Prestone "heavy duty cooling system cleaner" (p/n AS-100P) contains oxalic acid. Oxalic acid readily reacts with rust, but very slowly with iron/steel. Prestone has apparently discontinued the product, but it is still available at GM dealers under GM part#12346500. The cleaner is 9 oz (255g) and the neutralizer is 2 oz (56g) sodium carbonate. The acid has to be dissolved into hot water in a plastic container; it will not dissolve into cold water.

If you can't find the Prestone product, PM me. I recently bought a 50-pound sack of oxalic acid and could mail you some.

Sodium carbonate is available in the supermarket laundry soap aisle; look for "washing soda". Its also available at pool and spa suppliers as "pH Increaser". Its the same chemical used for electrolytic derusting. Google "electrolytic derusting" if you're not familiar with the process. There's also a video on www.moparmailinglist,com

Do not use hydrochloric acid (a/k/a muriatic acid) in a cooling system. It will attack the zinc in all of the brass components, and also cause chloride corrosion of iron and steel. i.e. it will destroy your radiator, thermostat, heater core, engine block and water pump impeller. Remember that zinc ia a copper/zinc alloy; the reason that old brass pipes develop pinhole leaks and become brittle is "dezincification", a corrosion process in which the zinc oxidizes and leaves the alloy, leaving behind a sponge-like matrix of copper.

Ken