Magnesium anodes
03-30-2016, 05:10 AM
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Magnesium anodes
Hi everyone - anyone have experience using magnesium instead of zinc anodes? I DO NOT boat in salt water and from what I've read magnesium is much better than zinc.
Thoughts??
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Thoughts??
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03-30-2016, 06:25 AM
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As fas as I know:
Zinc for salt water
Aluminum for fresh and salt water
Magnesium for fresh water
Typically, the new outdrives come with the aluminum anodes.
I have a few boats that sit in the water (fresh water) at a dock in fresh water all season every year, One is a 94 and another an 86. Original drives if you can believe it. The drives don't have much corrosion. I replace the Merc aluminum anodes (all of them) about every 4-6 yrs or when needed.
Now, many other boats in same cove, bay get corroded very fast and the owners are always (every handful of years) scraping and repainting , trying different primers and paints.
I can't seem to get thru there heads all they have to do is replace their damn anodes. Again, the Merc aluminum ones have done their job well.
Zinc for salt water
Aluminum for fresh and salt water
Magnesium for fresh water
Typically, the new outdrives come with the aluminum anodes.
I have a few boats that sit in the water (fresh water) at a dock in fresh water all season every year, One is a 94 and another an 86. Original drives if you can believe it. The drives don't have much corrosion. I replace the Merc aluminum anodes (all of them) about every 4-6 yrs or when needed.
Now, many other boats in same cove, bay get corroded very fast and the owners are always (every handful of years) scraping and repainting , trying different primers and paints.
I can't seem to get thru there heads all they have to do is replace their damn anodes. Again, the Merc aluminum ones have done their job well.
Last edited by SB; 03-30-2016 at 06:32 AM.
03-30-2016, 09:30 AM
#3
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As fas as I know:
Zinc for salt water
Aluminum for fresh and salt water
Magnesium for fresh water
Typically, the new outdrives come with the aluminum anodes.
I have a few boats that sit in the water (fresh water) at a dock in fresh water all season every year, One is a 94 and another an 86. Original drives if you can believe it. The drives don't have much corrosion. I replace the Merc aluminum anodes (all of them) about every 4-6 yrs or when needed.
Now, many other boats in same cove, bay get corroded very fast and the owners are always (every handful of years) scraping and repainting , trying different primers and paints.
I can't seem to get thru there heads all they have to do is replace their damn anodes. Again, the Merc aluminum ones have done their job well.
Zinc for salt water
Aluminum for fresh and salt water
Magnesium for fresh water
Typically, the new outdrives come with the aluminum anodes.
I have a few boats that sit in the water (fresh water) at a dock in fresh water all season every year, One is a 94 and another an 86. Original drives if you can believe it. The drives don't have much corrosion. I replace the Merc aluminum anodes (all of them) about every 4-6 yrs or when needed.
Now, many other boats in same cove, bay get corroded very fast and the owners are always (every handful of years) scraping and repainting , trying different primers and paints.
I can't seem to get thru there heads all they have to do is replace their damn anodes. Again, the Merc aluminum ones have done their job well.
SB is right on. Mercruiser puts alum anodes on new drives as they are dual usage anodes. While the alum does ok in fresh water, magnesium anodes are far superior. I leave the VR1 at my dock for maybe a week at a time depending on usage with no corrosion. In doing this the anodes are completely ate up at the end of the season. The drives, tabs look great.
03-30-2016, 02:12 PM
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I was always told magnesium and used to use them, I got sick of replacing them what seemed like ALL the time, where aluminum would last several years. Maybe they work better/more effective, but aluminum has always worked just fine in my experience.
03-30-2016, 03:52 PM
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They say Magnesium can sometimes over protect (what ever that means) and that if aluminum works fine than no need to move up to the faster dissolving (more noble) Magnesium.
03-31-2016, 08:44 AM
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If you don't mind changing them more often, there is certainly nothing wrong with using them.
Last edited by BBCLiberator; 03-31-2016 at 08:52 AM.
