What is Galvanic Erosion, is it serious and can it be prevented ?

Galvanic Erosion is corrosion/erosion of metals which is greatly accelerated by the influence of an electric current.

Two possibilities arise in a marine environment for this problem to ocurr. First is what is known as "stray current" erosion. This is caused by currents (usually DC earth return currents) being forced to use the hull (or any other conductor that is not specifically designed/installed as a current carrying medium) as the return path. This problem is tackled by installing the system correctly in the first place. ONE central earth point should be chosen and all systems should be 2 wire return circuits. On no account must the hull be used as an earth return in the manner of vehicle wiring. The second problem is explained as follows:-

Put as simply as possible "in the case of two dissimilar metals in electrical contact with each other, immersed in a conductive liquid, one of the metals will erode". In effect the 2 dissimilar metals in conjunction with the liquid (in our case water) produce a battery. The battery causes a current to flow, and this current (the galvanic current) is what causes the erosion

The metal which erodes is the one which is less noble. You don't really have to know what this means as long as you accept it. If you don't accept it, you can always go and look it up. Try an internet search for "galvanic series". That should get you plenty of hits.

This is why sacrifical anodes are fitted to metal hulls. The Anode is less noble than the hull and it's fittings so the Anode is the metal that erodes.

The Anodes take care of the electric currents that are produced as a result of (for example) a bronze screw and a steel hull). The Anode is far less noble than either of those two so the Anode is the metal that erodes.

Yes it is serious. Very serious. Without the Anodes, a hull will disintegrate far faster than anyone could believe possible without actually seeing it happen.

Recently a huge increase in Narrowboat hull corrosion/erosion has been reported. Many articles have been written in the popular magazines and books on the possible causes, including polluted water, poor steel quality etc.

Whilst there is no doubt that these can have a large effect on galvanic erosion there is little, if anything, that you, as a boat owner, can do about them.

One other point that has been mentioned is that the rate, occurence and severity of accelerated galvanic erosion seems to have been in direct proportion to the increased use of shorepower.

Please note that correctly installed, an elaborate electrical system will not cause or increase the effects of this erosion.

What will and does cause a problem is the earth connection running from the shorepower socket to the hull of the boat. This gives a direct path for the electric currents thus generated to flow and this is the cause of the problem. Obviously the shorepower earth connection cannot simply be removed. This would be highly dangerous.

There are actually 2 sides to this argument. One side argues that the entire electrical system should be isolated from the hull. The other side argues that it should be bonded but that certain precautions should be taken. Both systems have their pros and cons. This is an argument that has ensued for many years and probably will continue for many more however that is not our concern here. Our concern is preventing (or at least reducing) galvanic erosion when the installer/owner has made the decision to bond the AC earth conductor to the hull.

Please note that depending upon what type of vessel is involved, and when it was built, it may actually be a legal requirement to bond the ground conductor and the hull in order to comply with the required regulations for that particular vessel. If in any doubt then consult a qualified marine surveyor or marine law specialist.

Anyway back to the technical side of things ! What are the solutions ?

One solution is to fit an isolation transformer which totally isolates the shorepower 230 volts from the boat's 230 volts. In effect it creates it's own local 230 volt AC supply and earth. The problem with this solution is that the transformers are heavy and expensive.

The other solution is to fit a Galvanic isolator.

A Galvanic Isolator works by preventing the flow of the galvanic currents (which are below a certain threshold) but will still allow an AC earth fault current to flow. In this way, RCCDs, Circuit Breakers and overcurrent fuses will still operate exactly as previously therefore still providing those on board with the same level of safety but at the same time preventing the galvanic currents which slowly, but persistently, erode away at the hull and it's fitting. An immediate benefit of fitting a galvanic isolator is the increased life of the anodes.

A galvanic isolator is a 2 terminal device that is simply fitted in line with the earth connection from the shorepower socket. Typically, installation would take around 10 minutes in a Narrowboat.

Which is best? A galvanic isolator or an isolation transformer?

What are the pros and cons of each?

A new situation has arisen in recent years and it is now necessary to look into this matter in some detail in order to understand this question (and the answer to it).

Read on for a more detailed examination to discover why, now, an isolation transformer is a far better choice than a galvanic isolator.


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