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Originally Posted by unregistered
only magneisum, 99.9%, will function as an anode in a cu/al system
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That's a misleading statement. We were discussing the addition of aluminum to an otherwise copper system. In this case, a basic galvanic couple is formed. In any galvanic couple we use the terms "cathode" and "anode" to distinguish the two.
It's like labelling the positive and negative terminals in a circuit.
When you bring in a
third metal, your knowledgeable statements about the relationship are true enough. Yes, magnesium trumps aluminum as the anode when all three metals are together.
To say this invalidates what I've said is like poking an earthed wire into a battery-powered circuit. Looks like all I've said about differences in electrical potential is "without knowledge".
Magnesium is used purposefully as an anode against copper/aluminum pairing. Why?
Because when copper and aluminum alone are paired, the aluminum anode is sacrificed. So magnesium being way off on the galvanic scale takes the place of aluminum as the sacrificial anode.
Many water supply systems, where copper/brass (and plastic) are the only materials used, employ an aluminum rod as an anode. Its role is purely sacrificial. Without it there would be no anode, and the copper would corrode more rapidly (at the normal rate). Aluminum also trumps steel for the anode of copper, in case glazing on tank walls cracks.
When we're looking at a purely copper system, and we want a sacrificial anode just for the copper, an aluminum plug will serve. In our systems even anodised aluminum will do the trick.