Zinc or Swim
Engine manufacturers, as well as boatbuilders, attempt to save their aluminum products by attaching sacrificial anodes. As long as the anode is electrically connected to the part, either by direct contact or by wire, it stuffs the aluminum with excess electrons so it loses those rather than the aluminum giving up its own electrons. The zinc? It erodes — sacrificing itself for the benefit of the boat or motor.
Unfortunately, there is no formula to tell you how many anodes you will need and what size. In the beginning, it will be a matter of trial and error. A good place to start is the ABYC’s procedure. Inspect the anodes every month. Go with many small ones rather than a few large ones. If you have the right amount, your “zincs” should be about halfway gone by the end of the season, and replaced each spring. If they are not wearing away, they are either too large (rare) or not making good contact with the aluminum (common). Well-made anodes should have cast-in plates and fasteners to maintain good electrical connection throughout their lives, and they should meet military spec Mil-A-18001J (or higher last letter).
Don’t go nuts with zincs everywhere because aluminum won’t tolerate being overprotected. In mild cases, the zinc develops a crust and stops working. But if things get too far out of balance, you can generate an alkaline solution that will start eating away at the aluminum. An early sign of this is the softening or blistering of the bottom paint.
How Much Zinc?
Use too few anodes, and the fittings are not protected. Use too many, and the anodes erode quickly and may blister paint. A multimeter and a reference electrode help to nail down the right amount.
1. Place a silver/silver chloride electrode in the water near the item that is to be protected.
2. Touch the positive probe to the fitting. Note the meter’s reading in DC millivolts.
3. Connect a sacrificial zinc of the proposed size to the metal part to be protected. Put the zinc in the water and note the new meter reading.
4. Protection is adequate when the new voltage is 200 millivolts (0.20 volts) more negative than the reading noted without the zinc.