The demands of corrosion protection are one of the key topics in the field of maritime leisure. High standards regarding material selection and design must be observed, especially in a salt water environment, in order to ensure functionality and a long service life. This applies particularly to electric motors since the combination of electricity and salt water can have an extremely destructive effect in the event of a malfunction of improper handling (leakage currents, incorrect earthing).
A distinction is generally made between three types of corrosion: electrochemical, galvanic and electrolytic. All three types can occur in salt water as well as fresh water, but the effects in salt water are far more destructive. Beside salt content, other factors such as pH value and temperature also play a role.
Electrochemical corrosion occurs, when a nail is placed in water and rusts. It is, therefore, the degradation that results when an easily corroding material comes into contact with water. This type of corrosion can be completely avoided through careful material selection. This is why we only use A4-grade stainless steel, salt water-resistant aluminium and extremely high-quality and impact-resistant plastics such as PBT (polybutylene terephthalate) below the waterline.
In order to achieve the best possible compromise between corrosion resistance, strength and hardness, we make sure to use only the best of what leading-edge steel technology currently offers for mechanically critical parts. For the propeller shaft, for example, this means 1.4044-grade special purpose steel; for the shaft sheath on the Cruise we even use 1.4571 grade with titanium, a steel that is also used in the construction of drive shafts for container and cruise ships. Even when the the parts that are submerged in water often have the additional protection of coatings such as anodisation and seawater resistant paints, we do not rely on the coating (as coatings can suffer mechanical damage) but make sure we select only corrosion-resistant articles when choosing basic materials.
Galvanic corrosion occurs when two conductive materials with different chemical properties carry a current and touch under water.
Galvanic corrosion cannot occur if any of these conditions do not hold true. This should make it quite clear how galvanic corrosion can be prevented. For example, all conductive materials can be insulated from each other or electrochemically identical materials can be used (no galvanic corrosion at all can occur between an aluminium pylon support and an aluminium shaft sheath). Nevertheless, the complete exclusion of galvanic corrosion demands great care in the design phase and is very complex at a number of points (e.g. insulating the propeller shaft from the pylon support).
This is why the much simpler principle of the galvanic (sacrificial) anode has found greater acceptance in the boating industry. A galvanic anode is an electrochemically base metal (e.g. zinc or magnesium) that is attached to the motor in such as way that the more noble metals are protected against galvanic corrosion. Over time, the galvanic anode disintegrates and must be replaced after a certain period.
Torqeedo has chosen to take the more sophisticated path in this respect. That's why the Travel is today the only outboard motor that functions without a galvanic anode. It is designed from start to finish in such a way that galvanic corrosion can be completely ruled out.
Electrolytic corrosion is the potentially the most destructive type of corrosion. It acts around 10,000 times faster than galvanic corrosion and can literally dissolve entire motors within a matter of days. That's the bad news.
The good news is that electrolytic corrosion is always due to wiring faults, especially to problems with earthing.
A common error is, for example, to connect the Cruise 4.0 to 4 serially wired 12 V lead batteries and then the on-board radio, too – which needs 12 V supply voltage. And in such a way that it's connected between the 3rd and 4th lead batteries (i.e. between 36 and 48 V). Since most simple electronic devices have their earth on the housing, a radio attached to the hull of an aluminium boat will result in a voltage difference of 36 V between the boat's hull and the motor earthing, which can lead to dramatic corrosive effects. This problem does not occur if the on-board radio is connected between 0 and 12 V (i.e. between the 1st and 2nd batteries).