It’s a well-known fact that water corrodes metals: one needs to look no further than the nearest wishing well to find ample evidence. But when it comes to knowing what water may do to the metal on your boat, however, this simple fact becomes much more complicated. By developing a basic understanding of the science behind corrosion, you will better be able to protect your boat (and your pennies)!
The type of corrosion that is most harmful to your boat is called galvanic corrosion. It’s easy to make galvanic corrosion sound unfathomably complicated but its basic premise is a fairly simple one: essentially, whenever two different metals that are either physically or electrically connected are submerged in water a small amount of current will flow between them, creating a battery. The current between them is made up of electrons that are being supplied by one of the metals giving up its own metal ions into the water. This is galvanic corrosion and if it continues unchecked, the metal giving up bits of itself may eventually completely erode.
That, in a nutshell, is galvanic corrosion. With that basic understanding in place, let’s investigate the process a little more closely: different metals have different nobilities, with gold at the top of the scale and metals like zinc and magnesium at the bottom. When metals with different nobilities are submerged in water and create a battery, it is their position on the nobility scale that determines if they will sacrifice their own metal ions to provide electrons for the current. The metal with the lower nobility will be the metal that corrodes.
On a boat, a common galvanic circuit is created when a bronze propeller is attached to a stainless steel boat. In this case, bronze is the less noble metal, which means that when these two metals are in the water together, the expensive and necessary bronze propeller is donating electrons to the circuit and corroding itself. Wouldn’t it be nice if you could somehow just add an even less noble metal to this circuit so that the bronze propeller wouldn’t be the metal that suffers from corrosion? Enter the sacrificial anode: by simply attaching a less noble metal to the shaft, you break the circuit and it will be the less expensive metal that corrodes, not your propeller. Science can be pretty nifty!
Your propeller is not the only place that galvanic corrosion can take place: your engine is another prime place for galvanic circuits to set up shop because there are so many dissimilar metals there. For an outboard motor just attach anode tabs and for an inboard engine install pencil anodes, so you will break any harmful galvanic circuits and protect your engine, making the anodes bear the brunt of the corrosion.
As you can see, anodes are crucial players in the fight against corrosion. There are three materials used for anodes: aluminum, magnesium, and zinc. In order to make sure that you’re putting in the best anode for the job, it’s important to know that different types of water conduct electricity differently and different types of metal have different voltage potentials. Salt water is more electrically conductive than brackish water, which is more conductive than fresh water. Magnesium has the highest voltage potential, then aluminum, and then zinc. When you’re trying to decide which type of metal to select for your anode, you need to match the voltage potential of the metal with the conductivity of the water you’ll be in. In other words, if your boat is only in salt water, your anodes should be made of zinc, whereas if your boat is only in fresh water, your anodes should be magnesium. If your boat is exposed to salt water, brackish water, and fresh water, then aluminum anodes are best suited for the job. Bear in mind that magnesium anodes should only be used in fresh water because their high voltage potential can actually cause too much protection and extremely damage aluminum castings. Similarly, using zinc anodes in fresh or brackish water may result in a coat forming over the metal and diminish their ability to effectively function.
When it comes to installing an anode, remember that contact is key to break the circuit: make sure you either attach the anode directly to the metal you are trying to protect or that you connect the two with a wire. Moreover, anodes can only perform their function if they are completely exposed, which means you should never coat or paint them because it will render them useless. Finally, check your boat’s anodes about once a year and replace any of them that appear to have lost about half of themselves to corrosion.
Galvanic corrosion is both an extremely damaging but also highly preventable process. By having a basic understanding of the science behind the process you can keep your anodes in top sacrificial form, saving both your boat and your wallet from undue expense.
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