Monday, April 20, 2009

Galvanic Corrosion: What is it and how can we prevent it?

I was originally hoping to create a light and entertaining article about outdrive galvanic corrosion. Yet, in the end, it is corrosion. How sexy can it possibly be? Nonetheless this is an important issue that should concern ALL boaters.

First, what exactly is corrosion? Corrosion is a process of deterioration when metal components are exposed to a wet environment. This occurs both underwater and in the atmosphere. This deterioration is the process of the metal reverting to its oxide form. Steel, for example, will degrade (oxidize) back to its natural stable state – rust (iron ore).

Are you getting excited yet? If not, read on…

Galvanic corrosion occurs when two dissimilar metals (copper and steel in the example below) are connected and submersed in an electrolyte (water) whereby an electric charge is created, like a DC battery. Negative electrons from the negatively charged metal (anode) will flow to the more positively charged metal (cathode). Left alone, the charged metal will corrode away to nothing.

If protection for both types of metal is desired, then a third more active metal must be added – a Sacrificial Anode. The most common metal is zinc although magnesium and aluminum are also used. This active metal becomes the anode for both metals. In the example below, the zinc or aluminum sacrifices itself to protect the other two metals, hence the term “sacrificial anode”.

On your boat these sacrificial anodes will protect the aluminum outdrive from corroding away. Without proper protection your outdrive could look like this:

For your boat these anodes come in three common forms – zinc, magnesium, or aluminum. Zinc can be used in saltwater or heavily polluted waters; magnesium can be used in fresh – but NEVER in salt or brackish water; and aluminum, which can be used in ALL environments and last up to 50% longer. This begs the question of course, how can an aluminum anode protect my aluminum outdrive from corroding? The answer is the aluminum anode is composed of an aluminum alloy that is more anodic than the aluminum in the outdrive.

Which Anode Material is Right for Your Boat?

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.

You have now taken the time to protect your boat properly. You tied your boat up in the slip - ready for a season of boating - proud that you have taken the right precautions regarding galvanic corrosion. However, is the boat tied up along side of you protected? If not, why should you care?

Connecting into shore power connects your ground to neighboring boats. If those boats are not protected properly by suitable anodes, your boat will be protecting both yours and theirs resulting in a rapid wearing of your anodes.

Other factors that affect corrosion are pollutants in the water, salinity, temperature, and the presence of stray electrical currents from incorrectly wired or insulated electrical equipment onboard your boat.

I hope you found this explanation enlightening and maybe even a bit entertaining. A little basic knowledge about corrosion will ensure a long and happy relationship between you and your equipment.

Many thanks to the people of Performance Metals of Bechtelsville PA, makers of NavalloyTM Anodes, for the information and graphics contained in this article. For more information on their products, please visit

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1 comment: said...

This is important info for boat owners - protect that valuable investment!