- Galvanic corrosion is a reaction between two dissimilar metals commonly found on your boats.
- Zinc anodes are for use in saltwater while magnesium (or aluminum) are for freshwater use of your boat.
- When the anode has degraded by 1/3 to 1/2, it is recommended that you replace it.
What Is Galvanic Reaction?
Galvanic reaction is the principle upon which batteries are based. Think of your boat as a battery; the water you operate your boat in is the electrolyte. The less noble metals on your boat, such as a stainless steel propeller shaft, represent the negative post on a battery while the more noble metals on your boat, like a bronze propeller, are the positive post on a battery. The current flow between the dissimilar metals can damage or destroy the less noble metals on your boat.
Because dissimilar metals and alloys have different electrode potentials, when your boat is in the water, the different metals of the boat will act as anodes or cathodes. The water provides a means for ion migration whereby metallic ions move from the anode to the cathode within the metal. This leads to the metal at the anode corroding more quickly than it otherwise would. Subsequently, corrosion at the cathode is inhibited.
When two metals are submerged, the rate of corrosion is determined by the difference in nobility. Which is measured as a difference in voltage potential. The less noble metal, the one with the lower (more negative) electrode potential, will function as the anode and will experience galvanic corrosion.
Where Can These Metals Be Found?
The common cathodic and anodic medals that can be found on your boat are listed below. Remember, when dissimilar metals are closer together on the nobility scale, the less likely they are to corrode over time regardless of whether they are touching directly or connected via a conductor.
Relative Nobility Of Metals
Most Noble - Cathodic: less galvanic corrosion
Less Noble - Anodic: more galvanic corrosion
How To Protect Your Boat From Galvanic Corrosion
You may have noticed a series of green wires in your bilge running throughout your boat. These wires connect each of the metal components (thru hulls fittings, seacocks, engine, etc.) of the boat to a sacrificial anode. A zinc anode is preferred for saltwater boats and an anode made from magnesium is used for the fresh water ones. Typically, this anode is mounted outside of the boat on the transom; several bolts secure the anode to the boat. The green wires in your bilge are connected to the anode at the transom inside of the hull. These zinc and magnesium anodes sacrifice themselves by attracting the electric current flowing from the more noble metals in order to protect the critical metal components on your boat. Over time the anode will eventually break down and must be replaced.
The external metal components on your boat like propeller shafts, propellers, rudders, trim tabs and outdrives, should all have anodes directly attached to them to create this "controlled" galvanic corrosion. Again, the anode mounted to these pieces of equipment would be zinc for saltwater use and magnesium or aluminum for freshwater use of your boat. Sacrificial anodes will 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.
The connections of the green conducting wires you see in your bilge need to be checked regularly in order to help prevent premature damage. These wires and their connections are known as bonding wires and make up the bonding system on your boat when they are properly connected to the anode. Bonding of the metal components inside your boat will ensure that each of them will be at the same electrical potential. These wires should never lay in bilge water as that will accelerate the corrosion process. Any break in the bonding wires to the anode will cause the metal components to no longer be protected from galvanic corrosion. If working properly, these anodes will eventually degrade. Typically, when the anode has degraded by one third to one half, it is recommended that you replace it. This is also true for anodes directly connected to the external metal components.
You need to check your anodes if you see pitting on your propeller shaft or outdrive. If your bronze propeller is turning shades of pink, then your propeller is probably being attacked by galvanic corrosion. Again, this would be the time to check the anode. Your boat engine will have its own anode or sets of anodes depending on the size and type of engine in your boat. Your generator will also have at least one anode. Regular inspection and replacement of these anodes are critical to the performance of your engine and generator.
Understanding how and why galvanic corrosion occurs will allow you to help prevent damage and destruction to your boat. Regular inspection and replacement of your anodes will extend the life of all of your equipment both onboard and externally fitted. Controlling corrosion through sacrificial anodes can prevent a possible sinking of your boat. The lesson to be learned is that an "ounce of prevention" really does work when it comes to combating galvanic corrosion.