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 canal boat starter and leisure batteries, the corrosion can be chemically reversed and this process is known as recharging.
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 unreactive 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 canal 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.
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 boat 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 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 hull on a marina
beth 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 boat 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, esspecially those of us living aboard in marina
Therefore there are two pieces of equipment that are used to protect against galvanic corrosion caused by stray currents.
The Isolation Transformer
and the Galvanic Isolator