Originally Posted by manyboats
I was under the impression that zinc was much better but the switch to aluminum is a green thing. I made a point of getting all zinc before launch this fall.
Got nothing to do with green, it's all about voltage potential.
Zinc = -1.05v (salt water).
Aluminum indium = - 1.10v (brackish water, although some thruster) mfg's recommend for all water).
Magnesium = -1.60v (fresh water).
Basically salt water is more conductive than fresh so a lower voltage potential is required with brackish water in between. Either under or over protecting below the water line metals by use of incorrect anodes can cause damage.
These voltage potential differences may seem small but the effect can be considerable. Zinc used in fresh water will quickly (a couple of days) develop a calcereous scale which makes the anode useless. Magnesium in salt water will get eaten rapidly and again aluminum falls somewhere in the middle.
This is an extremely complex issue and if anyone is interested I am hosting the four day ABYC Corrosion Analysis
course at Port Credit Yacht Club