[mikeyt]

Just as a basic primer on outdrive protection: Anodes act as sacrificial lambs for the outdrive while Cathodes act as a protector of the outdrive against corrosion due to the electrical action between 2 dissimilar metals in water. Now that that is out of the way.....

This is straight from the Mercury website:

1. All MerCruiser sterndrives come standard with aluminum sacrificial anodes.
2. Bravo sterndrives are also equipped with a MerCathode which are optional on Alphas.
3. Magnesium anodes are suggested for fresh water use ONLY.

If you have a Bravo 3 outdrive, it already comes equipped with a Mercathode protection system.

What is a MerCathode and what does it do?

The Mercury Precision MerCathode system provides automatic protection against galvanic corrosion. It is a solid-state device that operates off a boat's 12-volt battery and provides protection by impressing a reverse blocking current that stops the destructive flow of galvanic currents. If your power package does not come already equipped with a MerCathode system, Mercury strongly recommends that you install one, particularly if your boat is equipped with a stainless steel propeller or other submerged stainless steel hardware.

What is a galvanic isolator and what does it do?

The Mercury Precision Galvanic Isolator is a solid-state device that is series connected in line into the boat's green safety grounding lead ahead of all grounding connections on the boat. This device functions as a filter, blocking the flow of destructive low voltage galvanic (DC) currents, but still maintaining the integrity of the safety grounding circuit. A galvanic isolator is only necessary on boats utilizing a shore power connection.


For those who dont have the Mercathode or other similar cathode protection systems then basically there are 3 main types of Anodes used on outdrives

Zinc Anodes
Zinc is the most common material used in saltwater - like boating at the Jersey Shore or the southern end of the Chesapeake Bay. When used in freshwater, Zinc anodes can form a coating after only a few months of use. This coating can stop them from working. It is a good policy to change them regularly, even if they look OK. Remember, if an anode doesn’t wear away it is NOT working!

NavalloyTM (Aluminum) Anodes*
The Aluminum alloy used in anodes is very different from normal aluminum. It includes about 5% zinc and a trace of Indium, which prevents the build up of an oxide layer.
Aluminum alloy anodes provide more protection and last longer than zinc. It will continue to work in freshwater and is safe for use in salt water. Aluminum is the only anode that is safe for all applications. Use these when boating in the Delaware River near Philadelphia, the northern parts of the Chesapeake Bay, or acidic lakes like Lake Wallenpaupack.

Magnesium Anodes
Magnesium is the most active metal on the galvanic scale. It can be used in freshwater, but care must be exercised. Magnesium can over-protect aluminum hulls or outdrives in salt or brackish water or even polluted freshwater, causing paint to be lifted with resulting corrosion. Even a few hours immersion can cause severe damage. Many boaters on Cayuga Lake in New York State are switching to magnesium for extra protection.

Here's a link to an easy to understand article on outdrive corrosion protection & how to choose which anode (or cathode) is right for your application.

Peters Marine Service Newsletter / Blog: Galvanic Corrosion: What is it and how can we prevent it?

Hope this helps!