National Homebrew Club Ireland

I left a mix of campden tablets (probably 6 or so) and a few litres of water in stainless steel pot with lid on. It was probably in there a couple of weeks. When I lifted the lid today there was a significant amount of corrosion on the lid and upper wall of the stainless steel pot.

I was a bit surprised. Any chemistry head care to explain how my stupidity led to what happened?

Should I clean it and wash it with something acidic now? It seems to be surface only.

I'm no stainless expert but worked with a guy who could have given you every detail for stainless and was a plant manager at a cheese factory and always complained about the use of 304 grade SS due too poor corrosive resistance as opposed too 316 explaining why they should never use 304 for walkways, railings etc especially near the cost as they always rust.
Here's a piece from Wikipedia

Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) of S30400 and S31600 stainless steel piping and tanks is a problem in water lines in brewery applications. A common form of stress corrosion cracking occurs at temperatures higher than ambient in the presence of chlorides. Duplex stainless steels and alloys containing molybdenum alloys are used as alternatives that are more resistant to stress corrosion cracking. Cracking may occur from the process or from the outside, for example under insulation. Reducing the amount of oxygen ingress and lowering the process temperature minimizes the possibility of stress corrosion cracking.
http://corrosion-doctors.org/Food-Industry/Food-corrosion.htm
Be careful when using this as I do work with a lot of nasty chemicals in my job but on an industrial scale and be sure to check your equipment storage preferably polytetrafluoroethylene ( PTFE) type containers as they are designed to hold different chemicals as well as Teflon material which is great for storage and there's numerous others


Gerry

As what was ritten.  :D . I was going to say something more layman but was beaten to it.

Thanks Gerry.

I cleaned it up and wiped it with phosphoric acid I had. Green/black gunk wiped off after an hour. Some pitting and tarnished unfortunately. Is the steel passivated now? Or should I expect more oxidisation? I have polishing discs. Would it be worth buffing up? I'm not worried about cosmetics but thinking more for reconditioning it so it doesn't deteriorate further.
I'm still not clear why campden solution did this? I put it in there as a storage solution. Similar to what guys do with fermentor.

I usually throw a Campden tablet with some water into every keg after I clean it for storage. Haven't noticed any issues like what you have noted though...

Maybe the keg is better grade stainless steel? Metabisulphite is listed as corrosive on that link Gerry shared earlier. Was news to me but interesting stuff. You might want to reconsider your current practice in light of this.

Yeah will stop doing that for sure!

Keep an eye on starsan which has phosphoric acid as anyone using swingtops will find it corroding the chrome with a blackening and a real metallic smell.
Give a good cleaning/buffing and boiling water.

Gerry