Metal Corrosion

How to identify and prevent galvanic corrosion on your boat

This article is not meant to be a scientific explanation of galvanic corrosion, but we do want to talk about how you can identify galvanic corrosion on your boat and what you can do to prevent it. We have seen corrosion on our clients’ boats that has completely damaged props, shafts and thru-hull fittings. In extreme cases it can reduce metals to powder and because it typically occurs below the waterline, it is hard to notice before it is too late.

If you are looking for more information on the science, there is an excellent book that was written quite a few years ago by Bob Ajeman called Boat and Yacht Corrosion Control, I would also recommend Corrosion, Parts One and Two by Nigel Calder and The Boatowner’s Guide to Corrosion: A Complete Reference for Boatowners and Marine Professionals by Everett Collier.

The definition of corrosion is the destruction of a metal or an alloy by a chemical or an electrochemical reaction with its environment. The Galvanic Series or Noble Scale lists metals in order of how active or noble they are. In electronics, the more active is called the anode and the less active is called the cathode. When two metals are placed in water, while electrically connected, the less noble or base metal will experience galvanic corrosion.

The Galvanic Series or Noble Scale

The two most common types of metal corrosion on your boat are galvanic action and electrolysis. Galvanic action is the electrical current formed when dissimilar metals are placed in saltwater, for example a copper propeller on a stainless steel shaft. Electrolysis, often referred to as induced current, stray current or voltage leak, can be caused by a number of different things including improperly grounded equipment, loose connections, a reversed polarity appliance or even the power on the dock.

The destructive effects of both look quite similar but the source is completely different. Galvanic corrosion tends to be a slow process that leaves encrusted corrosion while electrolysis damages equipment very quickly and leaves bright patches in the corroded metal as it leaves the raw surface exposed.

A permanently installed corrosion monitor is useful for instantly detecting stray current in a marina.

If you have seen any of these symptoms on your boat, we recommend starting with a Corrosion Test. You can perform this test yourself with a portable, analog unit such as the ProMariner Corrosion Test Meter (approx. $400). They are millivoltmeters with a probe that is attached to the metal (a thru-hull, for example) and a silver chloride half cell that is thrown over the side of your boat and immersed in the water. The readings will tell you, very quickly, whether the metals on your boat are being eroded or being protected. This simple test can also tell you if you are over or under zinced.

We sometimes recommend that our clients install a permanent corrosion tester such as the ProMariner Deluxe Corrosion Monitor (approx. $350). This will allow you to immediately determine if there is any stray current, which is particularly useful when you are pulling into a new marina or mooring next to another boat.

A Corrosion Survey is simply the process of performing this test and recording the readings for each metal fitting connected to the water. This can be a time consuming task so we also recommend bonding your boat. Basically, bonding provides an electrical path between all the metals on your boat that come in contact with water to a sacrificial anode. The sacrificial metal will erode first, protecting the boat metal that is bonded to it. If you have taken your boat out of the water and noticed that one zinc is worn far more than the others, then your boat is probably not bonded correctly.

How to identify galvanic corrosion?

Blistering of the hull
White powder on metals and pitting
Green colouring on metal

How do I prevent galvanic corrosion?

The first thing to do in your fight against corrosion is to install anodes on your boat. Anodes are made from zinc (saltwater), magnesium (fresh water) or aluminum (fresh and salt) and attach to the hull, shafts, trim tabs, and so on. Ensure that they are bonded.

The next step is to install a “fail-safe” Galvanic isolator, also known as a zinc saver, between the AC shore power cord and your DC bonding system. This will stop zinc loss to other boats and to the dock while maintaining AC safety ground protection. For more information on galvanic isolators, read the April 2012 Tech Talk column in Pacific Yachting.

More Prevention Tips

Use marine-rated electrical equipment only
Use quality multi-stranded, tinned wire with quality terminals
Keep electrical wiring out of damp areas
Eliminate fuses and switches on the grounded negative side
Return wiring should be at least as large as positive cable
Check your vessel for reversed polarity
Use high quality bottom paint

Galvanic corrosion is destructive and can amount to costly repairs, but our boats live in water so metal breaking down is a fact of life. However, as discussed, there are a number of things that you can do to protect your boat from corrosion.

About the author: Jeff Cote is the owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: www.pysystems.ca.