the silent enemy...
Some people rank electrolysis as one of the black arts. It can happen over time without you realising until systems fail. Being aware of the problem and ensuring that even the smallest fittings on your vessel are compatible are essential if the silent enemy is to be overcome. It's a small investment in a long future.

Electric current may be carried or conducted by a metal without changing the nature of the metal or its properties (unless heated to high heat). Some liquids known as electrolytes, like acid solutions, bases and salts can also conduct electric current. In this process, however, electrical energy is converted into chemical change. This chemical change can even take place in quite weak solutions.

Depending on the type of solution and electrode, the positive plate where current enters or anode may be corroded away with the dissolved metal deposited on the negative plate or cathode where the current exits. Where this occurs, the positive ions (or cations) are attracted to the negative cathode, the negative ions (anions) are attracted to the positive anode. These ions carry the electric current through the solution. The process is known as electrolysis.

Electrolysis does not affect all metals in the same way. The greater the number of ions present, the better the conductor and the stronger the current.

Platinum anode electrodes remain unaffected unless the solution they are in contact with contains a preferentially affected element like copper. Stainless steel is relatively immune but electrodes will corrode slowly over time.

Other elements are discharged preferentially according to how much energy is used up. The lower the energy required, the more easily it is attracted to the cathode. In the case of alloys, all metals in the alloy need to be considered as the base metal may have a low attraction but be alloyed with a metal of high attraction.

While pure water is a poor conductor, having few ions available, in sea water, because of its low resistivity/high conductivity, anodes and cathodes can exist when they are quite some distance apart.

While land communication systems are not generally very concerned with electrolytic damage, for marine vessels with metals constantly in contact with or immersed in sea water, it is a very real problem.

In ships using the hull as an earth, stray currents flowing along a metallic conductor can enter the water causing ionisation of the metal and creating an area of high anodic potential and rapid corrosion at the point of entry to the water. DC current is most susceptible but the phenomenon will also occur with AC current as it has a DC component. There is a tendency for protective coatings to be broken down with attacks concentrated against any weaknesses present and at grain boundaries. Typically for 1 ampere flowing for one year, stray currents can corrode approximately as much as 9kg/20 lbs of iron, 10kg/22 lbs of copper or 34kg/75 lbs of lead.

To avoid such corrosion all pipes carrying sea water should be bonded to earth at either end and as far as possible all electrical equipment should be earthed to isolated bus-bars and not to the hull. Radio earths should be kept separate.

Often where electrolysis occurs the electrodes are different metals. In this case the metal that is most prone to chemical change is affected. Notably copper, copper alloys, silver, silver solder, gold, platinum, and rhodium are cathodic to aluminium which suffers preferential corrosive attack. For this reason, it is important not to fit copper alloy radio earth plates where they may come into contact electrically with aluminium, such as outboard legs - even at a distance.

On the other hand zinc, magnesium and their alloys are anodic to aluminium and suffer attack themselves. This includes galvanised products which are zinc coated. Metals such as stainless steel (18/8, 18/2 and 13%Cr) and chrome plating (over 0.00254/0.0001 in) thick tend to reduce these bimetallic effects by forming protective films.

Wooden vessels are not immune to electrolysis. When wood absorbs sea water it can conduct electricity and corrosion between galvanically incompatible metals can arise, even when some distance apart. Therefore in the case of wooden boats it is normally better to use fittings less subject to electrolytic corrosion, such as gunmetals, phosphor bronzes and the like, rather than two-phase brasses (copper-base alloys) containing zinc which is prone to preferential attack.

Some ships use cathodic protection to prevent corrosion. In this method the current is neutralised by applying an equal and opposing current and/or use of sacrificial anodes of the correct polarity, in effect changing the anodes that are at risk into cathodes. However, this method is not always desirable, as it requires practical methods of monitoring the changing potential of the structure and trained personnel skilled in ensuring the correct electrical values are maintained. Where protection is not provided, it is necessary to exert great care in choice of materials and in electrical equipment installation to prevent the occurrence of electrolysis.

When choosing fittings and materials, Moonraker always takes into account factors of corrosion, especially where the marine environment is concerned. Moonraker E Plates are constructed from 94% low corrosion copper alloy, suitable for all waters including tropical, and are also available in low corrosion aluminium (at the same price) for installations where bimetallic attack may be a factor, for example vessels with aluminium outboard legs.