Preventing Electrolytic Corrosion

Preventing Electrolytic Corrosion
What is a trim tab anode? Where is it located on my outboard and what is its purpose? Are there other anodes on the typical outboard engine? Do they ever need to be replaced?

These all are great questions.

Depending on the engine manufacturer, the trim tab anode is located above the prop on the underside of the anti-ventilation plate. Other common locations are under the engine bracket, and depending upon the number of cylinders, one anode per cylinder is usually bolted to the engine block under the cowling.

In this context anodes are sacrificial. Electrical current that exists in the water from a variety of sources, and also current generated by the propeller spinning through the water, can create electrolysis; a process that consumes or erodes unprotected metal surfaces. The anode is offered sacrificially, interrupting the process by satisfying the electrolysis, and thereby protecting the aluminum alloy castings of the engine.

The anode must be in contact with the water to function correctly.

The anode is bolted to the engine which is bonded to the battery negative with battery cables; this transfer of current is absorbed by the battery, thinking of the anode as a current magnet makes it easier to understand.
When to change?

The exterior anodes that contact the water should be changed every year at annual service time or if you keep your boat in the water for extended period, you should inspect every six months.

The remaining anodes are inspected annually only and replacement is as needed.

There are some engines with no external anodes and they must be added at the time the boat is rigged. Flats boats are critical.

The Yamaha SHO in the photo is a good example of an engine that does not offer a factory installed trim tab anode. The engine has a trim tab in the location described above but it is not designed for cathodic protection.

The lower unit paint starting to bubble is an indicator of poor cathodic protection whether it is the lack of anodes or poor bonding.

Happy New Year!
Chris Mapp
Coastal Bend Marine
Port O'Connor, TX