That's for galvanic corrosion, and when copper and iron are both present, the 
iron corrodes first. With a magnesium or aluminum rod, it will corrode instead.

Chances are, it's the heat cycling that fatigues the copper over time to let in 
a bit of water. Any manufacturing defect or inconsistency would facilitate that 
too. Then steam generation inside the tube via the crack, and the cycling, 
erode further and 'pump' water in and out. Once water is in contact with the 
actual wire, then it's effects come into it. Note that there will be a 
potential gradient along the wire, beginning, and highest, with the hot 
connection. It should cause the worst looking damage there, and it appears to 
be so in the pic.

Sacrificial anode replacement will protect the tank lining, while electric 
elements will just need to be replaced periodically. A GFCI breaker would alert 
you to the element breach. Replacing the anode on a regular schedule will 
protect the tank.



Marcel Duchamp wrote:
> Jinx wrote:
> 
> Maybe not.  My limited understanding is that water heaters use a 
> sacrificial anode that has a lower potential than the rest of the 
> heater.  It slowly corrodes away during the life of the system.  When 
> it's gone, corrosion continues with whatever material has the next 
> higher potential.  This rapidly destroys the heater at that point.
> 
> Water heaters are sold with particular lifetimes - eg, 15 year, 20 year, 
> etc.  Same heaters, different longevity.  How?  Longer sacrificial 
> anode.  An electrician told me to buy the shorter lifetime heater at a 
> lower price and replace the anode with a 30 year one.
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