Galvanic corrosion in boats. What it is and how to combat it

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galvanic corrosion
Against galvanic corrosion: classic zinc placed near the propeller on an axle-line transmission.

When the boat is at rest, perhaps for winterization or long breaks, you must not forget about enemy number one of onboard metals (and thus propellers, shaft lines, sea intakes, rudder shafts…): galvanic corrosion.

Galvanic corrosion, what is it

Galvanic corrosion is a phenomenon generated by the passage of current between two different metals, defined as noble and less noble: when immersed in a salt solution, and electrically connected to each other, particles of the less noble metal migrate toward the more noble one affecting its integrity and essentially corroding it.

Think, for example, of submerged metal appendages: the propellers, shaft lines, sea intakes, rudders. Some of them are already electrically connected to each other: propeller and shaftline, for example. Other elements, however, can be connected through seawater in the bilge or a salt bridge.

Zincs, the allies against galvanic corrosion

To prevent corrosion from advancing and dangerously affecting your boat, you must ensure that the zincs are checked and replaced if they are worn out.

Their function is to take advantage of the natural propensity for corrosion before other metals. Once the boat is winged, if you see them worn out, it means they have done their job admirably-they have deservedly “sacrificed” themselves to save propeller, shaftline, or sea hold. Obviously, if the boat has no zincs or has run out of them, it will be subject to corrosion of its submerged metals to the point of compromising safety itself. Think of the damage of a sea socket corroding to the point of puncture.

When and how to change zincs

When the boat is dry-docked, check the wear condition of the zincs, both those on the transmission components and those mounted flush with the hull. If the thickness of the sacrificial anodes is less than half the original thickness, proceed with replacement.

To make this easier, it is sometimes useful to give a few light blows with a hammer to dislodge the zincs from the holder seat. Clean the ground plate or attachment area, such as the propeller shaft or rudder blade, with an iron brush.

The separator, another remedy

Another cause of galvanic current formation is external agents. Think about when you are connected with the 220 V cable at the dock. Its ground conductor, being common to all the posts, can actually make a connection between the metal parts of the other boats connected at the dock and yours. Please note that the connection via ground conductor is made even with the plug inserted in the column and with the socket switch disconnected.

galvanic corrosion
A galvanic separator to be installed between the dockside ground system and the boat ground circuit.

To prevent the spread of galvanic currents through the 220 V mains cable at the dock, a galvanic separator should be installed on the incoming 220 V line: there are several models on the market. Another solution to protect against dock currents is to use an electrical transformer, which by its nature is a galvanic separator, with a 1 to 1 ratio or better yet with input (dock side) 110 / 220 V and output (boat side) 220 V for those who moor their boats not only in Italy, and sized for the necessary power of the boat (on average 4 kW).

The shortcoming of the solution besides the high cost is the weight (about 40 kg), which makes it suitable only for large boats.

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