I think I'm headed this way....
I'll google it but if you don't mind, what is your source? what kind of concentration do you use? and what sort of work is this applied to?
The kind of stuff I'm working with is small and somewhat delicate. I'm not welding or brazing (for now) it's just silver solder with a small (most would consider it tiny!) Oxy-acetylene torch.
I really want to limit any abrasive / subtractive clean up to needle files for errant chunks of solder and fine sand paper for stubborn scale/carbonization or other staining due to temp. This is particularly problematic im tight corners and with fine details.
I'm not really concerned with passivation other than appearance. My understanding of the reading I've already done is that happens more or less automatically to clean SS as long as it's exposed to oxygen. (curious if flooding with Ox from my torch tank would speed this up - but see no reason to worry about it or need to "force" it).
I also understand that heat from welding/brazing/soldering can/will modify the chemical composition of the surface (reduce chromium) and affect passivization.
I'm not concerned about medical/food interactions - other than these objects will be handled a lot. I want the appearance to be lifetime durable and any oxidation etc. to be very limited.
-D
- Joined
- Feb 28, 2019
- Messages
- 499
This article very distinctly describes passivation vs pickling.
Clearly, passivation works best on clean "bare" metal:
And that's the part I need the most help with. The article above talks about using nitric-acid. So back to the beginning...
1. Reduce as much of the need to clean up thermal oxides by not producing them to begin with. Better solder technique, (different flux?)
2. Find the least toxic environmentally safe chemical way to remove any oxides produced that can't easily be removed mechanically.
Clearly, passivation works best on clean "bare" metal:
And that's the part I need the most help with. The article above talks about using nitric-acid. So back to the beginning...
1. Reduce as much of the need to clean up thermal oxides by not producing them to begin with. Better solder technique, (different flux?)
2. Find the least toxic environmentally safe chemical way to remove any oxides produced that can't easily be removed mechanically.
- Joined
- Apr 23, 2018
- Messages
- 6,533
Excellent choice. Substitute the hazardous stuff for the safe stuff. This does the job well enough.
I reviewed the SDS, that is a hydrofluoric acid product. Don't touch it, don't bring it into your home. Not worth it. I'd rather cook nitroglycerine in the kitchen than bring HF acid into my shop. None of us have any need whatsoever to use hydrofluoric acid, there are safer substitutes. That stuff is as scary as organophosphate nerve agent in terms of dermal route toxicity, and inhalation always trumps dermal.
Ah, this post makes your intentions clear. I think mechanical cleaning (scotchbrite) followed by a thorough cleaning with acetone will be fine for a hard solder join. You can brush some 20% HCL on as a prep if you need better adhesion. You can also dip treat in a warm weak solution acid (HCL works, nitric is better for metals, sulfuric may leave precipitate impurities) and wipe before soldering, that is the minimal strength/minimal quantity approach. I do think acid passivating will strengthen your hard solder joint on stainless, but it might not be all that necessary in the big picture.
- Joined
- Mar 25, 2013
- Messages
- 4,617
Electrochemical weld cleaning is fast and effective. It uses phosphoric acid which is not very toxic. I made my own setup using carbon fiber brushes. There are also expensive commercial kits depending on your production level.
Not my video
Not my video
- Joined
- Feb 28, 2019
- Messages
- 499
Bingo!
Now we are getting somewhere. I read about this technique but it looked like it required specific equipment beyond what I was willing to invest (at this point). Your response prompted me to find this:
DIY Stainless Steel Weld Cleaning
This looks like it's very much within bounds of what I want to try/use. The products mentioned need to be used with care but it seems manageable. "strong phosphoric acid 30-60%" thoughts on having that stuff kicking around?
@rwm Would love to hear more about your custom setup.
- Joined
- Mar 21, 2018
- Messages
- 1,597
Hydrofluoric acid is a fairly weak acid that has an affinity to only two things: glass and human skin. If you handle HFl, take super precautions and load up your PPE to the max, or it will EAT you and it will hurt the entire time it is eating you.
- Joined
- Dec 18, 2019
- Messages
- 6,447
https://emergency.cdc.gov/agent/hydrofluoricacid/basics/facts.asp A few selections, which I may have got out of order.
- Hydrogen fluoride gas, even at low levels, can irritate the eyes, nose, and respiratory tract. Breathing in hydrogen fluoride at high levels or in combination with skin contact can cause death from an irregular heartbeat or from fluid buildup in the lungs.
- Even small splashes of high-concentration hydrogen fluoride products on the skin can be fatal. Skin contact with hydrogen fluoride may not cause immediate pain or visible skin damage(signs of exposure).
- Often, patients exposed to low concentrations of hydrogen fluoride on the skin do not show effects or experience pain immediately. And, severe pain at the exposure site may be the only symptom for several hours. Visible damage may not appear until 12 to 24 hours after the exposure.
- Fingertip injuries from hydrogen fluoride may result in persistent pain, bone loss, and injury to the nail bed.
- Depending on the concentration of the chemical and the length of time of exposure, skin contact with hydrogen fluoride may cause severe pain at the point of contact; a rash; and deep, slow-healing burns. Severe pain can occur even if no burns can be seen.
- Joined
- Feb 28, 2019
- Messages
- 499
Read the full thread - perhaps it doesn't stand out, but I have no intention of working with hydrofluoric acid. The first thing I found contains it and the warning bells went off when I read the SDS for it. I'm also pretty hesitant to bring the other stuff into my house.
Will start with citric acid and see how that goes. The electro chemical process also looks promising with at least one company providing safer / less hazardous cleaning products.
Now that It's clear I can make my own "tig brush" with 30V 5amp power supply, that's probably the path I'll take if I need to.
Will start with citric acid and see how that goes. The electro chemical process also looks promising with at least one company providing safer / less hazardous cleaning products.
Now that It's clear I can make my own "tig brush" with 30V 5amp power supply, that's probably the path I'll take if I need to.