Galvanic Corrosion & How it Relates to Boaters





Galvanic Corrosion, What is It? ;  Galvanic corrosion (sometimes erroneously called electrolysis) can damage or destroy underwater metal parts of boats, dock hardware and other equipment.   When two different metals are touching each other or are electrically connected by a conductor, and are immersed in an electrolyte (an electrically conductive fluid, like salt water) an electro-chemical reaction can occur.  This is not totally dependent on being immersed, as the two different metals can be simply NEAR salt air (it just takes longer).  One of the metals (the “least noble” metal, called the anode) will corrode faster than it normally would, and the other (the “most noble” metal, or the cathode) will dissolve more slowly.  These being steel, even stainless, connected to say an aluminum boat hull.


The seawater Galvanic Sequence lists metals in order, based on their voltage potential and tendency to corrode. More active metals, the faster-dissolving anodes, are at the top of the series. Passive metals, the cathodes, are located at the bottom. Your boat might have a collection of submerged, electrically-connected metal parts immersed in salt water surrounding the boat, in bilge water or the water in the engine cooling system.  The stainless prop shaft, for example, because it is more active (less noble) than the bronze propeller, will begin to dissolve, leaving the prop intact.


It all has to do with the voltage of the anode when referenced against a standard. The standard is an electrode made of silver/silver chloride. That just means they are relative:  The higher the anode’s voltage, the more freely it gives up electrons and the more able it is to feed corrosion.  We are talking about negative voltage here, so wrap your mind around the concept of “more negative voltage.”


What metals are used in sacrificial anodes?  The three most active materials used in sacrificial anodes are zinc, aluminum and magnesium. They have different properties and uses.
The first property to consider is their electrical potential.  All metals generate a negative voltage (as compared to a reference electrode) when immersed in water.  The lower – the more
negative - the voltage, the more active the metal is considered to be, for example:
Magnesium generates -1.6 Volts, i.e. negative 1.6 volts.
Aluminum sacrificial anode alloy generates -1.1Volts
Zinc, -1.05 Volts
Now compare these voltages, by comparison, to metals commonly used in marine use:-
Bronze generates -0.3 Volts
Steel, -0.6 Volts
Aluminum, such as that used in hulls, stern-drives or outboard motors generates -0.75 Volts.
Here you see a lower unit's aluminum eaten away to the point it is not salvageable.  Notice the stainless steel prop seems untouched


How to Prevent it ;  Breaking the electrical circuit between exposed metals by connecting them to a sacrificial anode will prevent galvanic corrosion.  A sacrificial anode is an inexpensive piece of metal that is less noble than any other metal found on the boat, and it is electrically connected, bolted or wired into electrical contact with the other metal components.  Bolting a sacrificial anode made from zinc, aluminum or magnesium (located near the top of the Galvanic Sequence) to the stainless prop shaft will protect it from corrosion.  The anode and the stainless shaft will form a galvanic couple, and the anode will dissolve, keeping your prop shaft spinning happily. Sacrificial anodes can extend the life of your boat’s hull, engine, rudder, propeller shaft, engine cooling system, refrigeration condenser and other metal components by protecting them from the deterioration caused by galvanic corrosion.


Here a anode is mounted on a fiberglass hull Here is a anode designed to fit a large outboard motor  


The Importance of Marine-Grade Fasteners ;  Galvanic corrosion can also happen in a single piece of hardware, particularly alloys, which contain more than one metal.  Interactions between the differing metals in the alloy will dissolve the least noble, the most common example of this being the dissolution of zinc from many zinc/copper alloys (such as brass and some bronzes) leaving a weak, spongy copper residue. That’s why it’s so important to use marine-grade fasteners (bronze, monel or 316 stainless steel) in underwater applications so they are not destroyed, allowing a hose clamp to fail or your propeller to fall off.  Then the higher the salinity and temperature of the water, the greater the likelihood of corrosion.


Zinc or Aluminum for Salt Water ;  If you are a saltwater boater, you should install zinc or aluminum anodes to prevent galvanic corrosion on the engine and underwater parts of your boat.


Magnesium for Fresh Water ONLY ;  Since fresh water is much less conductive than salt water, magnesium anodes are the best choice because they’re more active (less noble) than zinc or aluminum so they will protect your engine parts more effectively.  Caution: DO NOT use magnesium anodes in any application other than fresh water because they will corrode rapidly, exposing your boat and engine to possible damage.


Aluminum for Brackish Water;   More active than zinc, aluminum anodes are a good compromise where fresh and salt water mix, such as the brackish water of river deltas, or if you use your boat in a variety of water types.  Zinc is too passive for brackish water, where it gets covered in a zinc oxide film and becomes inactive.  Magnesium corrodes too rapidly (due to the water’s salt content), leaving your boat without adequate protection.


When to Replace an Anode ;  The effectiveness of an anode depends on a good electrical connection, and is directly proportional to its surface area.  As it corrodes away its surface area and effectiveness diminishes.  Some manufacturers suggest replacing anodes when they are two thirds gone, but one major marine supply store recommends replacing them when half-eroded or half-dissolved.  If an anode is allowed to dissolve completely, the next least noble piece of metal in the circuit will start to dissolve.  And that might be a part of your engine.  Remember that polluted water, warm water temperatures and stray current corrosion can cause your anodes to wear away at an accelerated rate, and it’s smart to check them regularly.




On a aluminum boat, you will find usually find a Zinc anode bolted to a underwater bracket on the stern.  Some manufactures of aluminum boats weld two of these to each stern, and utilize one as a sonar transducer mount.


On your outboard motor, depending on manufacturer and age, if it as above about a 10 hp you may find a zinc anode attached to an underwater part.  This is a lead like looking material, either bolted onto or bolted into a cavity on the underwater part of the motor.


