Originally Posted By: Bluestream
Washing your vehicle undercarridge once a week will actually cause it to rust. (water + steel) not to mention what a huge PITA
Actually it's oxygen + steel in the presence of moisture (including humidity).
Water itself isn't corrosive (or corrodes very slowly) if allowed to dry quickly. But road salt/sand mix remains wet down to around 0F, essentially bathing the steel in a wet salt + oxygen environment that corrodes quickly.
You want to wash that off.
Back before digital cameras, standard service procedure (Nikon, Hassleblad, ect) for a mechanical film camera accidentally immersed in water was to remove battery if equipped, then put it in a sealable container of water and ship it to service.
Corrosion would begin after the wet camera was exposed to air; they want that to happen on the service desk where it can be dried and re-lubed. Film emulsion would swell but still processable.
A for being a PITA, well, it isn't. Adds a couple of minutes to your wash time.