ICQ>

There are two kinds of corrosion that you need to worry about. Normal oxidation occurs when oxygen disolved in the water oxidizes the metal. On steel we call it rust, on aluminum ot forms aluminum oxide - a thin black coating which has a high electrical resistance, but otherwise does not caus much problem - unless it is on the arm of your lawn chair and turns your hands or white shirt sleeves black.

Electrolytic corrosion occurs when dissimilar metals are in the presence of an electrolyte (any salt or acid solution, no matter how mild will do). The metal with the weaker electron bonds will be 'sacrificed' or eroded by the chemical action. In our engines, aluminum is such a 'weak' metal in the presence of steel, cast iron, etc.

The 'fix' is to use a sacrificial anode material in the system that is weaker than the aluminum on the electrolytic scale. Some cars (Saab) have used actual metal anodes installed in the coolant system that need to be replaced periodicallly. Volvo simply uses a good quality coolant designed for aluminum engines that contains some magnesium that acts as the sacrificial anode in the reaction. The magnesium is used up, preventing corrosion of the aluminum.

The key term here is 'used up' which is why it is very important to (a) use antifreeze designed for aluminum engines in your coolant system, and (b) be sure to flush and change the coolant at the recommended intervals (every two years) because once the coolant runs out of magnesium, the reaction will start working on your aluminum engine parts.