I found this thread as I have an MGB petrol tank with internal corrosion that I needed to remove. I learnt via the power of the internet that the Bilt Hamber deox C is likely anhydrous citric acid, and the quantity I need would likely be £100 at £20/kg. 5kg of citric acid for £20 on Internet, made up the solution with hot water and filled the tank. I made a little enclosure out of celotex and with a small tube heater was able to keep it at 20 degrees. Less than 48 hours later:
Before:
Before: