On Wed, 20 Aug 2008 09:21:42 -0700 (PDT), RichH
wrote:

Magnesium is less reactive than zinc;

They used magnesium in incendary bombs because it will burn easily.
But it's less reactive? What zinc is, is cheaper. Aluminum is the
standard fuel in firecrackers, and my boat is aluminum. Spirit Lake in
Iowa has a number of aluminum boats originally bought by the no longer
there boat rentals of fifty years ago. The businesses are long gone
but the boats remain. None have ever seen a zinc or anything like it
unless it was perhaps built into an outboard moter. None of these
boats have shore power, most have no battery. No bronze thru hull
fittings. No corrosion in any of these boats in fifty years. Active or
not, bare aluminum boats don't seem to corrode even in sea water, at
least not withour help from dissimilar metals or shore power. We have
a magnesium ladder that has been out in the weather, hung for storage
on the outside of a shed, for fifty years. It got darker, is the only
sign of weathering.

Casady