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Our boats are subject to several problems that have destructive effects. The first and best known of these is osmosis. Osmosis, through a chemical reaction, can affect the integrity of the hull, but it rarely creates serious damage if it is treated within a reasonable time, calculated in years. A very different matter, however, is that which concerns another reaction, this time an electrochemical one which, if not controlled, within a few months can create such damage as to cause the sinking of the boat, this is the corrosion of metals by galvanic current, or wandering currents. Galvanic currents Galvanic currents are one of the worst enemies that a shipyard has to fight when designing and building a boat. It is mistakenly thought that these are lethal for aluminum boats, but the owners of these boats are too aware of the danger to be caught off guard, when, instead, the owners of fiberglass boats underestimate it and often pay the consequences . Galvanic currents are an electrochemical reaction that is created between two metals of different electrical potential (aluminium – steel; bronze – aluminum etc.) immersed in an electrolyte and connected to each other. The electrolyte in question is sea water. To have a galvanic current between two metals it is not necessary for the two to be immersed in water, they can also be dry, on deck, where however salt has deposited which when it comes into contact with the humidity of the air forms the electrolyte. In the presence of galvanic current, the metal with the lower electrical potential is consumed and dissolves in the electrolyte, while the metal with the higher potential remains intact. The intensity of the currents Galvanic currents can be more or less intense and therefore cause different damage. Their intensity depends on: A) the potential of the two metals: the greater the potential difference, the greater the current; B) from the ambient temperature: the higher the temperature, the higher the current; C) by the salinity of the water: the saltier it is, the more it conducts; D) by the mass ratio of the two metals: if the anode, i.e. the metal with lower potential, the one being corroded, is much smaller than the cathode, the metal with higher potential, the current will be more intense and the anode will it will consume more quickly. Stray currents Stray currents are often confused with galvanic ones, but they are very different and the former are more dangerous than the latter. Galvanic currents are created by the simple presence of two metals somehow connected to each other within an electrolyte. The quantities of energy produced by this phenomenon are, in most cases, very small and it takes time to cause serious damage. Stray currents, on the other hand, are created when the return current to the batteries does not pass through the normal channels, i.e. the cable that reaches the negative of the battery, but takes other routes. This occurs whenever the cable through which the electric current should flow is damaged in some way. In this case, the return current towards the batteries, finding an obstacle, abandons the cable and takes a simpler route, such as the bilge water. These currents are much stronger than galvanic currents and do not need the presence of two metals, they can attack any metal, no matter its potential, this determines a greater destructive power than that of galvanic currents.

The damage It is not uncommon to see a mast base with green dust around or concentrated around the rivets, that green dust is one of the consequences of the galvanic current, if the owner does nothing, over time, the galvanic currents that are by exercising in that point of the tree, they will pierce it, weakening it until it causes its fall. However, if the owner notices it first, a drastic repair will be necessary which will change the elasticity of the mast forever. Once upon a time, in the 1970s and 1980s, several shipyards, especially the more artisanal ones, used to secure the bulb of boats with iron nuts on steel studs. They did so aware that the difference in material would cause corrosion of the iron nut, which would fuse with the stud, making the counter nut useless, which was indispensable if these were made of steel. This worked for many years, but today, in some cases, those iron nuts are so corroded by the currents that they give way and the bulbs fall, the boats turn over and, sometimes, sink. Some repair yards, paying little attention to the quality of the work and paying great attention to their pockets, mount reinforcing stainless steel plates under the anchor chain winches when the owner asks to fix a more powerful winch and forget to insulate the two materials . The bases of winches, in most cases, are made of cast iron. The stainless steel, in direct contact with the cast iron, corrodes it, the holes where the bolts holding the winch pass become deformed and widen and, one fine day, the winch flies into the water together with the chain. Another dangerous situation can be caused by a bronze propeller mounted on a boat moored near a steel boat. If both boats are connected to the shore power supply, the bronze propeller will suffer from the strong galvanic currents that the proximity to the steel hull creates. Protecting the boat To avoid damage from galvanic and stray currents, the potential ratios of the metals present in the boat must be changed. If aluminum is a metal with a contained and lower electrical potential than many other metals, therefore at risk of galvanic corrosion, it will be necessary to place it near it or connect it to a metal that is even less noble, that is, one that has an even lower potential , such as zinc. For this reason, zincs or, as they are commonly called, sacrificial anodes will be fixed under the hull and inside it. Their task is to consume themselves in place of other metals.

However, we cannot put anodes everywhere, so in many cases we must be careful that galvanic currents are not created. The best way to do this is to isolate the two metals of different potential. The electric winch with a cast iron base which is mounted on a stainless steel plate which reinforces the fiberglass base is subject to a strong galvanic current, given by the potential difference between the stainless steel and the cast iron. To avoid it, it will be sufficient to insert a third insulating material, such as a rubber or plastic sheet, between the two materials. If you need to fix something to the aluminum mast with screws or rivets, be careful to use screws and rivets made of a more noble material than aluminum, such as steel. A rivet is very small in relation to the mass of the shaft, the galvanic corrosion it can cause is almost imperceptible. However, if you also want to avoid this, insulate the screw with an insulating paste such as Duralac which, when applied with a brush, once dried creates an insulating rubbery layer between the two materials.

Bounding In larger boats, instead of placing anodes near the metal masses, the bounding system is used. This consists in connecting all the metal masses together and, therefore, placing a sacrificial anode at the end of the chain. Although one may instinctively think that in this way the galvanic cells are accentuated, in practice this is not the case, it will always be the least noble metal that is corroded by the galvanic currents, in this case the zinc which acts as an anode . Bounding also solves other problems, such as having to put a collar anode on the axis and on the propeller, which in some cases can lead to the unbalance of the axis and the propeller, due to the effect of the anode which does not it wears out evenly along its circumference. With bounding they come into contact with the axis of the brushes connected to the copper line, which unites all the other metal parts including the shaft. The bounding must then be connected to the negative of the batteries at the anode, and therefore to the earthing which on a boat becomes seagoing. To achieve grounding, the bounding can be connected to the bulb or to a dispersion plate. This system is also excellent in the event of lightning, at least as far as human life is concerned, because the electronic instruments, in the hypothesis that the boundary was crossed by lightning, would jump due to the enormous electromagnetic field that would be created.

The electrical system As we have seen, an even greater danger than galvanic currents are stray currents. These do not need two metals immersed in an electrolyte to exert their corrosive power, they only need one metal on which to discharge their destructive force. To protect yourself from these you need to have an electrical system in perfect condition. Any stripped wire or a rusty switch can cause significant electrical losses. This is why it is essential to take care of your electrical system on a boat. When making a joint between two copper cables, avoid coupling them by twisting them on themselves and sealing everything with adhesive tape, use clamps and insulating adhesive tape on these. Avoid running electrical cables too low in the bilge, where they can come into contact with water. Replace the switches when they show traces of rust. Good grounding will prevent stray currents, effectively extending the life of the boat. To maintain the grounding system, the braid terminals must be checked from time to time. These must be clean and not oxidized. Remember that the braid must not pass into the bilge water.

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Article by Stefano Cerulli and Federico Sennati