Post passivation question

[saulecker]

We just got 2 new tanks in and we are at the passivation stage. I ran a 4% citric acid solution in tank 1 for 4 hours (per our chemical guy). When the time was up I took a peek inside and expected everything to look all shiny and sparkly. Instead the tanks look pretty much exactly as they did before hand, kind of frosted metal with some buff spots.

Is post passivated stainless supposed to look shiny or pretty much the same?

Thanks!

Kevin

[yap]

passivation won't change the appearance much assuming the tank was clean to start. did you run caustic (or similar) through it first to remove any machining oil grease etc? was the 4% by weight or volume?

when i passivated my kettle it was recommended to use 10% by weight of powdered citric. after caustic the tank looked spic and span inside. after citric it may have been a bit shinier, but not by much...

[saulecker]

Yep, we ran a water rinse, followed by a long caustic session, followed by more water to rinse. It was 4% by weight, although it wasn't that different than by volume, unless my math was off. I did 20 lbs, 11 oz citric powder in 59.5 gallons water. The tank looks clean, just not shiny.

[saulecker]

Just talked to a brewer at another brewery and he said citric will passivate, but not do any kind of polishing or cleaning. He recommended I run our CIP acid (a phosphoric/nitric blend) in the tanks on a normal CIP cycle and if the tanks are still foggy/not shiny then get in touch with the tank manufacturer.

Good to know! We don't generally use citric here.

[dubo11]

from Birko does the job like a champ. Circulate @ 160 for 1 hour after a caustc and rinse cycle and you will be blinded from the shine.

[gitchegumee]

Passivation and/or use of acid will not make your tanks shiny if they aren't shiny to begin with. "Shiny" is a surface finish issue and acid will not change the surface finish. Surface finish is a mechanical issue for the most part. Unless you really hit it with hydrofluoric or some other nonsense--and you should NOT attempt that. Besides, heavy acid attack on metal doesn't make your tank "shiny", but instead frosted and pitted--so acid can change the surface finish but almost always for the worse. Instead, you must live with the finish you have. Buy quality equipment and you can have a shiny surface finish. Passivation just keeps the metal bright and subject to less staining and krausen-stuck goo. It also is helpful in keeping welds from corroding. But it will not improve surface finish.

[dick murton]

You would have to pay a lot of money to have polished internal finishes, for no gain under normal circumstances. The critical areas are where the paltes have been welded. Hopefully they have been ground back and then passivated.

5% caustic at 80 C for 30 minutes + degreasing, followed by nitric acid. It needs to be an oxidising acid. Not sure that citric is strongly oxidising. The passivation is to rebuild the relatively inert chromium dioxide layer back up again, which is what makes stainles steel stainless.