Corroded Stainless Kettle - Solutions?

[Sean Monaghan]

Hi All,

I tend to learn the hard way to preface this. I soldered in some tri-clamp fittings to a Brewer's Best kettle. My critical mistake was not wiping off the excess flux and leaving the kettle for months. There are some corrosion spots from the flux that was not wiped away. I've tried vigorously scrubbing it away with BKF, but the etched spots remained and rust quickly returned. I'm now considering attempting to sand and re-passivate the metal, but I wanted to seek out a little advice before doing so.

Would sanding going from 240 up to 400, then 600 and re-passivating with citric acid fix this issue? Any recommendations or tips would be appreciated.

Thanks!

 

[day_trippr]

I'm assuming with all this that the work area is now flux-free and other than the corroded area is basically bright. Based on that, and before removing any more metal, I'd try using hot citric acid solution - 8 percent by weight - let it cook overnight and see what happens. You might just get lucky, but if the result doesn't look "done" you can always try some light sanding and give it another try then.

Pictures might help as well as I'm mentally picturing minimal damage but maybe there's more to it...

Cheers!

 

[IslandLizard]

let it cook overnight and see what happens.

Cook?

 

[day_trippr]

"sit"?

Actually, I was too vague.
By hot I mean bring the solution up to working temperature - somewhere between say 150 and 180°F - then remove the heat and let it sit overnight...

Cheers!

 

[IslandLizard]

Soak?

 

[day_trippr]

Right, but with the heat source off. I've used citric enough that if it's going to work it'll work without constant heating...

Cheers!

 

[IslandLizard]

As @day_trippr said:

Pictures might help as well as I'm mentally picturing minimal damage but maybe there's more to it...

Pictures would really help, yes!

 

[IslandLizard]

Right, but with the heat source off. I've used citric enough that if it's going to work it'll work without constant heating...

Cheers!

That's why I was wondering why you used the word "cook" as that hints to applying heat. No?

 

[Sean Monaghan]

Thanks for all the discussion! This is helpful. Following up with some hopefully informative pics.

 

[day_trippr]

That's why I was wondering why you used the word "cook" as that hints to applying heat. No?

It's a not uncommonly used verb when talking about frying things in acid...

Cheers!

 

[day_trippr]

Can you feel those really dark spots in the middle of the largest splotches? Are they bumps or pits?

Cheers!

 

[Sean Monaghan]

Can you feel those really dark spots in the middle of the largest splotches? Are they bumps or pits?

Cheers!

Yes, it's a bit pitted. It would not scrub off with BKF and has a textured feel to it. I feel that I need a stronger abrasive.

 

[day_trippr]

I'd hit it with the citric acid treatment and see how much of that cleans up.
Any idea what the wall thickness of that kettle is?

Cheers!

 

[Sean Monaghan]

I'd hit it with the citric acid treatment and see how much of that cleans up.
Any idea what the wall thickness of that kettle is?

Cheers!

Manufacturer says a wall and bottom thickness of 0.8mm

 

[day_trippr]

Oof. That's three business cards thick.

Right then, let's hope the citric acid does enough to get things under control...

Cheers!

 

[IslandLizard]

[EDIT] Ah, just saw the posted pix.
The corrosion appears to be on the bottom.

For reference:
If there are also corrosions high(er) up the kettle wall, when giving it an acid batch, you don't have to fill the kettle all the way up. You can lie it on its side under a slight incline toward the top. The affected area placed on the bottom of course, effectively making a trough. Brace it with some wood, so it won't roll or move.
Then just fill the trough with a deep enough puddle of acid to cover the affected areas.
I'd say a mere gallon would probably cover them.

 

[Sean Monaghan]

Thank you. To confirm, no sanding? Acid bath at 8% might work?

 

[day_trippr]

Imo, it's definitely worth a shot to try that now and see what happens. There really is no down side. Citric acid powder is inexpensive, and if it cleans up those splotches without resorting to mechanical means you're likely going to be better off.

0.8mm/0.031" isn't a heck of a lot of metal. I'd want to see if a minimalistic approach might work before breaking out the Dremel with a cone grinder, for instance...

Cheers!

