Help please identify stainless dulling

[Pinkimark]

Hey,

I've got hold of 2 stainless steel pots, but they have some discolouration inside, can anyone help work out what it is and if anything can be done about it?

I don't imagine it would effect the brewing?

Thanks

Mark

 

[day_trippr]

I'd guess beer stone, mostly. A green scrubby with a thin paste of Bar Keepers Friend will cut that down to bright metal, at which point you might want to give it a citric acid treatment...

Cheers!

 

[friarsmith]

Search HBT for "passivating" or "passivation" for a variety of methods. Citric acid as mentioned above, BKF, 1oz StarSan per gallon distilled water in a spray bottle, etc.

 

[Pinkimark]

Thanks guys, I'll give them a try. I've talked to the previous owner who thinks he left something acidic in their too long? Does that change anything?

 

[day_trippr]

Nope.

Cheers!

 

[RPh_Guy]

Citric acid as mentioned above, BKF, 1oz StarSan per gallon distilled water in a spray bottle, etc.

Only citric acid and nitric acid are proven to passivate. Oxalic acid (BKF) and phosphoric acid (Star San) do not.

 

[Pinkimark]

So an update on this, I've scrubbed with BKF which hasn't really done a lot, I've soaked in citric acid and again no change in appearance - any other help is appreciated thanks!!

 

[wilserbrewer]

Perhaps scrubbing with a paste of Bar Keepers Friend would help restore the kettle.

Might someone have left a chlorine bleach solution soak in the kettle?

 

[Pinkimark]

I've already tried bar keepers friends without much luck.

It's quite possible it's been bleach soaked, although it's not pitted at all.

If it has been bleached what could be done?

 

[lump42]

Only citric acid and nitric acid are proven to passivate. Oxalic acid (BKF) and phosphoric acid (Star San) do not.

Not all national associations would agree. Australian SS Dev Assoc. don't seem to require acid passivation as the only method. It likely will give the thickest Cr oxide layer, but the pH and duration of brewing wouldn't normally be considered a very aggressive environment. I'm not a metallurgist or materials engineer, so I could easily be mistaken. We're brewing beer at home, there is little reason to follow ATSM standards when cleaning with BKF and air passivation is sufficient to keep my kettle from rusting.

PASSIVATION The treatment of the surface of stainless steel, often with acid solutions (or gels), to remove contaminants and promote the formation of the passive film on a surface that was freshly created, e.g. through grinding, machining or mechanical damage. It will remove acid soluble inclusions such as MnS.

Clean humid air will form a passive film on clean stainless steel and the appearance will not change.

Chemical passivation strengthens the passive film and typically takes an hour or so at ambient temperatures. Air passivation is adequate unless the environment is very aggressive for the grade.

source:https://assda.asn.au/blog/304-pickling-and-passivation-of-stainless-steel

 

[ramjam]

you could use lye, i use it to clean cast iron, this is what i use, be careful with this stuff if you decide to go this route it is pretty potent

 

[RPh_Guy]

Not all national associations would agree. Australian SS Dev Assoc. don't seem to require acid passivation as the only method. It likely will give the thickest Cr oxide layer, but the pH and duration of brewing wouldn't normally be considered a very aggressive environment. I'm not a metallurgist or materials engineer, so I could easily be mistaken. We're brewing beer at home, there is little reason to follow ATSM standards when cleaning with BKF and air passivation is sufficient to keep my kettle from rusting.

You're reading too much into what I said.

Passivation is selectively removing iron from the surface to leave it chromium-rich. Only nitric and citric acid are proven to do this. The oxide layer forms on its own via air exposure.
I didn't say anything about passivation being necessary for homebrewers, or it being the only method for cleaning.

There may even be other acids besides nitric and citric that passivate, but there is no scientific data to prove it.

I've soaked in citric acid

I would suggest using a 4% (40g/L) citric acid solution and holding it at 140°F or higher for 2 hours. Temperature is important!

Looking at your photos I think it might just be that the surface is pitted, giving it a matte appearance, rather than it being "discolored". If you want it to be shiny again you'd need to polish it.