There is a theory that the aluminum spacers might corrode due to natural electrolysis when joined to different metals. I have not heard of anyone having this problem. I have worked in the aviation industry and aluminum has been used with steel often. (although the steel and aluminum were zinc coated) My old house had aluminum wires in it and they did corrode when joined to copper or brass. (after 30 years)

 

I pulled this off another website:

Aluminum: Yes, its one piece. Does it have the chance to oxidize over time and fall apart in a harsh winter enviroment with chemicals/salt on the roads? Dont know! I wouldnt buy just for fear of that one thing! Aluminum doesnt weather well in my opinion.

Steel: Yes its welded from 3 pieces. Will it break? More than likely not. It may weigh all of 2-3 oz more over aluminum. I had the steel ProComp and never an issue. They may rust, but you can always spray them with undercoating before installing.

Poly: Yes they are flexible, but why would you want that? If your suspension components are flexible, whats the point of having shocks and coilovers? Poly will eventually smash in and/or be compressed to a smaller size. Steel and aluminum will more than likely stay the same height.

Alum.: lighter weight, costs $5 more, may oxidize from chemicals
steel: weighs more, is welded, $5 less, wont oxidize
poly: flexible (squishes around), will lose its height after a period of time from weight on it

(you have to register to view page...I think) http://www.fordtruckclub.net/forum/s...?t=1303&page=2