Corrosión galvánica

La conexión a tierra (toma de tierra o puesta a tierra) constituye una vía de retorno común para la corriente eléctrica de un circuito eléctrico
9 de marzo de 2020 por
Corrosión galvánica
Techno Sun, SLU, Hugo Rodrigo Zapata :

Galvanic corrosion

Galvanic corrosion is caused by an electric current entering a ship through the shore conductor of the port power supply and returning to the port through the water. These currents can cause corrosion on the metals of the ship that are underwater, such as the hull, propeller, shaft, etc. This current is called galvanic current.

Galvanic current is a DC current. It is caused by the natural difference in voltage between metals. A galvanic current can only exist if there is a closed electrical circuit. A conductor from another electrical circuit can be part of the galvanic corrosion circuit. If a boat with a metal hull is close to the shore, there is a natural voltage difference of 0.1-1 Vdc between the hull and the water.

This potential difference is of no consequence as long as the electrical circuit is not closed.

As soon as the port power is connected to the ship, the port ground is automatically connected to the ship's hull and the electrical circuit is closed. The following circuit is established: hull - water - port - ground spike - ground wire - hull. A galvanic current will pass through this circuit. The galvanic current passes partly through the AC circuit but is not related to that circuit. It will continue to pass current until the potential difference disappears. The intensity of the current depends on the resistance of the electrical circuit. The resistance is determined by factors such as the length of the port power cable and the resistance of the local ground.

Chemically speaking, the "weaker" metal in the galvanic circuit will be the first to separate its molecules to allow the current to pass. If the hull of the ship is part of the galvanic circuit and the hull contains the weaker metal, the hull will start to corrode over time. This can lead to a very unfavorable situation, which can be very expensive and unsafe if not checked. There are known cases of ships that have sunk due to galvanic corrosion. Aluminum hulls are particularly vulnerable to this type of corrosion.

Galvanic corrosion can also occur between the various metal parts that are attached to a ship, such as the propeller, engine and hull, among others. All these parts are grounded and therefore there will be small additional currents between them. For this reason sacrificial anodes are mounted.

A sacrificial anode is a piece of a metal that is weaker than the surrounding metals. So it is sacrificed to protect the rest of the metals. They only prevent corrosion by slowing it down. The type of sacrificial anode to use depends on the type of metal it protects and the type of water the boat is in. It is recommended to check these anodes regularly.

Prevention of galvanic corrosion

The answer is quite simple. To avoid corrosion the electrical circuit must be broken. Although this is almost impossible to achieve with the small circuits that are established between the different metal parts attached to the boat, it can be done with the connection to the port power supply.

The simplest way to break the circuit is not to connect the port ground to the hull. However, this is neither safe nor recommended, because it makes the hull insufficiently protected and therefore it can no longer be guaranteed that the ID will function properly, resulting in unsafe situations on board.

There are safe ways to avoid galvanic corrosion without compromising safety. This can be achieved by using galvanic isolation or by using an isolation transformer.

Galvanic isolation

Galvanic isolation prevents galvanic corrosion. It blocks low voltage DC currents that enter the ship through the shore conductor of the port. These currents can cause corrosion on the ship's underwater metals such as hull, propeller, shaft, etc.

The galvanic isolation consists of two diodes connected in antiparallel. The galvanic isolation is connected between the port ground connection and the ship's central ground point.

Diodes of this configuration conduct electricity in both directions only when a certain threshold voltage is reached. The threshold voltage is approximately 1.4 Vdc. The threshold voltage is greater than the galvanic potential difference between different metals. Thus, the galvanic current cannot pass. On the other hand, a higher ground fault voltage will be allowed to pass in the AC circuit, allowing the correct operation of the connected ID.

The advantage of galvanic isolation is its small weight and size, the disadvantage is that this unit needs a good grounding conductor. Another consideration is that galvanic corrosion can also appear through the neutral conductor. This is in cases where the neutral conductor has been grounded through one of the on-board electrical devices, such as a suppression filter.

Isolation transformer

A better solution to stop galvanic corrosion is to use an isolation transformer. In an isolation transformer the incoming electricity is converted into electromagnetism and then back into electricity.

The inlet and outlet are fully insulated and will break the electrical circuit between star point - ground conductor - hull - water - star point, thus effectively blocking the galvanic current.

Another feature of the isolation transformer is that in electrical terms it is a source of electricity, powered by another source of electricity. On the output side of the transformer one of the outgoing phases is connected to the shell, thus creating a phase, neutral and ground, which ensures the correct operation of the ID.

An isolation transformer will provide the same safety as in a domestic installation. The installation is also completely isolated from the electrical problems of the surrounding ships.

An additional advantage is that an isolation transformer can often increase or decrease the incoming port voltage. This can be useful when a 230 Vac boat has to be connected to a 120 Vac supply or the other way around.

Source: Wiring Unlimited de Victron Energy