Sodium metabisulphate on stainless steel

[Aarkhana]

Hi all. So I'm three brews into a new stainless steel kettle I purchased. Two days ago I did a brew and thought I'd fill it with water to soak to give it a better clean. I whacked in some sodium metabisulphate and left it. I only just got to cleaning it and to my horror, everywhere above the water line has been badly stained or oxidised. Not too happy about it - seems like $200 down the drain. I was told it's fine to leave this in a stainless steel vessel. Any ideas what happened?

 

[Lyrebird_Cycles]

This will happen if the grade of stainless in the vessel is 304 or similar.

What has happened is that the vapour pressure of SO2 is higher than the vapour pressure of water so the condensate on the inside of the vessel above the water line has a much higher concentration of SO2 than the solution*. This high concentration of SO2 will attack** grades of stainless that do not contain a stabiliser such as molybdenum. To prevent this you need to use a stabilised grade of stainless such as 316, which is ~4% molybdenum but much more expensive.

You can remove the damage by grinding the surface and then passivating it with a mixture of nitric and hydrofluoric acids, sold as pickling paste in welding supplies. Be careful, even other toxic chemicals are scared of hydrofluoric acid, it's nasty ****.



* This is also why whisky has more alcohol than beer.

**The attack is a reduction reaction, not oxidation, which is why treating it with a very strong oxidant will remove it.

 

[Yob]

fill it up?

I use a combo of metasillicate and percabonate for really good cleaning, but I always fill to the brim and leave overnight only.

Why metabisulphate?

 

[Aarkhana]

Thanks guys. I was recommended by a brew shop. I actually had some percarbonate but had much more of this. Thought I'd just soak it. So I'm guessing it will still be ok to use the pot though it'll be stained?

Also lyrebird based on what you've said, why would brew shops sell it and not mention the potential to for the gas to ruin stainless steel?

 

[Lyrebird_Cycles]

Because they are brewing shop attendants, not engineers.

 

[Aarkhana]

Yeah fair call. Pretty annoyed about it. Had lengthy conversations about it and it was only mentioned about bleach causing issues with SS.

 

[Zorco]

LC, thank you for every post you make. And for all those to come.

 

[evoo4u]

Yeah fair call. Pretty annoyed about it. Had lengthy conversations about it and it was only mentioned about bleach causing issues with SS.

Liquid soap left on SS will pit it quite quickly, also not a good idea to leave articles sprayed with Starsan to dry on a SS bench top.

 

[Aarkhana]

I think from now on I won't be doing too much soaking. I've cleaned some of the surface stains with a mixture of bicarb and liquid soap. Seemed to work ok. Might try the pickling paste though I'm a bit worried about it. Or... Should I just try to ignore the stains and keep brewing? Tough call though I would like to restore it.

 

[Aarkhana]

So after the bicarb and soap mix which did a little, I purchased bar keepers friend. It's not perfect but so much better.

Thanks for the input everyone.

 

[GalBrew]

Thanks guys. I was recommended by a brew shop. I actually had some percarbonate but had much more of this. Thought I'd just soak it. So I'm guessing it will still be ok to use the pot though it'll be stained?

Also lyrebird based on what you've said, why would brew shops sell it and not mention the potential to for the gas to ruin stainless steel?

That is **** advice. I'd be letting them know and sending them the pics.

 

[Lyrebird_Cycles]

Addendum:

The above was from memory, I looked up the actual mechanism and there's an extra step which I missed out: if oxygen is present it will oxidise the SO2 to SO3 which will dissolve in the condensate to form sulphuric acid which greatly increases the corrosion potential.

 

[timmi9191]

Because they are brewing shop attendants, not engineers.

Gotta love uneducated advice with cleaning chemicals.

 

[timmi9191]

Fwiw ive never had an issue with bleach for spot cleaning on 316 stainless with a good rinse. Wouldn't use it on 304 though..