SS passivation

[P69]

I've been making some of my own parts from stainless steel and figured that it's about time that I figured out how to passivate the parts after fabrication.

Here is what I understand. After machining/welding the parts, they have to be degreased, cleaned, then passivated to remove free iron from surfaces; otherwise they will "rust", although that "rust" is not an immediate threat to the integrity of the metal.

Nitric acid or citric acid is what is used, with citric being less toxic than nitric acid.

With citric acid, can  I effectively passivate using food grade acid  (5 lbs bag of 100% pure food grade citric acid on Amazon) for a food supply store (in correct concentration  of course) or do i need some kind of special formulation of citric acid and other mystery/proprietary ingredients (i.e. Citrisurf)?

Thanks

[Della and Dave]

You can do it with critic acid, but with food grade, you are going to have a harder time with controlling the concentration.  If it is relatively pure, 4-10% by weight is what ASTM A967 recommends with an immersion time of 20 minutes at 20-40 deg C, before neutralizing with water.  The part has to be cleaned and degreased first.   If you can't immerse the whole thing, just keep it wet and keep wiping it with fresh acid periodically and/or wrap it with a soaked towel.  You can get reactant or reagent grade at Carolina biological.  If you don't care if you get a few cosmetic rust stains, you can probably use bottled lemon juice, it just isn't as well controlled either, so time etc may not work the same.  It will naturally passivated, it just results in rust stains that are ugly. 

I agree with you on playing with nitric acid in it's concentrated form, it's really dangerous.  Citric may actually work better for passivating 316 anyway.  Getting nitric with shipping issues is more difficult as well.  Remember add acid to water, NOT the other way around.  (Drop acid, not water:-) I may have spent too much time in chem labs...

[P69]

Thanks Della/Dave

i had a hunch that Citrisurf was a bit much for a $50 gallon of 15% citric acid (according to their MSDS).

So if I want a half gallon of solution, I'd mix between 80 grams (4%) and 180 grams (10%) with the half gallon to get the right concentration, right? I don't think I'll need a half gallon because my parts are small for now, but that was easier on my mind to figure out.

I read A967 specs and it seems simple. Is this as simple as it seems? Clean it, Mix it, heat it, soak it, neutralize it, done!


Do you have a recommendation for cleaning/degreasing?

[Della and Dave]

It really is that simple.  The mix looks right to me, but I usually work in metric and use one liter is one kilogram to make the math easy.  Use a two liter soda bottle and add 80-200 grams of pure citric acid.  Or 6.6 times that if you use the cheap stuff.  I wonder what the other 85% is in the citrasurf,  might have an effect, but I don't know. 

I have had good luck with solvent grade denatured alcohol and I usually have some of it around, but mix with elbow grease. If you use isopropyl,look at the label, a lot of what you buy is heavily diluted.  Good Russian Vodka or Everclear works too.  There are probably other solvents like acetone and MEK that would work as well. I have made errors with not getting high quality materials though, so now I read the assay label carefully. Low quality solvents flash off and leave what ever was dissolved in them behind.  I like the alcohol, except for the flash point because MEK builds up in your liver and Acetone can give you Dain Bramage

If you don't want to heat the citric acid, add more time.  It isn't perfect, but it gets the job done. You can mix the citric acid with hot water. keep the alcohol away from open flame and don't use your wife's heirloom bowl to heat it.