Additional followup: a technical report on ground-rod lifetime at
http://www.erico.com/public/library/...ike/LT0540.pdf has some
interesting conclusions.

It cites a study or two which suggests that ground rods with a 10 mil
copper coating should be good for a service life of up to 40 years.
With a 13 mil coating, 50 years.

Galvanized ground rods with a 3.9 mil zinc coating don't seem to be
good for more than a 10-year service life. One sample dug up after 10
years was corroded through, almost to the point of breaking, not far
below the soil surface.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!