Hey, that's beautiful! ;-)
Or rather "citric acid", which I think will give you more hits when searching for this type of product. I've been using this for a long time as well, in powdered form, it's the best non-intrusive and non-toxic way of cleaning relics. Nice results indeed and you won't have to worry about potential toxic effects either!
Look forward to seeing more results!
Vinegar can also be used but it takes a long time.
did you clean that panzerschreck shield? and 50 mm mb ammo box