Magnesium Anodes
07-16-2002, 05:56 PM
#1
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Magnesium Anodes
I just bought magnesium anodes for my drives. The Mercury dealer recommended them since they perform better in fresh water than zinc does. Without getting too technical, can someone explain this.
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07-16-2002, 10:34 PM
#5
Toxic FORMULA
Platinum Member
There are three types available. Zinc for salt , magnesium for fresh and yes, aluminum for either. Since the aluminum is bare , it goes before the aluminum case (as long as it's painted)
07-17-2002, 08:35 AM
#6
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RLW,
They are Merc parts with a sticker on them that states " fresh water only". These are the flat style trim tab anodes.
They are Merc parts with a sticker on them that states " fresh water only". These are the flat style trim tab anodes.
07-17-2002, 08:59 AM
#8
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When you place two disimilar metals in a conducting fluid they become a battery. Particles will travel from the anode metal to the cathode metal. Which is the cathode and which is the anode depends on which metal has a higher anodic index.
In our case Aluminium has a fairly high anodic index .95V (for cast) and the lake full of iron docks, earth ground etc is neutral and so the aluminium in the drive tends to corode.
If you place a more active metal attached to the aluminium drive it becomes the anode of the battery and hence the part that corodes,, the aluminium now becomes the cathode.
Salt water is a good conductor of electricity and so Zinc with an anode index of 1.25 V works well as a sacrificial anode.
Fresh water is not a good conductor of electricity and so the low difference in anodic voltage 1.25 - .95 = 0.30V will protect the areas imediately around the anode but may not do as good job on parts further away, the other side of the drive for example. Zinc anodes do work in fresh water just not as well as magnesium. ( Pure water does not conduct electricity at all but few of us boat in distilled water)
Magnesium has a higher anodeic index 1.75V and so the difference 1.75-.95= 0.80V does a better job of protecting the whole drive in fresh water.
Magnesium is not used in salt water even tho it will always protect better because of the higher anodic voltage the high voltage difference and good conduction of the salt water would cause the magnesium anode to corode very quickly and so need continuous replacement. Zinc works well enough in salt water because of the better conduction of the water even though the anodic voltage difference is less and they will last long enough not to be inconvient.
In our case Aluminium has a fairly high anodic index .95V (for cast) and the lake full of iron docks, earth ground etc is neutral and so the aluminium in the drive tends to corode.
If you place a more active metal attached to the aluminium drive it becomes the anode of the battery and hence the part that corodes,, the aluminium now becomes the cathode.
Salt water is a good conductor of electricity and so Zinc with an anode index of 1.25 V works well as a sacrificial anode.
Fresh water is not a good conductor of electricity and so the low difference in anodic voltage 1.25 - .95 = 0.30V will protect the areas imediately around the anode but may not do as good job on parts further away, the other side of the drive for example. Zinc anodes do work in fresh water just not as well as magnesium. ( Pure water does not conduct electricity at all but few of us boat in distilled water)
Magnesium has a higher anodeic index 1.75V and so the difference 1.75-.95= 0.80V does a better job of protecting the whole drive in fresh water.
Magnesium is not used in salt water even tho it will always protect better because of the higher anodic voltage the high voltage difference and good conduction of the salt water would cause the magnesium anode to corode very quickly and so need continuous replacement. Zinc works well enough in salt water because of the better conduction of the water even though the anodic voltage difference is less and they will last long enough not to be inconvient.
Last edited by ursus; 07-17-2002 at 12:43 PM.
07-17-2002, 09:03 AM
#9
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It was explained to me a while back why magnesium is better. The salt in saltwater makes the water a better conductor of electricity. Its the electricity (anode and cathode thing) that causes the corrosion. The zinc works better in salt because of its electrical and corrosion properties in that enviroment, and vice versa for the magnesium in fresh.
07-17-2002, 09:45 AM
#10
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Thread Starter
Thanks for the explaination.
formula31,
I'll check for the number and post it later tonight.
formula31,
I'll check for the number and post it later tonight.
