There are many important parts of a boat that all have some functions that they perform. One of the most important parts is the Sacrificial Anode. It helps all other parts function protecting them from corrosion. What is a sacrificial anode?
A Sacrificial Anode is an Anode made from a less noble metal like aluminum or zinc. More electrically active, it protects the other two Metal Anodes when connected to them in seawater to make a circuit, & gives up its electrons, dissolving first while preventing Galvanic Corrosion on boat parts.
Easily missed by most people but a necessity for all sailors and boat owners to know about for the ultimate protection of their vessel. Here’s Why.
What Is Galvanic Corrosion
All metals immersed in an electrolyte sea water, for example, produce an electrical voltage. When two dissimilar metals are in contact or electrically connected then they produce a galvanic cell turning them into a battery, with the less noble metal, for example, a bronze propeller, on the boat, forming the anode and the more noble metal say a stainless steel shaft forming the cathode. Remember the cathode attracts cations or the cathode attracts (positive) + charge. The anode attracts (negative) – charge, creating a circuit.
One of the metals is constantly taking a beating and giving pieces of itself up as the current is existing uninterrupted. These pieces are called metal ions, used in order to make electrons that are needed for making an electric current. The process of a metal deteriorating over time due to electrical activity is known as Galvanic Corrosion, and on a boat, it can lead to some serious problems.
The most common places for galvanic corrosion to occur on a boat are on the propeller which is usually made of bronze or aluminum, the prop shaft, rudders, sterndrives, or outboard. If Galvanic Corrosion is not controlled, it can end up costing a boat owner a ton of money in damages, repairs, and even replacements.
In order to protect both metals, you need to connect a third metal that is more active than the first two. The most active metal zinc, for example, becomes the anode to the others and sacrifices itself by corroding giving up metal ions in order to protect the cathode hence the term sacrificial anode.
This is Galvanic Corrosion at work where one metal either (anode or cathode) makes a connection with another metal in Salt Water which is an Electrolyte solution that makes an Electrical current. Electrons are used in the process of deteriorating and destroying the anode that has more electrical potential eventually destroying metal parts under your boat.
To stop this from happening the solution is to add a third metal to the equation in between the connection of electric current. By doing this you sacrifice the important metals that are involved with the boat by giving themselves up. The Sacrifice Metal has to be more active than the other two. This metal becomes the Anode that is used by the other pair.
The three most Active materials that are used in Sacrificial Anodes are zinc, aluminum, and magnesium. The first property to consider is its 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
In order to provide protection, the highest practicable and the metal to be protected. The voltage difference possible is required between the sacrificial anode
For example, if zinc anode is used to protect a bronze propeller, a “driving or protecting voltage” of negative –0.75 volts will be available, i.e. zinc at -1.05 volts minus bronze at -0.3 = – 0.75 volts.
If aluminum anodes are used this increases to -0.8 Volts.
Magnesium anodes increase this to -1.3 volts.
The bigger the difference in voltage, the more protection you get.
How Does a Sacrificial Anode Work
Aluminum Alloy Anodes Are used on boats that travel in Brackish water. They are recommended for use on boats that are used in Salty water or Freshwater. They can generate 1.1 volts.
Magnesium Alloy Anodes are recommended and work well in the freshwater environment and are not recommended for Saltwater at all.
Sacrificial Anodes and Zincs- These types of Anodes are made to sacrifice themselves to the electrical current so your boat parts won’t have to. Sacrificial Anodes are typically made from zinc because it’s the best alloy that protects boats in the ocean. This is why Sacrificial Anodes are quite often referred
Sacrificial Anodes have a working life to them and will wear down and eventually be of no use leaving the boat owner at risk for damage to their boat. They also have to be installed the right way or the same bad result will happen
Besides the Electrical Potential, the next factor most important in Sacrificial Anodes is the Current Capacity of the anode material. The anode generates a voltage difference and this drives a current between the anode and the protected metal and through the water. The 3rd factor is Quality and that just means that you are getting what you are paying for. The most common material specs to look for are:
- Zinc: MIL-A-18001K
- Aluminum: MIL-A-24779(SH)
- Magnesium: MIL-A-21412
It’s like having a bigger battery, the more capacity you have the longer it will keep protecting. For a particular anode, the rate of current flow is dependent on the surface area of the anode and the longevity depends on the mass. So what they do and how long they do it is specified by the manufacturer and material size and shape.
