Anodes are mounted on an outboards lower unit, and sometimes on the lower edge of the outboards mounting bracket. referred to as Sacrificial Anodes, they are simply a disposable metal block attached to your boats underwater metal parts, that will sacrifice itself (corrode) first in order to prevent the important parts of your motor from corroding, such as your lower unit.
For Example, if two dissimilar metals are touching underwater, such as a stainless drive shaft, bronze bearings, and the aluminum housing of the lower unit, a naturally occurring transfer of electricity carries through the water from one metal to the other and causes the more active (softer) of the two metals to corrode. A sacrificial anode secured to the aluminum housing of the lower unit is now the most active metal in the system and corrodes before the aluminum of the lower unit. When the anode reaches 40%-50% of its original size, it is replaced to continue the protection cycle.
Sacrificial Anodes are commonly referred to as “a zinc”; however zinc is not the only anode type that protects underwater metals, aluminum and magnesium are also used; your outboards manufacturer will specify the proper material for a given environment.
In order for the Anode to do its job it must be able to conduct the electrical current from the water into the metal housing it’s protecting; in our case the aluminum of the outboard’s lower unit. As the Anode preforms its duty it will begin to “pit” and form a crust of oxidation that appears dull grey in most cases. The pitting process is normal and indicates the anode is doing its job. In some applications where the water is stained or muddy, the anode will form a coating of silica from the water. These coatings (silica and/or oxidation) have an effect to insulate the anode and inhibit its ability to conduct efficiently. Therefore anodes require periodic inspection and maintenance. The electrical connection between the anode and the outboard can be easily checked using an Ohm Meter; there should be zero ohms between the anode and the mounting surface on the outboard.
When maintenance is required, the anodes should be removed from the outboard and all surfaces’ cleaned to bear metal; the outboards mounting surface should be cleaned to bear metal also. Cleaning is accomplished by a vigorous buffing with a stiff wire brush until all oxidation or silica is removed, and bear metal is seen. (see photo’s)
Never paint a sacrificial anode, or the surface it mounts to. There needs to be a solid physical electrical contact between the anode and the metal requiring protection. If there is a break in this electrical continuity, the protection stops, resulting in corrosion of the important outboard parts.
Generally these are the rules for each body of water:
Salt Water: Use Zinc Anodes.
Brackish Water (Salt and Fresh mixed): Use Zinc Anodes.
Fresh Water: Use Magnesium Anodes.
Note:
The fresher the water, the less electrical current is transferred, requiring the more active anode. Magnesium anodes are ideal for fresh waters, as they are both economical and highly effective. They have excellent properties for corrosion protection in the higher resistive fresh water environment.
Bottom line, Anodes need to be clean and tight to efficiently do their job, so when preforming seasonal maintenance........
Don’t neglect your Anodes.
Notice the mounting area is cleaner that the rest of the anode but still oxidized
Before & after cleaning.
Wire brush can be found in painting or wielding departments of hardware stores.
Don't forget to clean the lower units mounting points.
IMHO, annual maintenance is fine, the big thing is to clean the contact points between the anode and the housing it's mounted to; if the contact point fails the system fails and your motor is unprotected.
Remember, we're talking about a very slow but steady process that only takes place while the boat is in the water; it's faster in saltwater, and slowest in freshwater.
EDIT 04/20/2013 15:18 EST - Added two pic's to cover items I missed above. Thanks for the reminder Jeff.
This is the Trolling Motor's anode on my MototGuide
This is the Trim-Tab anode on my Merc.
I don't have the common vertical fin, just a flat plate. This was a real bear to get off, I don't think its ever been off for maintenance; in fact it is so heavily covered with oxidation I could not get a meter reading, and the wire brush couldn't cut through it, I had to use 100 grit sandpaper.
