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Electrolysis on Boats: What You Need to Know

Ian Fortey by Ian Fortey Updated on June 25, 2021. In Boats

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New boaters are often unaware that electrolysis is something they need to be on the lookout for. Let’s be honest, a lot of us don’t even know what electrolysis is. So let’s start with the basics.

Electrolysis happens when an electrical current passes through water. That in turn causes a chemical reaction. That’s the simple, bare bones version. In terms of boating, the electrolytic reaction occurs between two metals. The electrical current strips away one metal and deposits it on another. Remember, it happens between two dissimilar metals.

Metals work at different voltages. If you have an aluminum boat but your prop is made of zinc, you are setting up a way for electrolysis to occur. Zinc has a higher electrolysis voltage than the aluminum boat hull. When a current flows through the water around your boat, it will strip away some electrons. It takes from the zinc and deposit them on the bare aluminum, which attracts the current.

Electrolysis causes corrosion. Over time, it will damage the metal parts of your boat and cause them to break down. In particularly bad cases, this could destroy an entire prop and shaft in a matter of a couple of weeks.

Electrolysis on Aluminum Boats

Aluminum boats are at greater risk from electrolysis than a boat not made from metal. The current will flow through the water, stripping electrons from the metal components. The end result is that it will be eating away at them as a result. This can happen very quickly. This kind of chemical reaction is almost unavoidable with an aluminum boat. That is, unless steps are taken to prevent or reduce the damage.

Will Electrolysis Affect a Non Aluminum Boat?

If you have a fiberglass hull or even an inflatable pontoon boat, electrolysis could still be a risk. Metal components in the water will still be susceptible. If you have any metal on your boat that is immersed in water, there is always a chance. This also becomes a risk when you are in dock. An electrolytic reaction can occur between your boat and a neighboring boat.

Electrolysis vs Galvanic Corrosion

These two terms come up frequently when discussing the issue. Sometimes they are mistakenly used interchangeably. Electrolysis and galvanic corrosion are not the same thing.

In order for electrolysis to occur, a current has to be forced into the reaction. That usually means bad wiring. It doesn’t have to be a high current, and not even one dangerous to living things.

Galvanic corrosion is pure chemistry. Two different metals in an electrolyte, which is salt water, will exchange electrons. Freshwater does not allow for this kind of corrosion. No current needs to be present to allow for this corrosion to occur. This is your boat essentially becoming a natural battery.

How to Stop Electrolysis on Boats with Zinc Anodes

Many people refer to anodes as “zincs.” That’s because they are very commonly made of zinc. But they are not only made of zinc, and it’s worth knowing that.

Zinc anodes are used in salt water. These are often called sacrificial anodes. Their purpose is to be sacrificed to electrolysis. Most often these are located on trim tabs, shafts and other places. They will give up electrons and weaken over time. This happens in order to protect the other, more important parts of your boat.

In a boat you may have any number of metal parts touching the water. Aluminum, stainless steel, bronze, and galvanized steel are all commonly used. The process of electrolysis sees electrons being stripped from the anode. Then they head towards the cathode. So what does that mean?

  • A cathode is a negatively charged electrode. The more noble metal in the reaction will always be the cathode.
  • The anode is a positively charged electrode. The less noble metal in a reaction will be the anode.
  • Electrons travel more easily through salt water from the anode to the cathode. This is why having something like zinc anodes is important. It allows those electrons to leave the unimportant anode. That saves the important parts of your boat, like the prop or the shaft.

In freshwater or brackish water, it’s best to use different anodes. Magnesium and aluminum are often used here.

The wires in your boat connect metal parts. That basically makes your boat a battery. Non-metal boats will have a copper bonding wire inside. This connects all metal parts so they can share anodes. This bonding connection also includes the metal frame of your boat’s hull which serves as the negative battery connection. And the engine and negative side of your boat battery. Stray current will damage this system.

If your anodes die quickly, your dock may have a faulty shore power ground lead. It should never be connected to the ground bonding system. If the shore power ground connects to the boat’s underwater bonding system, it can wear your anodes out. The shore power circuit is not typically something you can fix under normal circumstances. It can result in your boat being electrically connected to all the other boats. That stray current corrosion can be the most destructive form of electrolysis.

What is a Noble Metal?

There is a hierarchy of metals that rank nobility. It’s sometimes called the galvanic series. The least noble will always be the one that experiences corrosion. At least when compared to the more noble.

Many of these metals are never used on aluminum boats. The three most noble metals in the series are;

  • Graphite
  • Palladium
  • Platinum

There aren’t a lot of platinum fittings on aluminum boats, as you can imagine. But when you head down the list, you’ll see more common metals. The bottom three, and therefore the most likely to suffer corrosion, are;

  • Magnesium
  • Zinc
  • Beryllium

You can see why zinc is a popular choice, then. One thing to be aware of is that the greater the gap in nobility, the faster the reaction takes place. That means if you had underwater metal items that were platinum, your sacrificial anodes would wear out very quickly. Much faster than if you were using aluminum instead of platinum.

Other Ways for Preventing Corrosion

Basically, you need to prevent galvanic corrosion and electrolysis before they happen. Anodes are one of your best weapons, but there are others.

  • Paint. A good coat of protective paint over metal surfaces will greatly reduce the risk of a reaction. Just remember to never paint over an anode. If you do that, it makes the anode useless.
  • Corrosion inhibitors are another option. These make metal what they call “passive” and resistant to corrosion. You can buy corrosion inhibitors to do this. Chromium gel is one such solution.
  • Check your wiring. Electrolysis will only occur in the presence of a current. If you’re in freshwater, you will not have galvanic corrosion to worry about. That means neither method of corrosion should affect you. But if it is, then you’re getting stray current from somewhere. It could be as simple as a slightly frayed or loose wire touching the hull.
  • Maintain your anodes. Aside from not painting them, make sure you inspect your anodes regularly. Keep them clean of any grime or build up so they can do their job. Keeping your hull clean with a quality cleaner is helpful, too.
  • Don’t mix metals. That means if you have aluminum fittings, only use aluminum components with it. Things like fasteners. Don’t even put aluminum nuts on steel bolts or anything like that. It makes the chemical reaction that much easier when dissimilar metals touch. That also includes leaving metal items sitting around. A pair of pliers or even a sinker sitting against the hull can cause serious problems over time.
  • Keep the power off when your boat is penned. This will shut down magnetic fields. That in turn will prevent the reaction from occurring as easily. If the battery is disconnected when your boat is not in use then no stray current can leave the boat.
  • Do a test. Hull reference tests can measure the ambient voltage near your boat. This information can help you determine exactly how many anodes you need to install on your boat. That way you’ll know how to prevent any kind of corrosion from damaging it. Your local marina should be able to assist you with a test like this. A qualified marine electrician should be able to help you conduct the tests as well.
  • Zinc fish. If your marine is known for faulty wiring, you can put zinc fish in the water around your boat. These are cheaper than replacing anodes and work well. They’ll save your hull and prop shaft. Shore power is notorious for power leakage.

The Bottom Line

Electrical faults can be devastating. Bad shore power, especially the ground lead, are real dangers. Salt water is an electrically conducive solution and your boat is always in it. All the underwater items on your boat can fall prey to a stray electric current or a bad electrical circuit.

Although the process sounds both dangerous to deal with, preparation is key. This doesn’t have to ruin your fun on the water by any means. Prepare your boat ahead of time and you should be fine. Keep things clean and orderly, make use of anodes, and be aware of what metal is located where. Also make sure your wiring and power is being monitored.

About Ian

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.

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