interesting: this from wikipedia ( http://en.wikipedia.org/wiki/Antifreeze#Propylene_glycol ):

Propylene Glycol oxidizes when exposed to air and heat. When this occurs, organic acids are formed viz. Glycolic acid, Glyoxalic acid, Formic acid, Carbonic acid & Oxalic acid. If not properly inhibited, this fluid can be very corrosive. Protodin is added to Propylene Glycol to act as a buffer, preventing low pH attack on the system metals. It forms a protective skin inside the tank and pipelines which helps to prevent acid attack that cause corrosion.

Beside cooling system breakdown, biological fouling also occurs. Once bacterial slime starts the corrosion rate of the system increases. In system where a glycol solution is maintained on a continuous basis, regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color and biological contamination should be checked on routine basis.

Propylene glycol should be replaced when it turns reddish in color.