What is Galvanic Corrosion…?

Without delving too far into the depths of electrochemistry it is enough to know that metal atoms are full of electrically charged particles. Therefore, we can deduce that metals have a natural electrical charge.

Some metals are more naturally unstable than others, and want to give away bits of their atoms to become more stable.

If different metals are in contact and are placed in a substance that chemically encourages them to give away bits of their atoms, they will corrode.

It is this exchange of electrons between different metal types that are in electrical contact and placed in an electrolyte that is defined as Galvanic Corrosion.

In fact, that is exactly the principle of a battery. In this case the electrical currents being exchanged by the different metals used in the battery cell can produce useable amounts of electrical energy. In a primary battery, one that is used then discarded, once the corrosion and exchange of atomic particles from one metal to the other has taken place the flow of electricity stops.

In a secondary battery, such as our narrow boat starter and leisure batteries, the corrosion can be chemically reversed and this process is known as recharging.

Noble Metals

Metals can be arranged in an order that reflects their natural resistance to corrosion and are classified as more or less noble. A metal that is nonreactive and resistant to corrosion would be gold and it appears high in the list. A metal that is unstable and corrodes easily is known as reactive and an example is magnesium.

Gold is more “noble” than magnesium.

Back to Corrosion…

So we can see that if we have three metals in an electrolyte that are connected electrically then they will corrode at different rates.

The rate of corrosion is not only related to the metals noble position, it is also affected by the amount of electrical charge that flows between the metals. This is why we use sacrificial anodes made from the metal magnesium to protect our narrow boat hull. This metal is less noble, more reactive, than the steel hull or the bronze alloy propeller and therefore will corrode first thus “sacrificing” itself to corrosion before the other more noble metals around it.

Stray Currents…

We’ve seen that all metals have a natural electrical charge, therefore natural corrosion will inevitably take place albeit very slowly. Introduce an external current into the equation and corrosion can be dramatically accelerated.

A well designed narrowboat and shore electrical system will not dramatically affect the rate of galvanic corrosion. However, a stray electrical current will, and in extreme cases corrosion may be measured in hours rather than years.

Where can these stray currents come from?

Well, poorly specified wiring can cause current overload and the electricity will try and find a better path. Poor quality appliances can “leak” electricity. Different ground voltages between the battery and radio systems can cause problems and of course defective shore power wiring can also be a factor.

The first step for any narrow boat owner is to ensure that your electrical system is well designed. A correctly installed AC shore power system will neither cause or increase the effects of galvanic corrosion.  A correctly installed DC power system will neither cause or increase the effects of galvanic corrosion.

The most likely cause of accelerated galvanic corrosion on a well protected narrowboat hull on a marina berth would be a problem with the marina’s electrical system. Now before we all rush out and start pointing fingers at our friendly marina owners, this is a very complicated situation.

It’s up to You…

The emphasis is, correctly, on the narrowboat owner to protect his or her investment and the only real way to do this is to not be connected to AC shore power at all, have correctly installed DC and AC systems, and to have a full scheme of anodic hull protection in place. Of course in reality this is not practical for most boat owners, especially those of us living aboard in marinas.

Therefore there are two pieces of equipment that are used to protect against galvanic corrosion caused by stray currents.

Isolation transformers are the most effective way of disconnecting your narrowboat hull from shore power. These unit work by transferring electricity magnetically. There is no return path to earth and therefore you are effectively isolated from the earth system of the shore power provider. Out of the loop so to speak.

Galvanic Isolators detect stray currents flowing to earth. They do not totally isolate the AC system in the way an isolation transformer does but they are considerably less expensive and therefore better than nothing. As we have read in our section on Batteries and Anodes, stray currents will cause corrosion.

Galvanic isolators contain diodes which block these stray galvanic currents which flow at between 0.4 and 0.8 V and cause galvanic corrosion.

One of the potential considerations with galvanic isolators is how the design and build of modern electrical equipment has changed in recent years. Without getting too technical, a lot of modern equipment uses power supply electronics which can introduce excess radio frequency into the circuits. To deal with this extra frequency, capacitors are built in which divert the excess to earth, rather than allowing them to interfere with your radio or television etc. In a land based situation this is not an issue, but where a galvanic isolator is fitted this extra frequency can cause the isolator to be permanently conducting. Not so much of an issue with AC, as the voltages are so low, but if it is permanently conducting it will not be blocking stray DC currents.

Most up to date galvanic isolators will contain electronics to attempt to prevent the diodes conducting.

There is another potential problem for galvanic isolators which relates to the common shore earth conductor. It is possible that it can carry voltage. Only a small, non-life threatening voltage, but one sufficient to cause the isolator to conduct rather than block. This is simple to measure and if you suspect a problem with mooring connections, get the advice of a qualified electrcican who can measure if there is any voltage difference between the common earth cable and the ground itself.

As we can see, the only totally reliable way of preventing stray current flowing and causing corrosion is to disconnect the AC earth completely using an isolation transformer. And only then if the transformer is shore side, isolating the lea that leads onto the boat. If it is mounted in the narrow boat, the incoming lead is not grounded to earth!

So in practice, what does this mean?

A good quality galvanic isolator of the most modern design will do the job.
For those of you who like a belt and braces approach, fit an isolation transformer. Ideally on the pontoon beside the boat. There are portable products available but they are very expensive. Specialist transformer manufacturers can custom build units to order for a few hundred pounds, but make sure they understand that the earth has to be isolated, not all isolation transformers are built so.

If you decide to have a portable unit built the manufacturer will need to know the power requirements of your system and you will need to give some thought to the security of the unit if it is to be left “shore side”.