Background information – corrosion and corrosion protection
Corrosion protection is one of the key issues in the leisure boating sector. In order to ensure proper functioning and a long service life, material selection and design must meet stringent requirements, particularly in salt water environments. This is especially true for electric motors, as the combination of electricity and sea water can have extremely destructive effects in the event of a malfunction or operator error (leakage current, incorrect earthing).
In general, it is possible to distinguish three types of corrosion: electrochemical, galvanic and electrolytic corrosion. All three types can occur both in salt water and in fresh water, but are much more destructive in salt water. Apart from the salt concentration, other parameters such as pH and temperature also play a key role.
Electrochemical corrosion includes, for example, the rusting of a nail immersed in water as a result of the contact between easily corroded materials and water. This corrosion can be entirely prevented by the careful selection of materials. This is why we use only A4 stainless steels, seaworthy aluminium and extremely high-quality plastics with high impact toughness (such as PBT – polybutylene terephthalate) below the water line. To obtain the best possible compromise between corrosion resistance, strength and hardness, we use the best steels currently available, such as grade 1.4044 special steel for the propeller shaft and even 1.4571 with titanium for the shaft tube. This is the steel used for the shafts of container ships and cruise liners. Even though submerged parts are often protected by coatings such as industrial anodizing and seaworthy paint, we do not rely on the coating for corrosion protection (as coatings are subject to mechanical damage) but select corrosion-resistant materials.
Galvanic corrosion occurs when two conductive materials with different electrochemical properties are in electric contact and both materials are located in water.
If any of these three conditions is not met, galvanic corrosion cannot occur. This clearly indicates how galvanic corrosion may be prevented by the design of a system. For example, all conductive materials may be isolated from each other or materials with identical electrochemical properties may be used (no galvanic corrosion can occur between an aluminium pylon support and an aluminium shaft tube, for example). However, the complete exclusion of galvanic corrosion calls for considerable design efforts (e.g. isolation of the prop shaft from the pylon support, For this reason, it is normal practice to use sacrificial anodes on outboard motors. A sacrificial anode is an anode made from a less noble metal in the electrochemical series (e.g. zinc or magnesium) which is installed on the motor in such a way that the more noble metals are protected against galvanic corrosion. Over the course of time, the sacrificial (or galvanic) anode is dissolved and needs to be replaced.
Torqeedo has adopted the more complex approach, which is why the Travel is the only outboard motor in the world that does not need a sacrificial anode. It is designed throughout in such a way that galvanic corrosion is not possible.
Of all three types, electrolytic corrosion has the highest destruction potential, acting about 10,000 times faster than galvanic corrosion, and can literally dissolve an entire motor in a matter of days. This is the bad news, but there is also some good news: electrolytic corrosion is always the result of wiring faults, especially problems with earthing.
One common error, for example, it to connect the Cruise 4.0 to four 12 V lead-acid batteries connected in series, together with the on-board radio, which needs a 12 V power supply. Often, the radio is connected between the third and fourth batteries in the bank (i.e. between 36 V and 48 V). As most simple electronic devices have the earthing connection on their housing, this means, in the case of a radio bolted to an aluminium boat, that there is a potential difference of 36 V between the boat's hull and the motor earthing connection, which leads to dramatic corrosion effects. If the radio is connected between 0 and 12 V (between the first and second batteries), this problem does not arise.