The Importance of Sacrificial Anodes on a Boat
Sacrificial anodes help protect the metal parts of your boat that come in contact with salt water. They should be inspected regularly and changed as needed. Boat owners and operators should know what to look for to verify the sacrificial anodes are working properly, and know what not to do with their anodes.
What Do Sacrificial Anodes Do?
A sacrificial anode is a reactive piece of metal connected to a component to protect it from corrosion. They do this by being more reactive than whatever metal they are meant to protect. In boating, they are most common on engines and generators, but can be found on any metal component that comes in contact with water. The reason they are necessary is that all metal has some level of reaction to coming in contact with an electrolyte solution. An electrolyte solution is some type of fluid, typically water, with any amount of salt in it. In this case, salt is really anything other than the hydrogen and oxygen molecules that form water. Sodium chloride in particular makes an excellent electrolyte solution. Most motors are made out of aluminum, which is a fairly reactive metal and prone to corrosion. This means that the aluminum needs to be protected to prevent severe corrosion, where the water you are moving through will gradually eat away at the metal that composes the engine.
What Sacrificial Anodes Are Made Of
Sacrificial Anodes are commonly made out of two types of metal. The most common material is Zinc. Zinc is so commonly used that many people don’t refer to sacrificial anodes by their name and just call them “Zincs”. Zinc is commonly used because it is more reactive than aluminum, but not so reactive that it melts away too fast. Magnesium is occasionally used on boats that only ever touch fresh water because it is even more reactive than Zinc. The greater the difference in the metals' reactiveness, the more effective they are at protecting the metal they are connected to. The reason Magnesium is not commonly used in salt water applications is that it is so reactive that the Magnesium anodes would not last to the next service, and once the anode is degraded to a certain point, it is no longer protecting the component.
There are more sacrificial anodes on your engine than you probably realize. Most boaters are aware of the large anodes on their lower units and on the power trim and tilt parts of their outboard engines. Fewer boaters are aware of the anodes that manufacturers added to the raw water-cooling passages on many outboard engines. These are interesting because you should consider changing them more frequently than many manufacturers recommend, because an interesting issue occurs if you change them at the recommended interval. Part of the reaction process of Zinc and salt water is that the Zinc starts to expand, sometimes larger than the allotted space that the manufacturers allowed for the removal of the anode in the cooling passages. That means that if you wait to change these anodes to the regularly recommended maintenance interval, which is typically during the 200-hour or 300-hour service, the anodes may not be able to be removed. I typically recommend changing the anodes located in the cooling passages every 100-hour service; otherwise, there is a possibility that these will only be able to be changed during a major overhaul of the engines, which usually only occurs every 1000 hours or more.
Common Mistakes People Make with Anodes
One of the most common mistakes people make with sacrificial anodes is painting them. Painting an anode prevents it from working properly. You want the anode to be in as much contact with both the metal it is protecting and the electrolyte solution it is placed in. If you paint the anode, all you are doing is protecting the anode and preventing it from protecting the engine. Let the anode do its job and be a sacrifice. If the anode is not degrading over time, there is something wrong with how the anode is installed. They should degrade with time and use.
Another common issue that is seen is improper contact with the metal they are supposed to protect. Anodes only protect metal they are in contact with through some type of metal bond. If a piece of metal is isolated from the sacrificial anode, it will not be protected by the anode. Some manufacturers attempt to overcome this by using metal bonding wires to connect separated components; others use additional anodes to ensure as many components as possible have sacrificial anode protection. Both are viable means of protecting components, but if a component is not protected by one of these methods, then it will start to corrode, because it is not protected. It is important for the boat operator to regularly inspect metal components that come in contact with the water to verify everything is wearing appropriately. If sacrificial anodes are not degrading with use, then it means they may not be doing their job, which means the component they are meant to protect may be corroding more than it should. Some signs of this can be paint on the motor bubbling, especially below the water line.
Sacrificial anodes should be replaced when they are approximately 50% of their original size or mass. Once they are less than 70% of the original size, anodes become significantly less effective. This advice applies best for visible anodes. Anodes that are hidden in the engine cooling passages should be changed more regularly.
Why the Boat Operator Should Know
Knowing the basic components of your boat makes you a better boater and protects your boat for future use. Knowing what to look for helps keep your boat and its components in the best condition possible. Knowing this information is also useful bit of information to make sure whoever is maintaining your boat knows what they are doing.