Galvanic corrosion, also known as dissimilar metal corrosion or bimetallic corrosion, is an electrochemical process that occurs when two dissimilar metals are in electrical contact with each other in a corrosive electrolyte, resulting in the weaker metal becoming brittle and rusted (corrosion).
Different metals have varying inherent electrical properties. A galvanic series ranks metals according to their tendency to corrode (Anode / Zinc) or be protected (cathode) in a saltwater solution. Metals higher in the series are more noble (cathodic) and less likely to corrode, while metals lower in the series are more active (anodic) and more prone to corrosion.
For galvanic corrosion to occur, there must be electrical continuity between two dissimilar metals. An example of this that is commonly found on boats is the use of a stainless-steel bolt or screw coupled with an aluminum nut or washer. The contact between the two dissimilar metals creates a pathway for galvanic corrosion to occur. Galvanic corrosion can happen underwater or above.
Underwater, an electrolyte is a conductive solution such as saltwater that allows the flow of ions between the two metals. Electrolytes can be saltwater or freshwater. When these factors are present, an electrochemical reaction occurs. The more active metal (anode) corrodes at an accelerated rate compared to when it is alone. The electrons released by the corroding anode flow through the electrical contact to the more noble metal (cathode), which is typically protected from corrosion. To simplify, a Zinc Anode is what boaters install as a sacrificial metal to direct electrolysis to the anode instead of the more expensive (and important) metal that is part of the boat equipment. These zinc anodes must be monitored and replaced as needed. Your diver can inspect underwater anodes during the monthly underwater hull cleanings. The diver can also replace the zinc anodes as needed. It is also as important to monitor and replace zinc anodes that may be installed inside the engine and other parts of the hull. Your InterMarine Boats (954-894-9895) marine tech can perform this function.
Here are some real-world examples of galvanic corrosion found on boats:
- Steel nuts and bolts used with aluminum nuts or washers can cause the aluminum to corrode more quickly.
- Copper pipes connected to galvanized steel pipes can accelerate the corrosion of the steel pipe.
- A boat hull made of steel can corrode more quickly if it has a bronze propeller.
Galvanic corrosion can be mitigated by several methods, including:
- Using compatible metals: Select metals that are close together in the galvanic series to minimize the potential difference between them.
- Isolating the metals: Use non-conductive materials like gaskets, nylon washers/spacers, or sealants to electrically isolate the dissimilar metals from each other.
- Sacrificial anodes: Attach a more active metal (anode) to the protected metal (cathode). The sacrificial anode will corrode preferentially, thereby protecting the cathode. Zinc anodes are commonly used on boats.
- Cathodic protection: Apply a small electrical current to the protected metal (cathode) from an external source. This makes the cathode less reactive and reduces corrosion.
There are several ways to reduce electrolysis, also known as galvanic corrosion, on your boat:
- Sacrificial Anodes: Also known as Zincs, these are your first line of defense. Anodes are typically made of zinc, magnesium, or aluminum and are strategically attached to your boat's underwater hull, shafts, trim tabs, and engine components. Since they are more reactive than the hull metal, they corrode preferentially, sacrificing themselves to protect your boat. The type of metal used in the anode depends on whether your boat is in saltwater, freshwater, or brackish water. Make sure you have the correct type and quantity of anodes installed in the right locations and inspect them regularly for wear and tear. Replace them when they deteriorate significantly.
- Isolation Transformers: If you use shore power at a marina, a galvanic isolator device can be installed between the shore power connection and your boat's electrical system. This disrupts the weak stray electrical current that can travel through the marina's grounding system and contribute to electrolysis on your boat.
- Proper Grounding: Ensure all your boat's wiring, switches, equipment, and grounding points are properly installed and maintained. Faulty grounding can create unwanted electrical paths that could accelerate corrosion.
Paint and Coatings: Maintain good quality anti-fouling paint on your underwater hull. Chips and scratches can expose bare metal to the water and create prime spots for corrosion. For aluminum boats, consider using corrosion-resistant paint designed specifically for aluminum. - Battery Care: Disconnect your boat's batteries when not in use to minimize the chance of stray electrical currents. This is not always possible without extensive disassembly and risking memory loss on electrical equipment and joysticks on certain boats equipped as such.
- Haul Out (if applicable): If you store your boat out of the water such as on a trailer, lift, or dry storage rack for extended periods, it eliminates the possibility of electrolysis.
By implementing a combination of these methods, you can significantly reduce electrolysis and protect your boat from the damaging effects of galvanic corrosion. Remember, consulting with a qualified marine technician at InterMarine Boats is always recommended to ensure the proper solutions are implemented for your specific boat.