Casting a zinc anode for Alastor

July 2005

- Many people recommended, also MG Duff, as a source of commercial zinc anodes for the N32. It worked out at about $50 for the zinc and $50 for shipping, and I already had a mould and scrap zinc, so I decided to continue on the do-it-yourself route, but cast an entire new anode with new aluminium bar instead of patching up the old one as I had done on a previous occasion.

When I pulled Alastor from the water, I noticed that one of the studs holding the zinc (the forward one, connected to the engine and propeller shaft) was completely eaten away down to the square plate. There was also some very slight pitting (more a discolouration) on the stainless propeller shaft. I conjecture that when I had patched the existing zinc anode with new zinc, that the nuts were no longer in contact with the aluminium bar but instead were bearing on zinc. Dissolving of the zinc under the nut would lead to a progressively worsening electrical contact, until finally all contact was lost. Electrolytic action between the still-connected bronze propeller and mild steel stud would have promoted corrosion of the stud.

Alastor is kept at a fresh-water marina, so the zinc lasts a long time. I now gather that a passivating layer develops on zinc in fresh water, and that I should consider a magnesium anode for marina use.

Mould (made of cement) with aluminium strip in place. Plugs are 3/4" wood dowels on 9" centres; idea was to weigh them down during casting.
"Oven" is scrap breezeblock on barbecue burner, with propane torch added for extra heat.
Melting xinc scraps in a steel food tin. Stirring with an aluminium rod was a mistake - it melted, leaving clumps of alloy on the surface.
Casting the anode.
Out of the mould. Next, bolt holes were drilled, nut recesses drilled out down to the aluminium and zinc overspill ground away. Mould is not quite deep enough.
Finished anode in position.
Inside hull, showing electrical connections to engine and propeller tube. Wire joining two studs was added to give a redundant connection.

What worked

What didn't quite work


Some Galvanic Potentials

Aluminium bronze-0.41
(Zinc Handbook, Porter 1991)