Magnesium versus aluminum anodes

Ralph vaughn

Member
Nov 14, 2018
259
Atlanta Ga
Boat Info
2007 Sea Ray 290 radar & GPS, triple axle trailer. 2006 Sea Ray 280 radar & GPS & triple axle tlr
Engines
5.0 MPI closed cooling Sea Core engines & Bravo III outdrives
4.3 MPI with alpha outdrives
My 2008 290 is on s trailer and I will be splitting my boating between saltwater and freshwater probably about 100 hours in the water. The rest of the time the boat is on a trailer and parked in a covered RV storage lot. Should I use magnesium or aluminum anodes. Who sells the best anodes.

Thanks for answering another newby question
 
Magnesium - never in saltwater,
Aluminum - ok for freshwater but run the risk of not enough reactivity.
So, your only choice is Al but watch it closely. In my area, aluminum definitely does not work in our freshwater. One season with Al and the drive needs sanded and painted.
 
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Al is probably the best compromise, unless you want to buy a set of Zinc and Magnesiums, and then swap them each time you go into different waters (I think that's unnecessary as Al should do just fine).

I second the recommendation on boatzincs.com -- lightning fast order processing and fair prices.
 
If you want a better aluminum anode, look at those made by Performance Metals. Nothing wrong with BoatZincs, but they're just your normal, run of the mill anode.

As Paul mentioned, though, the "best anode" is often determined by the actual area. Al can work really well in fresh water - while in some areas, you may get paint lifting/bubbling. Local water quality makes all the difference.

Because you're trailer boating, you'll have much less issues - and can easily ID and fix any paint lifting issues before it becomes a real issue. Typically, the first place to start lifting would directly in front of the props.

Also... make sure your Mercathode system is working properly. That's very important with a B3. In some areas, guys upgrade to the HD Mercathode system, or add a transom mount kit, for better protection.
 
A high quality, needs-sensing, impressed current system may work for you. Electro Guard (out of California) specializes in these systems. The system we have constantly monitors the conditions to keep millivolts in the -50 to 150 range to protect marine aluminum.

Otherwise, magnesium in saltwater is too reactive and will blister you paint. Zinc in fresh water will form a coating and quit working. If you split-time you need to use aluminum, swap them, or get something like the Electro Guard system.
 
If you want a better aluminum anode, look at those made by Performance Metals. Nothing wrong with BoatZincs, but they're just your normal, run of the mill anode.

I use Performance Metals Navalloy AL anodes in Fresh water and they work fine. I can't find MG in all the sizes that I need. AL doesn't last as long as zinc in salt water, but zinc does not work in fresh water...

http://www.performancemetals.com/images/pdfs/PMI Catalog 15.pdf

Here's an interesting article that I saved:

Aluminum vs. Zinc
ZINC AGAIN: SO YOU THINK YOU KNOW THE DIFFERENCE BETWEEN ZINC AND ALUMINUM SACRIFICIAL ANODES?

BY Steve D' Antonio POSTED March 27, 2014



While commonly referred to simply as “zincs,” sacrificial anodes are in fact available in several different alloys including aluminum, magnesium and zinc, although the latter is the most common. (All anodes contain alloying and trace elements of other metals.) In fact, each metal is best suited to a different environment. Traditionally, fiberglass vessels operating in seawater employ anodes made from a zinc alloy; although ubiquitous, however, they are not necessarily the most desirable option.

Sacrificial anodes are used for one purpose: to prevent or deter corrosion of underwater metals such as propellers, bronze through-hull fittings, and stainless-steel propeller shafts and rudderstocks. (Once again contrary to popular belief, most props are made of a brass alloy such as manganese bronze, which contains zinc and is particularly susceptible to corrosion if not protected by an anode.) Each of these metals should be protected by a sacrificial anode, either directly or indirectly through the vessel’s bonding system.


