Do I need to bond all my skin fittings to an anode?
All electrically conductive materials have a galvanic potential shown on a scale of voltages compared to a neutral reference. Noble metals such as gold and titanium have positive potential, and base metals such as zinc and aluminium have a negative potential. When two materials with different potentials are linked by an electrolyte, such as seawater, current flows from the positive to the negative. Ions flow the other way resulting in erosion of the negative, or anodic material. It thus becomes sacrificial, protecting the more noble material from erosion.
It was common practice up to the 1980’s to link all metal underwater fittings with an electrical bonding circuit connected to a hull anode. There is about 0.3 volts potential difference between marine bronze used in fittings, and zinc used for anodes. With the introduction of more varied alloys and brasses used for marine fittings, the issues became more complicated. There can also be a potential difference of about 0.1 volts between noble copper alloys such as nickel bronze and manganese bronze, and cheaper copper alloys such as zinc brass used for popular valves and skin fittings.
For a sacrificial anode to be effective it needs to be close to the component being protected. This is because potential difference or voltages drop over a distance. It is termed ‘line of sight’ in that the anode needs to be able to easily ‘see’ the propeller or skin fitting it is intended to protect. This is why most anodes are located near the stern gear. The through hull skin fittings for heads compartments etc are often scattered around the hull bottom.
If all the skin fittings are bonded in one big circuit to an anode, they are also bonded together. This means that there is a possibility of an electrochemical reaction occurring between skin fittings which could be of slightly differing alloys. Although their potential difference will lower than to the anode, they are likely to be very much closer to each other than to the anode and therefore the potential difference can be actually be relatively greater. Best practice now is to ensure that most through hull skin fittings are electrically isolated within the vessel, unless they are mounted close to the anode.