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Anodes, Zincs, Magnesium, Aluminum and Galvanic Corrosion on Boats

There are many different types of corrosion and few as are misunderstood as those involving electricity. In this article we are going to stick exclusively to galvanic corrosion involving anodes on your boat. Galvanic corrosion is quite a slow process which may take months or even years to do serious damage. There are other forms of corrosion that may look like galvanism but actually are very fast acting and may show up in hours, days or weeks. The likelyhood of the local "dock expert" knowing the difference is quite slim. Words and phrases like electrolysis, grounded, grounding, grounding conductor, grounded conductor, bonding, electrolytic and stray current are among the most mis-used in our industry. Beware of all marine electrical advice on the internet ! Please, please, please do not ever listen to or hire anyone who uses the term "electrolysis" when discussing corrosion on boats. This is the most commonly mis-used and misunderstood term in our industry. To learn more I suggest you read "Electrolysis Mythology".

Lets not call them "zincs",,,,,, they are "anodes" that may or may not be zinc.

For the purpose of this article we are going to stick to strictly galvanic corrosion as it applies to your "anodes".

Anode materials in order of decreasing galvanic activity ...

MAGNESIUM anodes, generate -1.5 Volts, i.e. negative 1.5 Volts. Should be used only in fresh water as they are so active they don't last long enough in highly conductive salt or brackish water to be useful.

ALUMINUM anodes, generate -1.1 Volts, i.e. negative 1.1 Volts The only ones that work and work well in salt, fresh and brackish water. and outlast the other two regardless of water due to its higher density..

ZINC anodes, generate - 1.03 Volts, i.e. negative 1.03 Volts and do not have high enough voltage potential to function in fresh water and mediocre performance (if at all in brackish water. Zinc also acquires an insulating,calcareous coating in fresh water which breaks the galvanic cell and renders them useless. This can happen within a few days of entering fresh water.

PS. We are going to stick with fiberglass boats here, as steel, aluminum and wooden boats are different animals.
Chart from ABYC E-2 Cathodic Protection shows aluminum as the only anode material they recommend for all waters
Chart from Navalloy shows why voltage potential of aluminum is
recommended for all waters
It is critical to use the correct anode material for the type of water you cruise, salt, fresh or brackish. Buying the correct anode is not as simple as going to the chandlery and asking for an aluminum alloy, magnesium or zinc anode. "Zincs". have become the "Kleenex" of the boating world, a generic term many apply to all anodes. "Anode" is the proper term. Zinc, magnesium and aluminum alloy being the three most common types of anodes. When you hear the term "Cathodic Protection" it refers to the protection offered in a cathode-anode cell.

On a recent visits to Ontario chandleries I found shelves of anodes mostly unidentified as to manufacturer, material or part number. Almost half of the anodes on the shelves were zinc which is a completely inappropriate material for use in the fresh water of the Great Lakes, some were aluminum and some were magnesium. Even if you find a part number on an anode and check the manufacturers website, they rarely relate that part number to a material. If your preferred chandlery displays their anodes like the photos below, tell them to clean up their act.

Boxes of unidentified anodes are common in most chandleries

Bins of unidentified anodes

Three rare ianodes identified by material !

Material ???

If you buy cheap anodes they are likely coming from the same people that bulked up their powdered milk with Melamine (Google it). Or maybe the same people that caused a massive recall of Groco seacocks due to rapid corrosion with their questionable grasp of metallurgy (Google this one too).
Buy nothing but US Mil.Spec anodes from name brand manufacturers. Saving $3 here will cost you in the long run. Some low quality anodes contain impurities that can cause the anode to consume itself through galvanic corrosion.

A sound electrical connection between the anode and the metal it's protecting is absolutely critical and easily confirmed.

An inexpensive multimeter set to Ohms (resistance) will instantly confirm if your anode has a good electrical connection to the metal it is supposed to protect, Touch one probe to the anode and one to the cathode (the metal being protected) and if you see "O.L" it means there is no electrical connection and therefore no protection. If you see more than 1.0 Ohms you have poor conductivity and the system is not working as well as it should. Standards call for a reading of less than 1.0ohm with a reading of 0.0 ohm being perfect.

People often clean the exposed surface anodes with a wire brush but if a steel brush is used it can contaminate the anode with other metals that can render it useless. Cleaning the surface without cleaning the contact area (underneath) is also useless.

Many people paint their stainless steel shafts with anti-fouling paint or even paint their anodes ! The paint does two negative things.

1st it prevents a good electrical contact of the anode if mounted on a painted surface and prevents the flow of electrons if painted over.

2nd if you paint your shaft you are also enhancing corrosion of the stainless steel by depriving it of the oxygen it needs to maintain it's anti-corrosion surface.

The outdrive shown at right has it's anti-fouling paint applied properly, with a 1.5" unpainted border around it. I/O manufacturers will void your warranty if the paint touches the outdrive. Many anti-fouling paints contain some kind of metal content (cuprous oxide, carbon, titanium oxide etc.) If anti-fouling paint is in contact with the aluminum drive, the drive may become the anode and the entire bottom of the boat becomes the cathode. This turns everything below the waterline into a big battery and is a major contributor to drive corrosion. What is inexplicable is that so many so called "pro's" still paint right up to the drive.... Guess they never read the manufacturers warranty disclaimer.

