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Old 20-06-2016, 20:13   #1
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Magnesium anode for cast iron

Has anyone out there tried a magnesium anode on a cast-iron keel ( dont know what species of cast iron) in salt water?
I see they dont recommend it for steel in salt water but is that only because of the rate it is used up or are there other drawbacks?
The reason I ask is because I have access to a free source of junk magnesuim gas turbine casings & would not care if they get eaten very fast as I will be using them on a wire grounded to keel & hanging over the side in the marina. Have already cut pieces off casings with a grinder & cut off wheel. Makes for a spectacular flame if you press too hard on the grinder
Keel is painted with Primocon aluminium primer & Ultra anti-foul & currently using zinc anode bolted on keel. Anybody with expertise in cathodic protection please weigh in as well. Probably tell me it's a Baldrik plan but oh well just another of many
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Old 25-06-2016, 07:33   #2
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Re: Magnesium anode for cast iron

Magnesium is more reactive than zinc, so yeah it will go away faster than zinc. It will protect your zinc bolted to the hull. Don' t see the harm if the wire is bonded well to the keel. Trial and error on how much to use, but search out how to use multimeter and silver probe to measure to see if you are protected.

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Old 25-06-2016, 08:11   #3
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Re: Magnesium anode for cast iron

The concern with your iron keel is rust, not galvanic or electrolytic corrosion. An anode (be it zinc, magnesium or aluminum) is not going to protect you from that. Don't waste your time.
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Old 25-06-2016, 08:42   #4
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Re: Magnesium anode for cast iron

The diver dude is 100% correct. A anode only protects from galvanic corrosion when two or more dissimilar metals are in circuit / contact with each other. A zinc on a iron keel will not protect the keel from oxygen reaction to iron.
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Old 25-06-2016, 09:04   #5
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Re: Magnesium anode for cast iron

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The diver dude is 100% correct. A anode only protects from galvanic corrosion when two or more dissimilar metals are in circuit / contact with each other. A zinc on a iron keel will not protect the keel from oxygen reaction to iron.
Ah but the anti-foul paint is loaded with copper & the primer contains aluminium surely that would qualify for galvanic corrosion? Not to mention the keel bolts but dont know what alloy they are.
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Old 25-06-2016, 09:36   #6
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Re: Magnesium anode for cast iron

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Ah but the anti-foul paint is loaded with copper & the primer contains aluminium surely that would qualify for galvanic corrosion? Not to mention the keel bolts but dont know what alloy they are.
Generally the keel bolt metals ( SS) are close enough to iron in the galvanic series to not be a big problem. Aluminum is an anode to iron, so again not a problem for the keel.

Copper loaded bottom paint... well most of the time you have an epoxy barrier coat or three between the lead keel and copper bottom paint. The epoxy coat being there to prevent the formation of iron oxide. In any case the rate of corrosion from oxygen in the water would be FAR higher then galvanic corrosion from copper in the paint.
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Old 25-06-2016, 10:15   #7
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Re: Magnesium anode for cast iron

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Ah but the anti-foul paint is loaded with copper & the primer contains aluminium surely that would qualify for galvanic corrosion? Not to mention the keel bolts but dont know what alloy they are.
Anodic protection is a function of anode surface area relative to the item being protected. You hanging a piece of magnesium (and magnesium anodes are an alloy, not simply hunks of random machine casings. Magnesium is not appropriate for use as an anode in saltwater in any event) over the side isn't going to get the job done. And, once again, the only corrosion you are going to experience regarding your keel is rust.

You're trying to create a solution to a problem that doesn't exist.
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Old 25-06-2016, 16:37   #8
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Re: Magnesium anode for cast iron

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Generally the keel bolt metals ( SS) are close enough to iron in the galvanic series to not be a big problem. Aluminum is an anode to iron, so again not a problem for the keel.

Copper loaded bottom paint... well most of the time you have an epoxy barrier coat or three between the lead keel and copper bottom paint. The epoxy coat being there to prevent the formation of iron oxide. In any case the rate of corrosion from oxygen in the water would be FAR higher then galvanic corrosion from copper in the paint.
Na dont have epoxy barrier coat between/below paint layers but was planning to do that on next haul out. A friend of mine who had a steel keel left his zinc anode off ( took a fellow boaties advice) & his rate of rusting increased dramatically. According to your reasoning it should have no effect. Maybe there was other factors. Thanks for the input from the Bay area.
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Old 25-06-2016, 17:57   #9
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Re: Magnesium anode for cast iron

There's the option of copper free bottom paint next time.
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Old 25-06-2016, 18:01   #10
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Re: Magnesium anode for cast iron

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A friend of mine who had a steel keel left his zinc anode off ( took a fellow boaties advice) & his rate of rusting increased dramatically. According to your reasoning it should have no effect. Maybe there was other factors. Thanks for the input from the Bay area.
Oh brother
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Old 25-06-2016, 19:48   #11
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Re: Magnesium anode for cast iron

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Originally Posted by Compass790 View Post
Na dont have epoxy barrier coat between/below paint layers but was planning to do that on next haul out. A friend of mine who had a steel keel left his zinc anode off ( took a fellow boaties advice) & his rate of rusting increased dramatically. According to your reasoning it should have no effect. Maybe there was other factors. Thanks for the input from the Bay area.
The "friend of mine" thing carries no weight. I am a certified Corrosion Analyst and Fstbttms is 100% correct.
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Old 25-06-2016, 22:54   #12
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Re: Magnesium anode for cast iron

Pure magnesium burns. I can remember the teacher lighting magnesium ribbon to demonstrate this in high school chemistry. What I suspect you have is some sort of magnesium alloy which is becoming more common as the drive to lighten vehicles for gas mileage progresses. Whilst magnesium sits above zinc and aluminium re voltage potential difference generated I doubt the alloys it is used in would enjoy this position. Do the multimeter trick with a piece of it in an SS sink submerged in water. Zinc should give you about 0.6Volts. Try it with your stuff and see if you get somewhere near this. Easy one to check empirically (suck it and see in the vernacular)






I vaguely remember a story that the reason
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Old 26-06-2016, 07:31   #13
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Re: Magnesium anode for cast iron

So for my education, I misunderstood the intent of the OP to protect the keel from corrosion. Keeping the salt water away from the cast iron is probably the best way to do that. I assume in a fiberglass or wooden boat with a cast iron keel that there is still a need for anodes tied into the bonding system to protect thru-hulls, rudders, etc. Is the keel also tied into the bonding system, and if so, would it matter if the anodes are placed on the keel or some other location, like hanging over the side or on the transom?

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Old 26-06-2016, 07:40   #14
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Re: Magnesium anode for cast iron

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...would it matter if the anodes are placed on the keel or some other location, like hanging over the side or on the transom?
The longer the electrical path from the anode to the item being protected, the less protection is provided. Same goes for resistance. The more wire, connectors etc., the greater the resistance. Which means less protection.

BTW- the bonding of all underwater metals is far from universally accepted as a good idea.
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Old 26-06-2016, 09:26   #15
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Re: Magnesium anode for cast iron

I know. Case in point, I have one bronze thru-hulls in the bow of my 40 footer electrically isolated and it has held up well after 30 years. Thanks for the reply.

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