Casting a zinc anode for Alastor

- Many people recommended zincsmart.com, 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

• The basic process - casting zinc in a cement mould.
• Drilling out the nut recesses with a 3/4" spade-type wood drill. Slow, but it worked. A milling machine would have been easier and faster.

What didn't quite work

• Using a piece of scrap aluminium for a stir stick. It melted. An old steel spoon would have been better - oxide and impurities float to the surface and can be scooped off.
• The wooden plugs. Well, actually, they did work even if they did catch fire. What didn't work so well was the thin aluminum strip intended to hold them the correct distance apart - with weight on it and heat from the molten zinc, it sagged down, pulling the plugs out of alignment. The plugs should also have been more accurately aligned on the centre line of the mould. They did save a lot of effort drilling out the zinc when it had cooled, even if they weren't perfectly aligned.

Tips

• If you overheat the zinc, it will catch fire.
• Bolts, nuts and washers should be clean (sanded if necessary) and greased before assembly. Nuts should be tight and make good electrical contact with the aluminium bar inside the zinc.
• Check the electrical connections from zinc to propeller and propeller tube with an ohmmeter.

Some Galvanic Potentials