O.L = "Open Line" in other words there is no electrical connection between this zinc anode and the shaft

Unpainted border around outdrive
For an anode to work it requires a cathode which by definition is a metal more noble on the galvanic scale than the anode.

Are your anodes actually working ?
While a multi-meter set to Ohms will tell you if there is enough conductivity for the anode to work. It cannot tell you how much current is flowing and whether it is enough or too much, for that you need a
silver/silver chloride half cell (sometimes called a Corrosion Reference Electrode) which we should all have in our toolbox. I've provided the link so you can read up on that but It's really going beyond the scope of this article as my purpose here is just to get you using the correct anode material. This test can tell you by the voltage what material is on your boat but if you've bought the right anode material from a reputable manufacturer, you already know and the continuity test described above will suffice.
NOW THE SCIENCE (the simplified version)

Galvanic Corrosion - Electrolytic corrosion that occurs at the anode (zinc,magnesium,aluminum) of a galvanic cell. There are four elements required to form a galvanic cell, two metals of different potentials, a good electrical connection between those two metals and an electrolyte.

Electricity must go around in a circle. i.e. It must go back to where it came from or it simply will not flow therefore a galvanic cell requires these four things.
1. An anode.
2. A cathode.

3. An electrolyte (the water).
4. A electric connection (path) betweeen the anode and the cathode.

Galvanic couple (cell)
A galvanic couple (essentially a battery) occurs when there is a voltage potential difference between metals when these metals are in electrical contact and submersed in an electrolyte.

Note that "electrical contact" is underlined and the quality of this contact measured in Ohms is referred to as "continuity". The very definition of "galvanic couple" requires continuity between the metals involved. For an anode to do it's job there must be good continuity between the anode and the cathode (the metal it's supposed to protect) .Will the anode corrode without contact (continuity), of couse it will as all metals do by many other means, it just won't be protecting the other metals as it does.

Of two metals of different potential, say stainless steel and iron in a galvanic couple, the metal of lower nobility or lower on the galvanic scale (the iron) will give up electrons to the more noble metal and thereby corrode.

Anodes are designed to be the lowest nobility metals in the system and sacrifice (corrode) themselves to protect the other more expensive metals such as drive housings, propellers, propeller shafts, shaft struts and rudders. This action occurs faster in salt than fresh water due to the conductivity of the electrolyte involved although in rare cases, very polluted fresh water can be even more conductive than salt water. Water temperature also affects conductivity.

A vessel suffering from galvanic corrosion is the source of it's own problem. It is possible that corrosion may be caused by a neighboring vessel via "stray current corrosion" although far more rare than most believe. Even with stray current corrosion, in the vast majority of cases the subject vessel is the master of it's own disaster. Stray current corrosion is often referred to as the dreaded "electrolysis" ! Be wary of any advice from those who use that word. Electrolysis is an induced process found in industry and laboratories in which the anode is positive and the cathode is negative whereas on your boat it's the anode that is negative and the cathode which is positive Electrolysis simply cannot occur on a boat.

Sometimes people say "electrolysis" and mean "galvanic corrosion" , sometimes they mean "stray current corrosion" and sometimes they don't know what they mean.

Would you hire a technician to solve a corrosion issue if he didn't know the difference ?

Low quality anodes (often from foreign manufacturers) with impurities can actually corrode themselves as they can be cathodic and anodic due to contaminants in the metal.

If the aluminum (lower nobility) of an i/o drive or a saildrive is in contact with stainless steel (higher nobility) in fresh or salt water, the aluminum will corrode. This applies to any metals with differing potentials.

Pure water is not conductive but our fresh water lakes contain minerals and contaminents which are somewhat conductive and this may vary from harbour to harbour depending on local pollution from outflow or drainage etc. Salt water is much more conductive than our lakes and this is why we use different types of anodes as it takes more voltage to conduct through less conductive waters .... we are always trying to control the flow of electrons and this is affected by the electrolyte (water).

There must be contact (continuity) between the metals involved in order for current to flow. Because we are dealing with very low voltages (millivolts) resistance between the metals must be very low, preferably a fraction of an ohm or even 0.0ohms In order for the anode to sacrifice itself.

Serious galvanic corrosion is a lesser issue in fresh rather than salt water but it is inevitable and with the right (wrong) conditions it can cost big time ! Check yours before launch.

You may remember high school chemistry class where a battery was created by connecting two dissimilar metals with wire and immersing the whole contraption in water thereby activating magnetic fields and starting an electrochemical process causing current to flow.
On a boat with bronze, aluminum, manganese bronze, and stainless steel that are connected either with direct contact or with bonding wires and immersed in water...... you are in the same neighbourhood, you turn all of these metals into a big battery. The more noble metal is the "cathode", the less noble, the "anode". In this process the less noble metal gives up electrons to the more noble thus weakening the metal, otherwise known as "galvanic corrosion". The further apart these metals are on the galvanic scale (sometimes called the Noble Scale), the more corrosion occurs.

For some electrical horror stories take a look at AC/DC Nightmare Photos
The other type of corrosion involving Electricity on boats, Stray Current Corrosion often referred to improperly as "electrolysis".
If you are in fresh water and have concern for the safety of your family, take a look at Electric Shock Drowning.