You’ve already learned the importance of an active anode and a few things that can cause your anodes to have a slowed rate of burn, so let us take a minute to tell you about a “normal” rate of burn. Like everything else pertaining to this subject, “normal” is relative, but we’re going to give you some good guidelines to help you find your boat’s normal!
Most manufacturer guidelines state that anodes should be changed when they corrode to 50% of their original size. This may be surprising to you (and a little frustrating if you’re thinking “I paid for the whole thing, why do I only get to use half of it?”). Here’s why those guidelines are important.
- THE HARDWARE – An anode won’t stay on your boat unless it is attached (either bolted on, clamped on, or screwed) to the metal or vessel it is protecting. That means the hardware used to attach each anode is extremely important. Anodes are produced with this importance in mind, thus the hardware is usually placed in the middle of the anode. When the anode burns to 50% of its original size, that hardware becomes exposed. Once the hardware is exposed in will no longer fix the anode to your boat effectively.
As the anode corrodes, so do the areas around the hardware (or fasteners). That area is important to keep the anode tightly fixed to your boat (and remember, metal on metal contact is the best). When the material around the fasteners erodes far enough the anode may fall off. This often happens when you take your boat out to enjoy it. The movement can cause you to “throw” an anode leaving you without protection for your boat.
- SURFACE AREA – (Quick refresh - water is an electrolytic solution that conducts the flow of ions from an anode to a cathode to protect the cathode from corrosion)
Even though your anodes are completely submerged in water, the only actual contact it makes is with the surface area of the metal alloy. Since the surface area of the anode is the point of contact, that is where the ions get pulled from to protect the cathode. Therefore, the larger the surface area, the greater the corrosion protection potential.
As the anode erodes, it gets smaller in size, also decreasing the overall surface area. In order to protect your boat efficiently, it is safest to change your anodes once they reach 50% of their original size/surface area.
If you’re in a boating environment that allows you to keep your boat in the water year-round (lucky!) we hope you have a diving service. Never be afraid to ask your service to show you the used anodes when they replace them with new anodes. If you can see bare fasteners or the anode surface area is less than 50% they should have changed them sooner. Know your boat’s “NORMAL” by looking at the anodes when they are changed and pay attention to the frequency of replacement.
Remember, there are many factors that can cause your anodes to corrode at an increased rate, so be aware of the normal rate of burn for your boat. If that timeframe suddenly changes it means some corrosive property around your boat has changed as well. Protect you boat by regularly changing your Martyr Sacrificial Anodes.