I got through around 20 replies before I replied. Not my norm, usually read them all.
The CO2 bought at welding shops and such is very, very clean. And the tanks they are put in, if from a reputable shop have to have a DOT inspection before being certified or recertified.
Having said that, steel tanks can corrode under the right conditions.
But I really doubt you have a corroded tank. Probably more likely carbonic acid is what you are tasting.
I've gotten the off flavor before with an aluminum tank that I over carbed the beer with when I first started kegging..
Just my two pennies. Counts for not much.