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

Among the dozens of different types of corrosion, the two types of general concern to boaters are Stray Current Corrosion and Galvanic Corrosion. For this excercise we are going to stick to Galvanic Corrosion which is a simple process made more complicated by the widespread bunk on the internet. Words like electrolysis, grounding, bonding and other electrical terms are commonly misused and cause much confusion. Stray current corrosion is another discussion.

So it is with "zincs". which have become the "Kleenex" of the boating world. "Anode" is the proper term and zinc, magnesium and aluminum alloys are variants for use in different types of water. Curiously most new (almost all) boats are delivered to dealers in the Great Lakes with totally inappropriate zinc anodes !

On a recent visit to an Ontario chandlery I found shelves of anodes mostly unidentified as to material. Almost half of the anodes on the shelf were zinc which is a completely inappropriate material for use in the fresh water of the Great Lakes, some were aluminum-indium and some were magnesium, the only appropriate material for use in fresh water.
From the moment a metal is created it starts to return to it's orginal form. We call that corrosion. Some do it very quickly and some take a long time. Corrosion can be accelerated or delayed by various environmental conditions (salt water vs. fresh water etc.), electrical involvement and various other factors. All metals have a natural voltage potential and these potential voltages are listed in the Noble Scale (sometimes called Galvanic Scale).

When two metals with different potentials are in contact and submerged in an electrolyte (lake or seawater) current begins to flow from the less noble metal to the more noble metal. The further apart they are on the galvanic scale, the more dramatic the corrosion. This current carries electrons away from the less noble metal and we call this electrolytic corrosion. Electrolytic corrosion simply means corrosion involving electricity whether from natural voltage potentials or external sources.

Galvanic couple (cell)
If the aluminum (lower nobility) of an i/o drive or a saildrive is in contact with stainless steel (higher nobility), the aluminum will corrode. This applies to any metal with differing potentials. Anode materials 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.

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.

Pure water is not conductive but our fresh water lakes contain 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 different types of anodes .... we are always trying to balance the flow of electrons and this is affected by the electrolyte (water).

The Mercury Marine website says all drives are shipped with aluminum anodes but advises magnesium for fresh water !

I could not even get continuity across the surface of the anode. Bottom line... about 96% of the anodes were not doing their job. Check yours before launch.

Galvanic Corrosion - Corrosion that occurs at the anode (zinc,magnesium,aluminum) of a galvanic cell.

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, galvanized 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.

Galvanic Couple - All metals have a natural voltage potential. A galvanic couple occurs when there is a voltage potential difference between metals. This occurs when these metals are immersed in a conductive or corrosive solution (lake or sea) and the metals are put in contact or electrically connected and an electron flow between them results.

Note that "contact or electrically connected" are 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 metal (cathode) it's supposed to protect.Will the anode corrode without contact (continuity), of couse it will as all metals do, it just won't be protecting the other metals as it does.

Get the correct material. For the Great Lakes use magnesium anodes. If you are heading south switch to zinc and if you are going to keep you boat in the brackish water of the upper Chesapeake you'd be wise to switch to aluminum. Getting the correct type of anode is only half the story. There must be a galvanic couple for the anode to do its job. So first a couple of definitions .........

In a galvanic couple, the higher resistance metal turns cathodic while the less resistant one becomes anodic. Typically, the cathodic material undergoes little or no corrosion at all in a galvanic couple. Due to the unlike metals that are involved and the electric currents, the type of corrosion is referred to as two-metal or galvanic corrosion. .

The "sacrificial" anodes on your shafts, trim tabs etc. are supposed to sacrifice themselves thereby protecting expensive metal parts. This is why it's important to keep your anodes or "zinc's" in good condition and never paint them.

A vessel suffering from galvanic corrosion is usually the source of it's own problem, although two vessel's linked by shore power grounds can create a galvanic cell between two very close boats, more on this later.

Low quality metals with impurities can cause corrosion to themselves as they can be cathodic and anodic due to contaminants in the metal.

Stray current corrosion and electrolysis (please don't use that word, it is simply incorrect in discussion of boat corrosion) are related topics that can be read about on other pages.

Complicating this picture somewhat is the fact that DC can be super-imposed on your AC wiring through the common ground on board or the ground in the shore power pedestal we all share on the dock. As all vessels in the marina are connected through shorepower grounds there is potential for widespread damage. Aside from concerns of corrosion there is also potential for electrocution if shorepower cords are allowed to lie in the water let alone the fools that leave their shorepower cord plugged in at the dock while they go out for an afternoon cruise.

Recent studies have shown that AC current from shorepower in the water can also cause corrosion to underwater parts although at a much slower rate than DC. This has been a long argued issue by people who know a lot more about this than me. Ground fault protection systems, galvanic isolators, isolation transformers and impressed current systems are some of the various methods used in an attempt to combat corrosion.

Salt water is a more serious breeding ground for marine corrosion as the salt makes it more conductive however, polluted fresh water however can be even more conductive with the right contaminants although pure fresh water is not conductive at all..

With our aging fleet of pleasure craft it's likely that at some time, less than expert hands have played with your electrical system. If your vessel is suffering from any electrical faults or unusual corrosion consult with an American Boat and Yacht Council Certified marine electrical technician with specific corrosion control training or give me a call and I will try to set you up with an expert in this field.

When two different metals are in contact in a corrosive environment, one of the metals experiences accelerated galvanic corrosion while the other metal remains galvanically protected.

Metals near each other in the galvanic series have little effect on each other. Generally, as the separation between metals in the series increases, the corroding effect on the metal higher in the series increases as well.

Relative surface areas of contacting dissimilar metals is also relevant in determining which metal exhibits accelerated corrosion. It is undesirable to have a large cathode surface in contact with a relatively small anode surface.

Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals experiences an accelerated corrosion rate. The contacting metals form a bimetallic couple because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow.