Choosing a Sacrificial Anode
What is a sacrificial anode, and how does it work?
A sacrificial anode is a metal rod or electrode that is made from a material that is more easily corroded than the metal it is protecting. When the anode is connected to the metal object, it attracts corrosion away from the object, sacrificing itself in the process and extending the lifespan of the protected metal. The current will flow from the newly introduced anode, and the protected metal becomes cathodic, creating a galvanic cell. The oxidation reactions are transferred from the metal surface to the galvanic anode and will be sacrificed in favor of the protected metal structure. This process occurs via "galvanic corrosion". Galvanic corrosion occurs when two metals with different levels of electrical activity, say a bronze propeller and a stainless steel prop shaft, are immersed in the same conductive liquid (like seawater). The liquid allows a weak electric current (electrons) to flow from the more active metal (the anode) to the less active one (the cathode). This is the same type of chemical reaction that occurs in a battery circuit. As the electric current flows, the more active metal (the anode) gives up electrons to the other and slowly dissolves in the process – not good if the anode happens to be your propeller!
A sacrificial anode is a third type of metal, say aluminum or zinc, which is installed because it is even more electrically active than either of the two original metals. When electrically connected to them by seawater, it becomes the material that gives up electrons and dissolves – thus sacrificing itself while preserving the original two metals. As long as you keep replacing the sacrificial anode, such as an aluminum anode rod before it has dissolved, the other less active metal components of your boat remain protected.
Often, non-zinc anodes are actually the better choice. Here's a quick summary, followed by a more in-depth look at this topic:
- Zinc works well in saltwater and has been the traditional anode metal for a long time, but it's also the heaviest option and is more toxic for the environment.
- Aluminum is lightweight and works in both salt AND brackish waters. In addition, they may last up to 50% longer in saltwater.
- Magnesium is the best anode to use in freshwater but does not perform as well in salt water.
The Metals
- Zinc is the traditional anode material because, in the old days, it was relatively easy to obtain. While zinc works adequately in saltwater, it will not protect your boat in fresh or brackish water. Additionally, not only is zinc itself toxic in the environment, zinc anodes must also contain extremely toxic cadmium metal as an activator. Zinc anodes are available for both recreational and commercial applications.
- Aluminum anodes are rapidly replacing zinc for use on ALL hull materials. With the recent advent of alloys developed by the US Navy, anode aluminum is very different from the aluminum alloys used for boat hulls, outboards, and stern drives. These "mil-spec" aluminum anodes are more electrically active and protect better than zinc; plus, they last longer! Aluminum has been used in the offshore industry for years to protect installations where long-term corrosion protection is essential. Aluminum anodes also use a much less toxic activator – which makes them better for the environment. To top it off, aluminum anodes are really the only choice that will work in both brackish AND salt water. So, if you keep your boat in a place that is exposed to both (such as at the mouth of a river that empties into the ocean), you should definitely be using aluminum. Aluminum has become the anode of choice for the US Navy, as well as for large commercial fleets. This is due to the money they save (aluminum anodes last longer, so ships go longer between replacements) and because aluminum is so much lighter that it can help provide better fuel economy in larger fleets. Aluminum anodes are available for both recreational and commercial applications.
- Magnesium anodes are the most active and are the only anodes that work well in the low conductivity of fresh water. Magnesium is also relatively non-toxic to aquatic life. Being so active, magnesium doesn't last long in salt or brackish water, and so is not recommended for these waters. Magnesium anodes are only available for recreational applications.
Which Metal Should I Choose?
- Saltwater: Aluminum anodes are more active, protect better, and last longer than zinc anodes in saltwater – a win/win situation. Magnesium anodes are so active in this highly conductive liquid that they completely corrode in only a few months, resulting in prohibitively high replacement costs. While zinc has been the traditional anode for use in saltwater, it doesn't provide as much protection or last as long as aluminum.
