How do anodes protect my boat from electrolytic corrosion?
What are sacrificial anodes and why do I need to replace them?
Any time you have exposed metals on your boat, you’ll want to protect them with sacrificial anodes. Anodes are often referred to as “zincs,” although different materials are used in different applications. Anodes are alloys intended to take the damage caused by electrolysis in the water before the metals on your boat are damaged. Electrolysis in water is the decomposition of water into oxygen and hydrogen gas due to the passage of an electric current. This can be caused by your own boat, or boats around you- by anything from faulty wiring to an electrical box that isn’t fully insulated. The danger may not be enough to harm anyone who gets into the water, but electrolysis doesn’t need much power behind its current to do damage.
Commonly, your boat will have a combination of exposed metals. The shaft is likely stainless steel, while your propeller may be bronze, nibral, stainless, or aluminum. When dissimilar metals are electrically connected and submerged, a chemical reaction takes place. This chemical reaction forms a circuit where the least noble metal (the anode) will begin to break down leaving the noblest metal (the cathode) intact. When we apply this to our boat, the bronze propeller is more active (less noble) than the stainless steel propeller shaft. In this case, the bronze propeller will naturally begin to breakdown leaving the steel shaft intact.
Since propellers and shafts are expensive to replace, it is easy to protect these metals by adding a zinc collar anode to the shaft. The correct anodes, which are highly active, will corrode instead of the other metals they are electrically connected to. Replace your anodes as needed to keep protecting the metal components of your boat. Letting them go too long risks damage to expensive boat parts from propellers to shafting to outdrives.
Will any anode work in any water?
The short answer is no. Choose your anode material according to the water where your boat is floating. Throughout the world, there are many different water environments that may require different solutions. The three basic water types are salt, fresh and brackish. Zinc anodes work best in saltwater. Aluminum anodes protect very well in brackish, whereas magnesium anodes protect best in freshwater. Whenever possible, measure electrolytic corrosion around your boat by using corrosion test meter. This will quickly indicate if your anodes are providing adequate protection.
NOTE: Essentially, freshwater is a much less conductive environment than salt water, therefore magnesium anodes are the best choice as they are much more active (less noble) than zinc or aluminum anodes. The result is increased efficiency thus superior protection for your underwater metal components. It is not recommended to use magnesium anodes in salt or brackish water. The result may be an accelerated corrosion rate, which may damage the metal parts of your boat and leave you with no anode protection in a short period of time.
How often should anodes be changed?
- Change all anodes at 50% corrosion.
- Zinc is the easiest to identify corrosion. If bolts are exposed, or material is corroded half way in-whether from the inner points or the outer points-it’s time to change! When the corners of a diver’s dream are rounded out, or there is corrosion around the bolt holes-change them! If there are barnacles growing on the anode-change it!
- Aluminum anodes are hard to judge because of how the material breaks down. These seem to corrode from the contact point (inside-where we can’t see the corrosion) outward. Aluminum anodes can also form a cloud of crystal-like corrosion in the process of corroding. Err on the side of caution with aluminum anode replacement.
- Diver’s recommend quarterly inspections to ensure anode efficiency and timely replacement.
If you’re unsure-replace them! The cost of a few anodes is easily justifiable when you find yourself spending
thousands on shafting and propellers due to electrolytic corrosion.