What is galvanic corrosion?
Galvanic corrosion (sometimes erroneously called electrolysis) can damage or destroy underwater metal parts of boats, dock hardware and other equipment. When two different metals are touching each other or are electrically connected by a conductor, and are immersed in an electrolyte (an electrically conductive fluid, like salt water) an electro-chemical reaction can occur. One of the metals (the “least noble” metal, called the anode) will corrode faster than it normally would, and the other (The “most noble” metal, or the cathode) will dissolve more slowly. The seawater Galvanic Sequence lists metals in order, based on their voltage potential and tendency to corrode. More active metals, the faster-dissolving anodes, are at the top of the series. Passive metals, the cathodes, are located at the bottom. Your boat might have a collection of submerged electrically connected metal parts immersed in salt water surrounding the boat, in bilgewater or the water in the engine cooling system. The stainless prop shaft, for example, because it is more active (less noble) than the bronze propeller, will begin to dissolve, leaving the prop intact.
How to prevent it
Breaking the electrical circuit between exposed metals by connecting them to a sacrificial anode will prevent galvanic corrosion. A sacrificial anode is an inexpensive piece of metal that is less noble than any other metal found on the boat, and it is electrically connected, bolted or wired into electrical contact with the other metal components. Bolting a sacrificial anode made from zinc (located near the top of the Galvanic Sequence) to the stainless prop shaft will protect it from corrosion. The zinc anode and the stainless shaft will form a galvanic couple, and the anode will dissolve, keeping your prop shaft spinning happily. Sacrificial anodes can extend the life of your boat’s hull, engine, rudder, propeller shaft, engine cooling system, refrigeration condenser and other metal components by protecting them from the deterioration caused by galvanic corrosion.
When to replace an anode
The effectiveness of an anode depends on a good electrical connection, and is directly proportional to its surface area. As it corrodes away its surface area and effectiveness diminishes. Some manufacturers suggest replacing anodes when they are two thirds gone, but we recommend replacing them when half-eroded or half-dissolved. If an anode is allowed to dissolve completely, the next least noble piece of metal in the circuit will start to dissolve. And that might be a part of your engine.
The importance of marine grade fasteners
Galvanic corrosion can also happen in a single piece of hardware, particularly alloys, which contain more than one metal. Interactions between the differing metals in the alloy will dissolve the least noble, the most common example of this being the dissolution of zinc from many zinc/copper alloys (such as brass and some bronzes) leaving a weak, spongy copper residue. That’s why it’s so important to use marine-grade fasteners (bronze, monel and 316 stainless steel) in underwater applications so they are not destroyed, allowing a hose clamp to fail or your propeller to fall off. The higher the salinity and temperature of the water, the greater the likelihood of corrosion.
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