I pull mine every service (100 hours), and usually just wire wheel them as long as they are at least 50% or so. I think I replaced them around 1000 hours, but every outboard will be different depending on use and the anode condition on the boat itself. For a long time aluminum boat manufacturers were using zinc anodes, which are pretty much useless. The aluminum outboard anodes took all of the abuse in that case.

 

Anodes just need to be visually checked. If you run in salt or brackish water, and they look clean/pristine, then something is very wrong (lousy mechanical or electrical connection?) and you need to get to the bottom of it. They should like white from the galvanic action in salt water. If they are white, but they are not all cruddy lucking or pitted and they have not notably reduced in size, they are good to go after a little cleaning. Anodes get 'eaten up' if they are doing their job, so if they are obviously undersized from the original size, time to replace them. Usually not very expensive, so I always replace once any wear is noticeable. Frequency of replacement is a function of how long you moor your boat, where you moor your boat, how 'hot' the marina is where you moor your boat, and whether your boat is properly wired/bonded. Vessels with shore power are a different story and it gets more complicated: a properly installed galvanic isolator will help keep corrosion at a minimum on those vessels.

 

Anodes just need to be visually checked. If you run in salt or brackish water, and they look clean/pristine, then something is very wrong (lousy mechanical or electrical connection?) and you need to get to the bottom of it. They should like white from the galvanic action in salt water. If they are white, but they are not all cruddy lucking or pitted and they have not notably reduced in size, they are good to go after a little cleaning. Anodes get 'eaten up' if they are doing their job, so if they are obviously undersized from the original size, time to replace them. Usually not very expensive, so I always replace once any wear is noticeable. Frequency of replacement is a function of how long you moor your boat, where you moor your boat, how 'hot' the marina is where you moor your boat, and whether your boat is properly wired/bonded. Vessels with shore power are a different story and it gets more complicated: a properly installed galvanic isolator will help keep galvanic action at a minimum on those vessels.

Thanks for the thorough explanation. It now leads me to another question.

I moor my boat in the Willamette River near Portland for about four months each Spring during Springer season.
I then moor my boat on Dock C of the West Mooring Basin at Astoria for the month of August.
I do not have electricity at either moorage,
The remainder of the time my boat is kept in my garage.
The external anodes on my Yamaha 150 have minimal wear. The aluminum anodes on the stern of my boat show much more wear/corrosion. I touch them up with a wire brush and have them replaced (thanks Quiet Riot) when needed.

For the past five years or so, I've been experiencing what I assume is electrolysis. The paint is bubbling and in some cases flaking. Is this and my question about anodes connected? Is galvanic action and electrolysis the same thing?

Sorry if I omitted important details.

 

All sorts of terms get bandied about on the Internet: galvanic corrosion, electrolytic corrosion, stray current corrosion, electrolysis, crevice corrosion and some authors get adamant about when each term should be used.

First is electrolysis. This describes the reaction when you submerge two electrodes (metals) in an electrolyte (such as sulphuric acid in a lead acid battery or seawater when talking about our vessel corrosion). In seawater electrolysis, as you pass current through the electrolyte, you will get hydrogen at the cathode and oxygen plus chlorine at the anode: the anode oxidizes and the cathode reduces chemical species. I used this chemistry to design and implement an onboard chlorine generator that produces chlorine by passing 8000 amps DC through seawater. Fun project that used the principles of electrolysis to kill foreign species in ballast water tanks.

So electrolysis is occurring any time you create current through seawater. This current can be from 'stray current' corrosion or galvanic corrosion.

Galvanic corrosion occurs whenever you submerge dissimilar metals in an electrolyte. Such as an aluminum hull with a stainless fastener.

Stray current corrosion, sometimes called electrolytic corrosion, occurs from applying currents improperly to any metal in contact with the water.

From a practical perspective, galvanic corrosion is something you want to control by establishing your hull and engines at one potential so the galvanic action of your anodes bear the entire brunt of the galvanic corrosion. On your aluminum vessel you want to ensure your hull and your motors are at the same potential by bonding them all to one common bonding bar, and making sure your hull and anodes are thoroughly mechanically/electrically connected to that bond. Ideally, your aluminum alloy anodes should be welded to your aluminum hull to ensure a corrosion free bond. This ensures your hull will have minimal galvanic corrosion.

However when you moor your boat, you become susceptible to stray current corrosion caused by your own boat (miswiring) and any nearby boats that are improperly wired. Your boat should be wired so the hull and all metal are bonded, but wired so no current flows through the hull. Lots of items can be miswired to cause current flow through your hull: fuel level sensors are culprits, wiper motors are culprits, led lights are culprits all because they incorrectly have chassis electrical connections (usually because they are designed for automotive use, not marine use). And the moment you add any AC wiring to your vessel, you have a way more complex (and dangerous) grounding/bonding scenario.

The simple answer to your situation is to do a review of your grounding and bonding wiring on your boat and to determine if your moorage is at fault for your corrosion. The definitive way to check your boat at moorage is to use a DVM and a silver silver chloride reference cell to verify all potentials of all metal in contact with the water. This measurement will show improper voltages that need to be addressed by rewiring and/or bonding of anodes.

 

Thank you Positize and Matt Park for the detailed responses.

Positize, you suggested: The simple answer to your situation is to do a review of your grounding and bonding wiring on your boat

Any suggestions where to begin? I have minimal mechanical aptitude and ability and when it comes to electricity, I'm at a total loss. Are there individuals or companies that offer this service?

FYI, For the last four years, I have used Jon's Mobile Marine Repair for my maintenance and repair needs.

Thanks for taking the time to help educate me.