What's going on in this keg?

[Mr. No-Tip]

The pictured keg has had a spontaneously fermented cider in it for 6+ months. It was a crazy thing that made the whole thing milky white and continued to ferment even at very cool temperatures. It eventually finished and flocked out.

I finished it last weekend and hosed it out before filling it with caustic for 24 hours and the rinsing it.

It's been drying for 24 hours and I turned it around to find this rusty looking stuff at the bottom.

Pretty sure it wasn't there before adding the caustic (but it was dark). 100% sure it wasn't there before the cider.

Any ideas?

 

[MHB]

If it looks like rust, it probably is. Remember its Stainless Steel, not Stain Proof Steel.
If you use (or if anyone ever has) bleach or other Chlorinated cleaners on Stainless you can start rust happening.
Don't know what sort of bugs were working on your cider but some do produce acids that can react with stainless if the surface of the metal has been disrupted.
You should Passivate the surface before using the keg, a commercial paste or a bit of Phosphoric acid would be best, Citric acid and other often talked about "cheap" measures really wont do the job.
Mark

 

[Mr. No-Tip]

Ta. So if it is rust, is the acid going to remove the rust and passivate or effectively cover the rust? I used concentrated star san to passivate an SS fermenter as per the manufacturers instructions. Would that work here or is that another cheap solution?

 

[MHB]

If you get the rust off, using Phosphoric acid converts the surface Chromium into Chromium Phosphate that is really tuff and protects the underlying metal.
Whether or not Star San has enough phosphoric acid in it to do the job or not is something I cant comment on. If the manufacture says so, it might be worth testing. If it doesn't work the rust will be back and you can try something else, personally I use a commercial pickling paste, had the jar for 10 years and used it on lots of commercial builds, need to get a new jar soon.
Mark