- Brackish Water: Aluminum anodes provide superior protection here. They do not suffer the fast corrosion rates of magnesium and protect better than the less active zinc.
- Freshwater: Magnesium is the clear anode of choice. It offers superior protection in this low-conductivity liquid. Zinc anodes are not suitable for use in freshwater because they build up a hard, dense coating over a period of months – rendering the anode less effective. Aluminum alloys offer some protection in freshwater but not as much as magnesium – so aluminum would only be recommended if your boat is intermittently in fresh water.
Additional FAQs
- How often should I replace my anodes? Adequate anodic protection is so important to the integrity of your vessel that you should err on the side of caution when scheduling anode replacements. A common rule of thumb is that an anode is approaching the end of its life when it appears to be half its original size, as its electrical connection has often degraded so much that it may no longer be providing adequate protection. Manufacturers recommend that you change anodes yearly – even if they still look okay – it's cheap insurance.
- How do I install my anodes? It is critical to ensure good connectivity with the metal being protected. A sure sign of poor connectivity is an anode that still looks brand new after a few months in the water – so make sure there is nothing between your anode and the metal it's supposed to protect. Anodes are supposed to dissolve – if they aren't corroding, they aren't working! Additionally – you should never paint your anodes or mix anodes of different metals (only the most active one will actually be working). Protect trim tabs individually, and do not bond them to the rest of the boat. It's also a good idea to always use new fasteners when installing anodes.
- How many anodes should I use, and what shape? Anodes come in a variety of generalized and specialized shapes. Their sizing, placement, and number tend to be a combination of science, art, and experimentation. Consult a marine corrosion specialist if you have concerns about the anodes on your boat. Remember that more is not necessarily better – it IS possible to overprotect with too many anodes. This is especially true on wood and metal-hulled boats.
- How do I protect my aluminum hull, outboard motor lower units, or I/O stern drive? Outboards and I/Os are made from aluminum alloys, which are particularly prone to corrosion. Zinc anodes have electrical activity levels barely above these aluminum parts, and while they do work, aluminum and magnesium anodes are better suited for this task. In fact, most manufacturers have switched to installing aluminum anodes on these units right in the factory. Remember – the military-spec aluminum anode alloy is much more electrically active than the alloys used in aluminum hulls and outboards. Think of the difference between steel and stainless steel.
- Does aluminum corrode faster than zinc? While some people believe this - it's not actually true. Aluminum anodes usually last longer than their zinc counterparts. That said, aluminum is more active than zinc, so it more readily detects stray currents around the boat. If your aluminum anodes are corroding overly fast, it's likely you have a stray current problem on your boat (or on a boat nearby), which should be investigated.
- Why is there green corrosion on electrical connections? Green corrosion on electrical connections is often caused by the presence of moisture and the reaction between copper and oxygen in the air. This reaction forms copper oxide, which appears green. Proper insulation, regular maintenance, and the use of sacrificial anodes can help prevent this corrosion.
- Aluminum or zinc anodes for salt water? Aluminum and zinc anodes can both work in salt water, but they have pros and cons. Aluminum anodes are lighter, cheaper, and more eco-friendly, but they need regular cleaning to avoid passivation. Zinc anodes are heavier, more expensive, and more toxic, but they are more reliable in salt water.
Every boat is different, and while the above information is typical, with so many variables, it may not apply in all situations. When buying a new boat, it's important to keep in mind that what worked in the old marina might not be right for your boat's new home as needs can change from marina to marina (or even dock to dock!). To protect your investment, it may be best to consult with your local experts to make sure your current anode coverage is sufficient for the warranty of your water heater or other marine appliances.
Fisheries Supply carries all types of sacrificial anodes from all the best manufacturers - including Martyr and Sea Shield - so we've got everything you need to protect your boat. We hope you've enjoyed this Navigator, but if you have any further questions, please don't hesitate to contact our product experts at (800) 426-6930.