Just had a look at the safety data sheet for DEOX-C to see if gave any glue as to ingredients. It's a white crystalline solid giving a pH of 3.5 -4. So it's most likely a weak organic acid such as citric or sulphamic acid, if it froths a bit there's probably a bit of dish soap in as a wetting agent.
Citric acid is cheap as chips.
https://www.waitrose.com/ecom/products/citric-acid/384290-845523-845524?gclid=65b0eccedcf5188fcdaa8445a2d6205a&gclsrc=3p.ds&msclkid=65b0eccedcf5188fcdaa8445a2d6205aI use it for pickling prior to zinc plating both to remove any residual rust in any pitting and strip off any residual zinc plate. Works best the warmer the solution is.
Like many of the acid based products if there's any plated coating on the inside it will have that as well, but if the tank is so rusty inside any plating is long gone.
Rust removal by electrolysis or acid will release hydrogen so take the same precautions as you would charging a lead acid battery. Don't have a fag hanging out the corner of your mouth when you peer in to see if it's working. Otherwise near an open window where there's a bit of airflow to mitigate any build up.
Regards
Dave
