Anodes

R

Roy

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Jan 1, 2000
RO Number
114
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186
Can anyone explain how an aluminum anode protects an aluminum outdrive?

Thanks.......Roy
 
The difference in an aluminum anode vs. the drive is the alloy. Aluminum anodes meet MIL-A-24779 which means one of the primary alloying metals is indium as well as some zinc. With that alloy as you can see, the anode is more reactive and thus protective, as well as has a longer life (about 20%) over zinc.

http://boatzincs.blogspot.com/2013/03/what-are-technical-differences-between.html

If you want the specifics of the alloy, down load the spec here:

http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-A/MIL-A-24779_29464/
 
Wow - thanks. Does that mean that aluminum is actually a better material for salt water anodes than zinc?
 
It may be better in salt water than zinc. Depends on what you are trying to protect. It's more active than zinc and will last longer than zinc like Gee Bee said. I don't know anyone who has used it and wonder what the cost is?
 
"I don't know anyone who has used it and wonder what the cost is?"

Aluminum cost is about the same as zinc - I have never used aluminum so I cannot say how it works.
 
Boat zincs.com has them and you're correct about pricing. I've been thinking about switching.
 
If you read the "Mercruiser Guide to Corrosion" they will tell you that Al anodes are an absolute requirement in salt water (and Mg in fresh). They say it protects better and if you are in warranty they require it. All Mercruiser units come from the factory with Al anodes installed, so unless you put the new unit in fresh water without changing over to Mg, you most likely have seen them in use. So yes Roy, they are superior to standard zinc.

That all said, after fighting Bravo III corrosion for some time I would highly recommend anyone with a stern drive in salt or brackish water do a hull potential survey to make sure you have sufficient protection. For instance on my unit (fresh water), I require two Mercathodes in operating in tandem and Mg anodes including some extra units. Mercruiser IMHO has really good advice in their "Guide to Corrosion" but their engineers do not put that advice into their products. :(
 
As a mfr of "both" Zinc (Zn) and Aluminum (Al) COMMERCIAL anodes, sometimes we recommend zinc, sometimes aluminum, sometimes either one, sometimes neither one. There are general rules to go by, but there are also exceptions to those rules. The first determination is the water type (fresh, brackish, salt). There's also vessel usage habits and types of metal to protect.
Al anodes are 50 m/v more negative than Zn anodes and Al also has a longer life span than Zn. Furthermore, Al is lighter and costs less. Advantage Al! The negative with Al is the sustainability of current output over time in some situations. Al anodes can become less efficient and even passify over time without maintenance (cleaning). Vessel owners should be aware of this "potential" maintenance issue so protection is not lost if the anode quits working. The cause is the lack of sluffing, creating an oxide build up that can eventually crust over in times when low current transfer exists.

If the boat next to you is using one type of anode, it doesn't mean you should automatically be using the same. Boating habits can make a difference in what anodes to use. If a vessel is rarely used, say during the winter months, Al may need cleaning, even in salt water application and more-so in brackish situations, but certainly in fresh waters.

I'm not advocating any type of anode, just informing on real-world cases.
 
I have found with Mg anodes, a good sanding brings them back nicely.
 
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