G'day chief, sorry I didn't see this earlier. If you want some Deoxidine 624, then let me know & will grab some for you. BTW, Deox 624 is also the spec pre-cleaning/etching process for all Boeing & Mil aircraft prior to Alodine treatment & then re-painting maintenance/repair.
Do not use hydrochloric acid.
Can you post or send me some pics of the inside of the tank so I can see how severe the corrosion is, cheers.