[Kobuchi]

Originally Posted by AngryAlpaca

Zinc is higher on the electrochemical series. It'd own aluminum ever so slowly.

Own? Aluminum eats zinc, but yes slowly.

Originally Posted by jman1310

ah, well aluminum isn't normally used on the running gear of a boat so I didn't know that!
but what of the question in general, is a sacrificial metal a good idea?

I actually have a workhorse powerboat made entirely of unpainted aluminum. On second thought, I think the steering cable to the outboard is steel... *shudders*. I've never seen aluminum used deliberately in craft without aluminum hulls.

Is a sacrificial metal a good idea? Nothing that would react quickly, I think. We don't want haze and particles. I wouldn't add a third metal like zinc to give copper better fodder than aluminum, because copper will just chew into that zinc and make a mess. With steel bridges and cranes we don't mind a bar of zinc crumbling in the elements, but inside a closed cooling loop we do.

I would consider a sacrificial metal if very slow to corrode - and in practice this means it wouldn't be a third metal but a second. This seems pointless until we consider that the cathode metal of a galvanic couple corrodes less slowly than just by itself. An all copper & brass loop might benefit from, say, tin. The tin would act as a weak corrosion inhibitor.