n5160u - 9/14/2004 2:31 PM
N2 is also dry. Have you ever seeen the moisture around the valve stem when adding air to a tire? That moisture ends up inside the tire and stays there until the tire is removed from the rim. I used to have some alloys that were real "mags" ie: MG and they corroded really badly from humid air being pumped into them.
Metal in the alloys or Mg as you point out can only corrode when in contact with another metal of a different potential, like the wheel nuts, the hub and balancing weights. Yes, the small weights on the rims can corrode the metal.
The contained water on the other hand does nothing because it can not form a circuit.
The same thing happens when you mismatch the metals in any type of multi-component system. The several metals form a battery where electrons migrate (from the rims in your case) to the car or from the car. That and the fact that under enormous stress metals are constantly reworked and the crystals fatigue.
The most likely cause of metal degradation in alloys is
- electrolytic oxidation
- uneven cooling or poor casting
Water inside the air inside the tire would only do something if the rim was attached to ground with an earthing strap, such as might happen in storage.
That would be the Science chapter on Galvanni and electricity.