Regarding galvanic corrosion, each metal possesses its own natural corrosion potential, measured in millivolts: the more “negative” they are, the more prone to corrosion. The goal of the anode, then, is always to be more negative than the metal it’s protecting. The relative energy capacity of zinc is 368 amp hours per pound with a voltage of negative 1,050 millivolts. While that’s adequate in most cases, zinc anodes possess one flaw: When used in brackish or fresh water they are prone to developing a calcareous coating, a whitish material that essentially puts a zinc anode to sleep. Many sailors mistakenly perceive these especially “long-lived” zincs as effective. In reality, they’re anything but; even where anodes are still present, they provide no anodic protection at all. An anode that lasts for an inordinately long time probably isn’t working for any number of reasons, calcareous coating or otherwise.

Aluminum anodes, alternatively, offer several advantages. They’re immune to the calcareous coating menace and are therefore well suited for use in seawater, as well as brackish and fresh water. Aluminum anodes also pack more of a punch; they either last longer than zinc anodes of the same weight, or provide the same protection as zinc in a lighter package. Their relative energy capacity is 1,108 amp hours per pound (significantly more than zinc), with a voltage of negative 1,100 millivolts. If you opt for aluminum anodes, be sure that the change is universal for all anodes used within the same bonding system.

Surprisingly, aluminum anodes are often cost no more than zinc ones. Also, because most are cadmium-free, aluminum anodes are less of a hazard to the marine environment. So, regardless of what a diver, boatyard manager or other industry pro tells you, there are several advantages, and no drawbacks, to switching to aluminum anodes.

As for magnesium anodes, these work in fresh water only. Don’t use them if operating your boat in the briny.

Finally, when purchasing anodes, select those that meet stringent military specification standards: A-24779, A-18001 or A-21412 (for aluminum, zinc and magnesium, respectively). These standards ensure the purity of the alloy, an important factor in an anode’s effectiveness, resistance to self-fouling and longevity. Inexpensive anodes often represent false economy.


_Steve D’Antonio offers services for boat owners and boat buyers through his company, Steve D’Antonio Marine Consulting. This article first appeared in the January 2014 issue of Cruising World.
_
 
Good article. Thanks.
I have some skepticism on the claim that aluminum is superior to zincs in all cases in saltwater. I have read articles explaining zinc, in some circumstances, is superior. https://citimarinestore.com/citiguide/when-to-use-zinc-anodes-over-aluminum-anodes-on-a-boat/

Not being a scientist myself, I’m just a dumb boat-breaker that reads a lot.

For the original posters purpose, trailered mostly, split-time salt/fresh, clearly aluminum is the best choice, in my opinion.
 
A high quality, needs-sensing, impressed current system may work for you. Electro Guard (out of California) specializes in these systems. The system we have constantly monitors the conditions to keep millivolts in the -50 to 150 range to protect marine aluminum.
I didn't google or look into what you're writing about, but is this similar to the standard Mercathode system he already has?
 
Good article. Thanks.
I have some skepticism on the claim that aluminum is superior to zincs in all cases in saltwater. I have read articles explaining zinc, in some circumstances, is superior. https://citimarinestore.com/citiguide/when-to-use-zinc-anodes-over-aluminum-anodes-on-a-boat/

Not being a scientist myself, I’m just a dumb boat-breaker that reads a lot.

For the original posters purpose, trailered mostly, split-time salt/fresh, clearly aluminum is the best choice, in my opinion.
I think zinc is always superior in salt water...when I would move from fresh to salt, my AL anodes would be worn out at my first month's diver's report and I would have him replace with zinc and they would last much longer...
 
I didn't google or look into what you're writing about, but is this similar to the standard Mercathode system he already has?
I believe the Mercathode system is less variable in its output, from my understanding. Sort of a dumb system. It sort of puts out X millivolts. Probably fine since Mercury designs the system for a complete engine package in a known boat. The EG has an electronic control box that senses the need and varies output according to conditions. Since EG is a small company specializing in marine corrosion, they custom design the system for client needs. Since he is drastically changing his environment from highly conductive saltwater to non-conductive fresh water, he may be better off with a more variable system. Just a suggestion to research.
 
Thanks everyone for the plethora of good information.
 

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