Acids to passivate SS?

[And1129]

Hi all,

I'm thinking about how best to passivate my stainless steel equipment. I have a new SS mashtun arriving soon, and I have lots of other stainless equipment that it has been a few years since i've passivated.
I thoroughly clean and sanitize my fermenter and boil kettle after each use, but I am feeling that it's time for a deep clean and passivation.
For the new mash tun, i will of course do a TSP wash and i'll follow that by a PBW wash, but then i'm wandering what acid to use for passivation.
I know that traditional homebrewer advice is to use StarSan at 1oz/gallon concentration, and I could do that, but star san is rather expensive. I'd rather reserve the Star San for sanitization and use something else for passivation.

I DO have plenty of 85% Phosphoric acid, and concentrated sulfuric acid, nitric acid, hydrochloric acid, and a few other acids. It would actually be fairly convenient for me to use one of those instead of my StarSan.

I would rather run a passivation cycle for longer time and higher concentration of acids than heating solutions up to high temperatures. I regularly work with strong acids, so i'm comfortable dealing with them and making solutions at precise concentrations.

Can anyone offer advice or resources outlining which acid at which concentration and time is best for passivating 304 stainless? I might just use 1oz of 85% H3PO4 per gallon and let it sit overnight and then air dry?

Hoping I can find a good industrial SOP where the science makes sense.

Thanks. Cheers.

 

[day_trippr]

The top two choices for passivation are nitric and citric acid, with the latter being the most practical (and safe) choice for home use for most of us. I use a 5-6% solution by weight heated to ~180°F and kept there for a few hours before rinsing with RO water (for the low mineral content).

That said, if you actually have enough nitric acid and are comfortable using it, it is the preferred choice wrt industrial use. Must be a reason I do not know the SOP for that, though...

Cheers!

 

[shoengine]

https://www.besttechnologyinc.com/passivation-systems/what-is-passivation/

 

[day_trippr]

^Good link^
The nitric SOP looks more complex than the citric path...

Cheers!

 

[whattabrau]

There's a slide deck from NASA where they compare citric and nitric acids: Citric Acid Passivation of Stainless Steel - NASA Technical Reports Server (NTRS)

One takeaway is that passivating incorrectly may be worse than not doing it at all.

That said, I have only gotten rust and pitting on the *outside* of homebrew hardware (e.g. old scratched + dented kegs after half a year in the keezer), and it's easier to show them a bit of sandpaper and/or an acid wipe than try to passivate them. (my opinion, in all its worth)

 

[day_trippr]

Yeah, I can imagine trying to find a way to immerse one of my 20g kettles would be a challenge - plus would need a crapton of acid

The biggest thing I've passivated to date is my 50' 1/2" IC, next was a 22" x 6" SS hop spider...

Cheers!

 

[Silver_Is_Money]

When I was involved in chemical synthesis, we passivated 2,000 to 5,000 gallon reaction and blending vessels with a combination of Phosphoric and Nitric Acid.

 

[whattabrau]

Yeah, I can imagine trying to find a way to immerse one of my 20g kettles would be a challenge - plus would need a crapton of acid

Plus, you'd need an even bigger kettle to submerge the entire thing to passivate the outside. Then you'd need a bigger kettle to passivate that one, ... At some point one might start considering plastic.

The biggest thing I've passivated to date is my 50' 1/2" IC, next was a 22" x 6" SS hop spider...

Cheers!

The biggest thing I passivated was my SS brewbucket. My current understanding is that soaking it in starsan does nothing, except maybe if you need to warranty it there's some advantage in saying that you followed their instructions in preparing it for use.

I did also passivate part of a kettle onto which I etched volume markings using the acid+electricity method. I soaked a kitchen paper in citric acid solution and slapped it on the etched area. Again, it probably did exactly nothing, but there's no sign of rust on the kettle, so at least it didn't significantly make it worse.

 

[pvtpublic]

Use wooden barrels and you'll never have to worry about it again!

 

[Bad Bubba]

I would not mess with Nitric acid at home. Citric acid will work just fine.

 

[sbro]

I use a 5-6% solution by weight heated to ~180°F

I have some dry food grade citric acid. Do I mix a ratio of 5 grams of this with 95 grams of water to get the right concentration to passivate stainless steel? Obviously I would scale that ratio up to get the volume needed.

 

[day_trippr]

Yup, you got it. Using metric weights makes it easy, just scale it to whatever volume you need...

Cheers!

 

[MaxStout]

I would not mess with Nitric acid at home. Citric acid will work just fine.

And it's cheap on Amazon.
https://www.amazon.com/Granular-Con...687048505&sprefix=citric+acid,aps,180&sr=8-12

 

[bruce_the_loon]

And it's cheap on Amazon.
https://www.amazon.com/Granular-Con...687048505&sprefix=citric+acid,aps,180&sr=8-12

I love the search engine manipulation going on in the product title. DIY Bath Bombs FTW