More On "Antifreeze: Red or Green?"In a well formulated US green coolant, phosphate works to provide the benefits you describe. However, if the silicate is not well stabilized and the silicate protection is lost for an aluminum head, a phenomenon know as either "hot surface corrosion" or "aluminum transport deposition", depending on what car company you are talking to, can occur.
In this form of corrosion, the aluminum is corroded from the hot spots in the head, usually near the exhaust ports, and then reacts with the phosphate in solution to form aluminum phosphate. The aluminum phosphate is then deposited on a cool surface in a heat exchanger leading to over heat problems.
It has been reported as either a gelatinous precipitate that blocks the tubes or as a thin ceramic deposit on the radiator (original finding of the problem when Chrysler was testing VW engines to use in Omni/ Horizon in the late 70's) in the shape of the fan. If you write the chemical reactions that occur when the aluminum phosphate is precipitated, you will find that caustic is generated. The corrosion process then increases exponentially.
That is the very reason Dexcool was created. It can affect anything aluminum in the cooling system (LIM, blower housing, water pump, etc). Now we have the yellow, which is better than both.
