Chemistry lesson for the day: (He sez with an embarrassed grin on his
homely face) Aluminum blocks, etc., will tend to corrode, to some extent,
no matter what you do to your car. This corrosion will result in the
formation of Al+3 ions in the cooling system. (OK - no big deal - it's just
a normal long-term, slow, and not very sigificant, process which will
happen. Period.)

Al+3, in the presence of PO4-3 (phosphate), however, forms AlPO4, aluminum
phosphate, which has solubility properties similar to granite. If phosphate
is present in the cooling system, AlPO4 will be continually forming as the
Al block corrodes. Deposits of AlPO4 will form whereever the reaction takes
place. If left to go on long enough these deposits can block passages in
the radiator and limit the surface area through which heat can be
dissipated. The deposits are also less conductive than the radiator itself
which further reduces the effectiveness of the radiator by reducing the rate
of heat flow even through portions of the radiator which are still open
enough for collant to pass through. Eventually, the radiator will need to
be reoved and either replaced with a new one or cleaned and replaced after
cleaning. I don't have information on cleaning AlPO4 deposits from
radiators. It may work OK and then again...

An additional effect of the phosphate will be to reduce the concentration of
Al+3 ions in the coolant. The presence of Al+3 ions will tend to reduce the
rate of the corrosion. This is a benefit in that this will prolong the life
of aluminum parts. But if Al+3 ions are removed as AlPO4 this corrosion
rate will be accelerated somewhat. This will decrease (a bit) the life of
the aluminum block. This probably will not be a major consideration.