Different types of corrosion:

Corrosion is one of the biggest enemies of a yachtsman. There is a lot of misunderstanding about corrosion. One often mistakes the corrosion taking place under water for the corrosion taking place above deck.
It's very important to identify two types of corrosion:

Example 1 of electrochemical corrosion caused by an electrical connection of the rudder stock
with another underwater object like the propeller shaft.

Example 2 of electrochemical corrosion caused by an electrical connection of the rudder stock
with another underwater object like the propeller shaft.

Example 3 of electrochemical corrosion caused by using copper based anti-fauling in contact with the rudder shaft.

Example 4 of extreme electrochemical corrosion caused by an electrical connection of the rudder stock
with another underwater object like the propeller shaft.

Example 5 of extremely heavy electrochemical corrosion caused by an electrical connection of the rudder stock with another underwater object like the propeller shaft.
Please note the rudder shaft material is stainless steel proving electrolysis is not an aluminium rudder stock problem, but a general material independent problem.

Example of a 25+ year old aluminium 6082 rudder shaft which served on a X99. When proper measures are taken to prevent electrochemical corrosion,
aluminium rudderstocks will last more than a lifetime.

The principle of electrolysis:

The principle of electrolysis is based on the fact that when a metal is put into water it will create a non neutral voltage. On the surface a very small quantity of positive charged metal ions will break out leaving the negative charge in the form of electrodes behind in the metal. The reaction is (Me=metal) Me -> Mez+ + z e. The metal will be negatively charged. As long as this metal is not connected to another object with a different charge, nothing will happen and a balance will have been reached. 
The reaction as described above is not enough for electrolysis to arise. The problem starts because all existing metals have a different charge when put underwater. Some positive, some negative. The absolute charge varies and increases when the temperature rises, the amount of salt in the water is higher and if more oxygen is present in the water. The absolute worst place for electrolysis is the Mediterranean as all factors are on it's worst.
The phenomena described above is still not enough for electrolysis to arise. Only when the two different metals are electrically connected, electrolysis can start. One metal (the one with the most negative charge) will act as anode and sacrifice itself. The other metal with act as cathode. As the anode is electrically connected to the positively charged cathode, it can't reach it's natural negatively charged balance as described above. But it will constantly try to reach it. To get a more negative charge, the metal will split constantly into positively charged metal ions breaking out of the surface and leaving behind the negatively charged electrons. The electrons will be transported through the wire connection the two metals to the more positive charged cathode, and the whole process will start over again. It will only end when the complete anode is dissolved.

Electrolysis on GRP and composite yachts:

Lets now project this theoretical phenomena to your sailing yacht. First we have to determine which metal will start acting as anode. Following list is a sum up of metals in order of electro negativity, starting with the most negatively charged metal: Magnesium, Zinc, Aluminium, Steel and Iron, cast Iron, Stainless Steel (active) , Lead, Brass, Copper, Bronze, Stainless Steel (passive), Carbon. (One can make Stainless Steel passive by chemical treatment after machining). Second, the hull material is important. Lets start with composite hulls.
Typical rudder stock metals are aluminium and (active) stainless steel. Typical propeller shaft metal is (passive) stainless steel. Typical propeller metal is bronze. Typical keel material is lead or cast iron or a combination of both. (The propeller shaft can be made out of passive stainless steel as the geometry is often very simple, no welding will have to be done and can be ordered as passive stainless steel).
Lets take a yacht with an aluminium rudder shaft, standard propeller drive system and a cast iron keel. Some boat builders choose to connect all tree parts with a copper wire (see red line in illustration). They protect the complete system with a zinc anode on the propeller or propeller shaft. The zinc anode has the lowest electro negativity and will act as anode. The propeller shaft, made of passive stainless steel, will act as cathode. As long as the zinc anode is present, it will all go well. Problems will arise when the zinc anode either is completely dissolved or falls off. The new anode will be the aluminium rudder shaft, which will start to sacrifice itself. Even if one had chosen to use a stainless steel (active) rudder shaft, the same would happen: The rudder shaft will start to dissolve in the water. A typical example is a Scandinavian boat owner, having sailed years without any problems, now take's his yacht to the Mediterranean and is confronted after one year in the water with the fact that the zinc anode has been completely dissolved in the water and his rudder shaft shows signs of electrolysis. The extreme salty and warm environment highly increases the electro potential difference between the different metals, making the reaction to go much quicker.

Solution to avoid electrolysis:

This is the reason that we can only advise to make sure the rudder shaft is completely electrically disconnected from the rest of the vessel. Often this connection in made without the awareness of the boat owner. Other parts on the yacht, like the steering system and autopilot drive, are linked to the rudder shaft. Parts like the engine control and instruments are again linked to the steering system. Without any precautions a connection to the engine and propeller shaft is easily made. As the general minus pole of the power supply on the yacht is mostly connected to the engine, every electrical function on board is connected to the propeller shaft. (An exception to this is when a trust bearing in combination with an electrically isolated flexible coupling is used in the propeller drive system). Lets closely look at the parts that could connect to your rudder shaft:

Test methods:

When the yacht is ashore, these connections can be tested by using an accurate resistance meter. One should connect one pole to the rudder shaft and the other pole to the propeller shaft or keel. The resistance should be infinite. If the resistance is less, one has a current leak and should test all above possible connections.
When the yacht is in the water, one can do the same test. A second test is to measure the voltage difference between the rudder and propeller shaft. Depending on the factors water type, temperature, used metals as described above, the voltage difference can vary from a couple of millivolts to tenths of volts. If a voltage difference is present, there will be no connection between the measured parts.

Electrolysis on aluminium and steel yachts:

The rules for avoiding electrolysis on aluminium and steel yachts are the same as on composite yachts: Disconnect the rudder from the rest of the ship. Mostly metal ships will have metal rudder blades. It's advisable to protect these blades with a separate zinc anode on the rudder blade. Especially when different types of metals or metal mixture are used. For example a stainless steel rudderstock with a steel blade, or a aluminium rudder stock with an aluminium blade. The aluminium of the rudder stock is not the same as the aluminium of the blade. A small galvanic element is created that should be protected with an anode.

Electrolysis between the yacht and the outside world:

This type of electrolysis is the most feared type. A complete aluminium hull can dissolve in one month making the yacht a total loss. Instances are known even on composite yachts that lost a complete stainless steel rudder in one month time.
As this explanation of electrolysis is mainly based on protecting the rudder system, and as long as the rudder shaft is electrically disconnected from the rest of the yacht, the rudder shaft can't suffer from electrolysis.
The only way to prevent electrolysis between your yacht and the outside world is to completely insolate the power supply via a galvanic isolator.
Following links provide more information on electrolysis and galvanic isolators.






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