You really need to inspect them often, especially if you moor your boat.  And do not paint over them during your spring revitalization as this restricts the anode from function as designed.   Even a wire wheel may not remove much of the oxidation, so you may have to use a angle head grinder with a sander pad to get down to new metal.


Another thing that can be very detrimental is if you happen to moor your boat in a marina, especially in salt water, if other more permanently moored boats are using a battery charger to maintain their batteries, AND do not have proper grounding or zincs, there may be a electrical charge in the water more than normal.


An important thing to remember is that you must complete a "circuit" for the anode to be effective, which means that the zinc must be in direct contact with the metal components of your boat located below the waterline.  This is the reason that a hanging grouper-style of anode comes with a wire and metal clip that is intended for attachment to a "ground" within your boat. When attaching a zinc anode to a prop or shaft, please be certain that the point of surface attachment is clean and "bright" so a good contact is made, completing the circuit.  Also, never paint over a zinc with bottom paint, which will only isolate and protect the anode from its intended purpose.


For an inboard motored boat that is left moored most of the time, keeping these anodes in good shape could be the difference of being a proud boat owner and being a sunken boat owner if ANY of the through the hull fittings corrode to the point you have a very bad leak.






 Now What is Electrolytic Corrosion?   Now what is the difference between this and galvanic corrosion?   If you see an accelerated corrosion on the metal parts of your boat located below the waterline, you are the victim of "electrolytic corrosion".  The scientific term "electrolytic" corrosion describes an accelerated type of corrosion that occurs when an electric current is added to the water surrounding your boat (usually at a dock). This corrosion is typically caused by faulty wiring that permits an electric current to enter the water.  This corrosion, combined with galvanic corrosion, is also an electromechanical action which causes metal parts to decompose, but at a very accelerated level.  This destructive process is also caused by electrolysis, which is an electric current set up between the metal parts of your boat, with salt water as the electrolyte, but it can be much more damaging in a very short time.

You can see the eating away on shafts, propellers, rudders, and other fittings and drive parts under the water as a pitting on the surface.   The results are unattractive, affect operating performance, and cause excessive maintenance of hull, propellers, engine, accessories and fittings.  Some metals react more actively than others and will be eaten up first.  This activeness is the reason that a steel fitting can corrode away while a brass one next to it will not (at first).  Both these parts could be protected, however, by a properly installed alloy anode.  The anode will be eaten away and the steel and brass fittings will be untouched.  The sacrificing action of the anode is the reason it is called a sacrificial anode, and the source of the name "Martyr".

The protection of the underwater metal parts on your boat or motor from corrosive electrolytic action is a very real and necessary concern.  The use of an activated alloy such as an “anode” will help.  Anodes will distract the corrosion action to itself and away from expensive metal parts.  Fortunately anodes are relatively inexpensive and are readily available.  Factors such as warm water temperatures, polluted water and stray current corrosion can cause your anodes to waste away at an accelerated rate so it is wise to check them on a regular basis.

Aluminum boats regularly used in salt water, the owner needs to be especially informed and act accordingly.  Not just the visible parts but the hidden surfaces, like under the floorboards.  If your boat is like most small boats under 22' or so where any water on the deck goes into the bilge to then be pumped out, this should probably le looked at every two to three years.  And especially if you are buying a used boat that has saltwater experience.


Corrosion Under the Convertible Top or Bow Cover Snaps ;   When I bought the used aluminum open bowed boat, since I occasionally fish the ocean and have seen water come over the bow, this could be disastrous in this instance, so soon afterwards I had a upholsterer fabricate a bow cover.  Since this aluminum boat was new to me as a somewhat ocean boat, I noticed that the convertible top's male snaps had a Nylon washer between the snap and the aluminum hull.   I mentioned this to the upholster.   His comment was that they are not needed.  OK, I found out 3 years later that he was right, FOR FIBERGLASS BOATS, but he did not know what he was talking about for aluminum boats and of course he is now out of business.


I got a corrosion under about all of these snaps.  This entailed drilling all of them out, sanding down to bare metal, spraying zinc chromate on the aluminum, priming and then repainting much of the forward deck.    After the painting was done, I used anti-seize compound on any surface that came in contact with the aluminum.  This was coated on the new Nylon washer that went under the snaps and all the pop rivets before they were installed.


Then the open center of the male rivets were filled with Silicone caulking to again try to keep out any salt corrosion.  Any bolt on fastener was also coated with anti-seize.  And any bolt on unit was separated from the hull by placing sections of plastic milk jugs between it and the hull.

Here you can see the corrosion under the paint around the snaps More corrosion at this one Here is the one on the left, showing all the corrosion that was scraped loose
Not Zinc ?

In recent years cadmium in zinc has become an environmental concern, leading to a movement in the direction of aluminum anodes.  Such anodes are effective even for protecting aluminum components--lower end cases, for example--because the aluminum used in the anode is a more anodic alloy.  Aluminum alloy anodes are almost certainly to become more common.  It has not happened already only because the cost of aluminum anodes has been higher than zinc without any discernable benefit to the boat owner.  Today aluminum is actually cheaper than zinc.  In addition, aluminum anodes tend to last longer, they work better than zinc in brackish water (and maybe in salt water as well) and they appear to be better for the environment.  When making the switch from zinc to aluminum, ALL of your anodes must be aluminum.  This can be a problem in some locales as many local marine suppliers still do not stock a wide selection of aluminum anodes.  That however will eventually change.



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Copyright © 2015 LeeRoy Wisner  All Rights Reserved


Originated 06-28-2015, Last updated 07-03-2015
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