Citric is less toxic but equally effective as far as I remember but oxalic definitely does the trick and in my experience doesn't blacken the metal in the way that oven pride or the like do. Having said that I've only used OA on big chrome parts (frames forks etc) whereas I use overn pride to de-anno bits n bobs and yeh if you aren't on top of it it will blacken alloys. You can usually salvage even these by polishing tho. Can't remember how much OA I used but I do remember just putting some in to the water bath and watching how fast it was acting and topping up if it seemd slow..no very scientific but no disasters to date!!
