The Boating Forum - Zinc or Aluminum Anodes

I am a little confused on which anodes are the correct ones for me. I have always used Zinc in the past, but now I am reading where aluminum will provide the same amount of protection, and last longer.
The anodes will be going on 225 opti's, soul use will be in salt water. Northern Gulf

I am wanting to replace all of them, are there kits available?

Is one brand better than the rest? ;?

Koilane - 4/15/2008 4:02 AM

I am a little confused on which anodes are the correct ones for me. I have always used Zinc in the past, but now I am reading where aluminum will provide the same amount of protection, and last longer.
The anodes will be going on 225 opti's, soul use will be in salt water. Northern Gulf

I am wanting to replace all of them, are there kits available?

Is one brand better than the rest? ;?

Zinc is for salt water. magnesium is for freshwater...at least I think that's wha I've heard.

BTW: Boating is good for the "soul"; but I'm sure the "sole" use of your boat will be in salt water. :)

Good catch!

One of my sources is West Marine

Quote

Zinc Anodes for Salt &Brackish Water:

Zinc Anodes are intended for us in highly corrosive saltwater environments


Aluminum Anodes for Salt & Brackish Water:

Aluminum Anodes for salt and brackish water will "last longer while providing the same level protection as Zinc". Because Martyr Aluminum Anodes are more active than zinc, they are able to work in a mix of fresh and salt water, where zinc will passivate and magnesium will corrode too rapidly.

End Quote

This quote is specifically speaking of Martyr Anodes by CMP Global

I have also run across other sources where they suggest Aluminum because of life span instead of the Zinc

No one type is better or more durable - the statement by Martyr is a little misleading, but I understand why they say that.
"last longer while providing the same protection" is not an entirely true statement. Aluminum lasts longer than magnesium and zinc last longer than aluminum, but if you have an anode that lasts too long for your environment, is doesn't dissolve and just gets crudded up.

So zinc in brackish waters wouldn't dissolve quickly enough, and as a result get crud all over it, at which point it is no longer working effectviely, so in that case, aluminum is a better choice for brackish. Magnesium works well but dissolves too quicly in brackish, so it too wouldn't last as long as aluminum in brackish. The net net is that alum is good for brackish.


However, for pure salt water applications, aluminum will act just like magnesium in brackish. They will dissolve too quickly. Do your anodes currently get a whitish crud that builds up over time?

I mixed and matched brands for zinc anodes, and noticed no difference. For aluminum, there was only one brand readily available as a kit for my outdrive so I went with them.


In a nutshell, zinc dissolves slowly, aluminum a little faster, and magnesium even faster, so zinc is the least active, and magnesium is the most active. Salt water conducts electricity better than brackish, and brackish water conducts better than fresh water. The saltier the water, the less active your anodes need to be. If your anodes are too active, you have more protection than you need, but they will dissolve too quickly. If your anodes are not active enough, you have inadequate protection, but the anodes will last forever, but get all cruddy.

So there is no one type that is better, but there are certain types that fit some cases better than others. If you are in 100% salt water, I think that the aluminum would dissolve too quickly. Keep in mind that aluminum anodes also cost more but weigh less, so Martyr makes a bigger profit if they can get you to buy alum instead of zinc.

That said, when I was in brackish water, my zinc anodes weren't dissolving quickly enough, so I switched to aluminum and changed then every spring.

Does it matter that you put aluminum zincs on aluminum drives? Seems counter productive.

All zinks are made out of zink. The brand doesnt matter.

Your motor lower unit is aluminum. Using an aluminum sacrificial anode on such in salt water is worthless.

Recommended by my motor mfgr:
Magnesium in fresh water
ZINC in salt water.

The Aluminum used in the Anodes is not the same type used in the outdrive. The Aluminum anodes on an outdrive will work better in Salt water than the Zinc ones will. Aluminum anodes do last longer than an equal sized zinc anode. In salt water.

I am a certified ABYC Corrosion Technician.

What Rival1 said. I know these things may be hard to understand; but a zinc anode is not pure zinc. The same goes for aluminum and Mg. These anodes are alloys and designed to protect hulls, shafts, props, outdrives in many different circumstances. I have seen lots of aluminum outdrives with blisters under the paint. This corrosion is caused by using zinc anodes in freshwater. You want the antode to be less "nobel" then the alloy you want to protect. Aluminum outdrives are best protected by Aluminum anodes in saltwater.
For my 35Bertram with stainless steel and nibral bronze below the waterline I can choose zinc anodes or Aluminum in saltwater. If I choose to mix them the aluminum anodes would protect my zinc anodes. So mixing anode types is stupid. I think mixing zinc anodes is not to smart either; but I do it. They may different alloys and one anode could end protection the others. Therefore not lasting long?
One more thing to think about is it is possible to have too much zinc protection.
Talk to Rival1 about that one or Nigel Calder.

I agree with Rival. I assume that the aluminum in the anode is of a different make up than the outdrive thus designed to work and be effective against corrosion. Thanks to all.

As stated, aluminum anodes are not just aluminum. They are, typically an alloy of aluminum, zinc, and indium. From what research I have done, the very small amount of indium in the alloy inhibits passivation formation (a non-conductive oxide layer causing the anode to not work). "Dissolving" is a bit of a simplification. It is a "sacrificial" anode. Anode material is being electrochemically attacked instead of you motor, fittings, etc. If it does not corrode/dissolve it is not working.

Woody

I once heard someone talking about how cheap his anodes were because they didn't last a whole season (pretty short up here, by the way).

I told him quality wasn't his problem, but something at his dock.

He had no idea what I was talking about.

I am a little confused on which anodes are the correct ones for me. I have always used Zinc in the past, but now I am reading where aluminum will provide the same amount of protection, and last longer.
The anodes will be going on 225 opti's, soul use will be in salt water. Northern Gulf

I am wanting to replace all of them, are there kits available?

Is one brand better than the rest? ;?

Zinc or aluminum. Aluminum will last longer that zinc and will not be a problem for aluminum castings on outboards or outdrives.
Do not mix the types of anodes. Either all zinc or all aluminum. That said, some engines (like my Yami) have sacraficial anodes bolted to the heads. Since they are zinc, I use zinc for the other anodes.
I have only seen zincs as separate parts, not as 'kits'

The Aluminum used in the Anodes is not the same type used in the outdrive. The Aluminum anodes on an outdrive will work better in Salt water than the Zinc ones will. Aluminum anodes do last longer than an equal sized zinc anode. In salt water.

I am a certified ABYC Corrosion Technician.


Aluminum is twice as expensive.

Do not mix the types of anodes. Either all zinc or all aluminum. That said, some engines (like my Yami) have sacraficial anodes bolted to the heads. Since they are zinc, I use zinc for the other anodes.
I have only seen zincs as separate parts, not as 'kits'

God point.

The anodes are alloys not pure Zinc anyway. Stay with the Manufacturer's Anodes and be assured to have consistency with the internal anodes.

You don't want one type of anode protected the other one. Could be false economy

Good stuff to read :

http://www.boatzincs.com/?gclid=CLXN3JCj8poCFRBbagod2Xj5cw