03-31-2016, 09:17 AM
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As SB indicates, Magnesium can "overprotect". A galvanic cell exists when ALL of the following are present: An anode, a cathode, a direct electrical connection and an electrolyte. Notice we have just described a battery (as an aside the electrical potential between carbon and zinc is 1.9 volts, hence the traditional 1.5 volt battery is born). Note also that if you remove any one of the above mentioned components, corrosion stops. In the example of a battery, when it is stored on a shelf the direct electrical connection is removed and the battery lasts a long time. When a boat is out of the water, the electrolyte is removed and corrosion stops.
One simple method to prevent corrosion is to prevent contact between the electrolyte (water) and the metals to be protected. A common way to do this is by painting or otherwise coating the metals. With a perfect coating, there is no corrosion. Salt water is considered more "corrosive" than fresh water because it is more electrically conductive (is a better electrolyte). It is not corrosive in an of itself.
So given the absence of perfect coatings and given our preference for sexy uncoated (bare aluminum) lowers and such, we typically mount sacrificial anodes on our drives to protect them. Any material will do for an anode as long as it has a lower electrical potential than the material we are trying to protect (the cathode). So a simple choice right? Choose the lowest potential material for the anode and just replace them often. Well, perhaps not so fast. As the anode corrodes to protect the cathode, the material from that anode tries to plate the cathode (now you've not only invented the battery, but also chrome plating). This can be reactive enough to blister the cathode under the paint and lift the protective coating right off the drive which will in turn require more cathodic protection and faster anodic material loss as well as making your drive really ugly.
Another way to protect objects is to impress an electrical current on the parts to be protected and force small potential anodes (one grade of aluminum can protect another grade of aluminum even though the electrical potential between them is very small) to corrode. This is the basis of the Mercathode system and is why low potential aluminum anodes work well. It is also why it is very important to ensure all the bonding straps (electrical connections between each drive component) are in place. Recall that for corrosion to occur there must be a direct electrical connection between dissimilar materials, the same holds for protection.
So in short, aluminum anodes and a functional Mercathode system function very well together. The anodes last a long time, the paint doesn't get blasted off the drive and everybody is happy. Having said that, if you boat in exceptionally fresh water and have a lot of unprotected (unpainted) material in the water, then a good argument can be made for using a more aggressive anode, but be sure that you really need it.
One simple method to prevent corrosion is to prevent contact between the electrolyte (water) and the metals to be protected. A common way to do this is by painting or otherwise coating the metals. With a perfect coating, there is no corrosion. Salt water is considered more "corrosive" than fresh water because it is more electrically conductive (is a better electrolyte). It is not corrosive in an of itself.
So given the absence of perfect coatings and given our preference for sexy uncoated (bare aluminum) lowers and such, we typically mount sacrificial anodes on our drives to protect them. Any material will do for an anode as long as it has a lower electrical potential than the material we are trying to protect (the cathode). So a simple choice right? Choose the lowest potential material for the anode and just replace them often. Well, perhaps not so fast. As the anode corrodes to protect the cathode, the material from that anode tries to plate the cathode (now you've not only invented the battery, but also chrome plating). This can be reactive enough to blister the cathode under the paint and lift the protective coating right off the drive which will in turn require more cathodic protection and faster anodic material loss as well as making your drive really ugly.
Another way to protect objects is to impress an electrical current on the parts to be protected and force small potential anodes (one grade of aluminum can protect another grade of aluminum even though the electrical potential between them is very small) to corrode. This is the basis of the Mercathode system and is why low potential aluminum anodes work well. It is also why it is very important to ensure all the bonding straps (electrical connections between each drive component) are in place. Recall that for corrosion to occur there must be a direct electrical connection between dissimilar materials, the same holds for protection.
So in short, aluminum anodes and a functional Mercathode system function very well together. The anodes last a long time, the paint doesn't get blasted off the drive and everybody is happy. Having said that, if you boat in exceptionally fresh water and have a lot of unprotected (unpainted) material in the water, then a good argument can be made for using a more aggressive anode, but be sure that you really need it.
