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On Feb 3, 10:55 pm, "Bob F" wrote:
wrote:
A friend has a house that came with two identical water
heaters hooked up in parallel. He doesn't need all the capacity,
so we turned one of them off and turned off the water going
into it. This was several years ago. The water heater was
about 4 years old at the time, it's now 7 years old.

Given that a tank has already had some service, what would
you guys do to try to keep the spare tank available for as long
as possible?

Leave it full of water?

Drain it?

My thought was that draining it would be worse, because
allowing air in, it would rust.......

2 thoughts"

Hot water furnaces and their piping don't corrode because the O2 that
rusts them
gets used up, and the rusting stops, as I understand it.

As long as the anode is good, rust shouldn't happen, right?- Hide quoted
text -

- Show quoted text -

Rust and what the anode is there for are two seperate things.
Rust involves oxygen combining with iron. The sacrificial anode
involves two dissimilar metals in an electrolyte
================================================== ==

A good observation, BUT the (dissolved) oxygen would/should combine with
sacrificial magnesium, zinc etc first, as well.

Magnesium, zinc, alum appear not to "rust" because the nature of those
oxides is *mechanically* stable, whereas iron oxides are not, constantly
exposing more fresh iron, ergo a deterioating process.
--
EA




. Think about boats.
They use a similar approach, with zinc being used
as the sacrificial anode to protect the underwater metals. Zinc is
more reactive than bronze, stainless steel, etc so it comes off
instead.