 

[impact.walter]

As to sanding, I assess that as pit corrosion, for which sanding isn’t really appropriate as sanding is a surface process. For marine stainless (316) patient very fine sanding is applicable to remove shallow surface marring which presents as an unsightly surface ‘rust’, ie micro crevice corrosion in the absence of, or low O2. Pit corrosion in marine environments is typically associated with electrolysis (unlikely in your situation) or galvanic action (possible in your situation). Sanding will merely feather the creator edges by removing more material. Try to store stainless dry to avoid crevice corrosion and galvanic action, especially in the presence of minerals and/or salts. I mention marine only to qualify my knowledge of metals in that specific environment.

 

[Sean Monaghan]

As to sanding, I assess that as pit corrosion, for which sanding isn’t really appropriate as sanding is a surface process. For marine stainless (316) patient very fine sanding is applicable to remove shallow surface marring which presents as an unsightly surface ‘rust’, ie micro crevice corrosion in the absence of, or low O2. Pit corrosion in marine environments is typically associated with electrolysis (unlikely in your situation) or galvanic action (possible in your situation). Sanding will merely feather the creator edges by removing more material. Try to store stainless dry to avoid crevice corrosion and galvanic action, especially in the presence of minerals and/or salts. I mention marine only to qualify my knowledge of metals in that specific environment.

Thanks! I really appreciate this response. Any guidance on how to proceed? Or just get a new kettle?

 

[impact.walter]

Pitting (especially multiple small) is awkward. I’m not approaching this from a sanitary perspective as there are advisers here who are more knowledgeable than I in that department. The brown colour is likely ferris oxide. I would personally work on the premise that ‘iron’ oxide isn’t bad for me, and I’d assume the same for my brew. However, I would get a small sharpish metal needle and probe as much out as possible and then sanitise thoroughly. You might combine this approach with a small (stainless, not brass) wire brush. They look just like a toothbrush- same size & geometry. The risk is IMHO a sanitary one. If those pits can be kept dry between uses, any further degradation is likely to be very slow, but still an obvious sanitation challenge. I hesitate to suggest a chemical solution which might not be food-grade. In any case the sanitary challenge would remain.

 

[day_trippr]

So, "scaling" by any other name.
Still a mechanical process that likely will be insufficient wrt stopping the erosion the OP has described...which if not arrested will lead to a breach...

Cheers!

 

[impact.walter]

I see others have mentioned citric acid already. I think the suggestion is good. Passivating 304/316 stainless with citric acid is now quite common in several industries, including CIP in dairy factories. I haven’t personally used citric, as my applications have been marine and chemical industry vessels, not food, and with phosphoric acid (hence my reluctance to mention).
Should you decide to passivate with citric acid, I’ll just mention that you can’t possibly rinse too much afterwards. Especially in those pits. Your objective is to remove all traces of the citric in those pits, else the acid will continue to erode your vessel. That just about exhausts any light I may shine on your problem. I hope you are successful.

 

[day_trippr]

For the record, nitric followed by citric acid are the top two processes used for passivation of stainless steel...

Cheers!

 

[impact.walter]

Nitric is nasty though.

 

[day_trippr]

Indeed, which is why we recommend using citric - which is amazingly effective and quite safe in the 8% concentration recommended...

Cheers!

 

[Sean Monaghan]

So if I hit it with 8% citric it sounds like there will still be a sanitation issue?

 

[sibelman]

It seems there's no way to repair this damage, though maybe it will look better after an acid passivation treatment.

My intuition is that sanitation concerns are small to non-existent in a boil kettle, where everything gets hot enough to quickly kill any bacteria. I wouldn't use the pitted kettle as a fermenter, or to make a no-boil beer.

The pitting could worsen over time, but the kettle should make fine beer until/unless a pit becomes a hole. Drying after cleaning might slow further degradation.

 

[pvtpublic]

Here's an excerpt from Five Star Chemicals' product guide. Take note that the Acid #3ab mentioned is hydrochloric acid, something you can get at any hardware store. They call it muriatic acid, though.

 

[tracer bullet]

phosphoric acid

10% phosphoric acid isn't uncommon to add to mash water to bring down the pH. I don't know, so I'll ask, would that be an alternative to the citric acid? I'm just thinking that it might be something already on hand, or useful as a thing to own in the future if it had to be bought (and could easily be bought the next time ingredients are bought).