Originally Posted by Red_Chili
Corrosion kills. Doesn't old coolant contribute to galvanic corrosion due to chemical changes in the coolant itself?
Yup, probably. Galvanic corrosion happens when dissimilar metals are immersed in an electrolyte. In the case of our engines, iron block and aluminum head bolted on (thus great electrical contact) are just waiting for the electrolyte to flow through, which we provide with coolant passages. That could be the acids left over from combustion, impurities in the water, etc. That's mainly why I think my HG went the distance, the distilled water that got flushed often. That is also why I think Toyota Red, Prestone green, whatever, is more of a non-factor if you use distilled water. Using tap water full of incident salt is probably the common denominator in HG failures, but I dunno. I do think the red stuff with phosphates works better with distilled water, where as the silicate green stuff is probably safer with water of unknown quality. The silicates are more active at preventing aluminum corrosion at the expense of wearing out pumps from the abrasion. But doing a phosphate wash or coat is a common corrosion protection technique, so I don't think Toyota Red is a bad coolant from a galvanic protection viewpoint.