pishta answered that - "pool acid". Go to any pool supply store, or even Ace Hardware, and ask for "muriatic acid". It is a mild mixture of HCl acid and water. Most radiator cleaners, like Prestone Super Flush use citric acid. One guy posted here on using an industrial version, with photos showing excellent results. If the chemical flush causes pinhole leaks, that is probably good since they were already there and hidden. Once clean, you can usually repair them w/ solder, if copper tubes.
