Sparge Kettle Discolouration

[Jonno2003]

Brewed today for the first time this year. Left sparge water in the sparge kettle (Brewmonk 25 litre sparge kettle) overnight with half a Camden tablet in it. Heated it up to normal sparge temperature and tapped it - slight blue tinge to the water. Didn’t use it, improvised with a normal kettle instead, ditched the water in it to find the inside is discoloured, almost black, where the water sat overnight but shiny stainless elsewhere, as it always has been up to now. Photo shows. Any ideas? Is it a gunner?

 

[JockyBrewer]

That’s oxidation of the steel. Should come off with a steel cleaner or something like white vinegar.

But I have no idea what would cause it. Hydrochloric acid can, but I don’t know much more than that.