Inboard boats with bronze or stainless metal parts can be protected using Zinc or Aluminum Anodes. Don’t worry about overkill from overprotecting. It doesn’t matter. The voltage added by Zinc or Aluminum won’t cause any damage. No matter how much anode material is added the maximum voltage won’t change. It will stay the voltage generated by the anode itself.
Sterndrives and Outboard Motors
Can require a little more care. The sacrificial anodes have a difficult task since they have to protect what is already a very active aluminum assembly. Initially, the anodes for these units were made of zinc, but in response to corrosion problems, Mercury and Johnson/Evinrude/OMC started selling the aluminum anodes in the early 1990s.
Other manufacturers are switching to aluminum too. The small increase in protective voltage helps ensure that the sterndrive is protected. If you use zinc anodes you may even invalidate your warranty! Again, be careful using magnesium anodes since you can overprotect your sterndrive or outboard.
Keep paint on Sterndrives and engines in good condition and keep Sterndrive immersed in water. If you don’t then the Anodes won’t be able to work. Don’t mix zinc anodes on the hull with aluminum anodes on the drive. The aluminum anodes will protect the zinc anodes in addition to the unit.
Where possible Navalloy™ is a combination of (aluminum/zinc/indium alloy) these anodes are recommended over zinc. Zinc anodes can become inactive after only a few months due to the build-up of an insulating film of zinc hydroxide. Aluminum anodes will remain active.
The ABYC (American Boat and Yacht Council), which set the standards for the industry, clarified their recommendations on anode materials in the Standards and Technical Information Reports for Small Craft (July 2008-2009)
How to Install Anodes On a Boat
For Sacrificial Anodes to be effective they have to be installed in the direct vicinity and as close to the metal that it is meant to be protected as possible. Metal to metal is the best option. To do this, mount the Anode itself to the metal that needs to be protected. You can also direct the two by using a wire.
It’s important that the surface of the metal is clean when you mount the Anode to it. This ensures the best electrical contact is made. When using Sacrificial anodes, you have to be sure that you don’t use too much or too little for total protection. Most boaters agree that 1-2% coverage of the surface area of the metal to be protected must be covered.
If you use too much zinc to protect your metal, it can crust over and stop working completely. Be sure that you are not over or under-using your anode! Sacrificial Anodes should be replaced once a year or when they are corroded to half their size. Premium Anodes have a Red Spot marker that will show when it’s time to replace them. Never paint over an Anode. If they are cover-up they won’t work.
Boat Parts at Risk of Galvanic Corrosion
- Propellers and Rudders- protected typically by Zinc
- Hull Plates-Multiple Zinc Plates
- Outdrives- Multiple Anodes are used for protection
- Heat Exchangers- preemptively fitted with a zinc “pencil” anode. They are usually installed beneath a brass plug in the exchanger.
The advantage of Sacrificial Anode systems over others is they need no external power source, are easy to install, the low voltage and current between the anode and the surface it is protecting infrequently generate stray current, overprotection is unlikely, and inspection and monitoring are simple if you know what to look for. To protect your investment.
Oxidation wears out metal parts frequently, causing them to deteriorate at faster rates if they are not protected by a Sacrificial Anode.
Saltwater contains electrolytes that create an electrically conductive solution. When metal parts from your boat are introduced into this solution, such as iron, bronze, or aluminum like the hulls, ship propellers, and outboard engines of a boat, they wouldn’t last very long.
References: Performance Metals- The Basics of Corrosion