Anodes For Yachts,fishing boats, trawlers

ANODE INFORMATION AND GALVANIC CORROSION.

Galvanic corrosion is something you probably remember something about from school but by now may have forgotten most of what it actually means.

Basically it is the process whereby, if two dissimilar metals are in contact in an electrolyte (e.g. Salt water), then the most reactive one will corrode first.

The Basics

There is a great deal of chemistry behind it which is well worth looking into if you have time, but if not, then the quickest way to understand it is to look at the Galvanic Series which essentially lists the most common metals in a chart where the most reactive are at one end and the least reactive are at the other.

As can be seen from the Galvanic chart below, Magnesium is at the top(most reactive) and Graphite is at the bottom (least reactive). If any two of the metals within this chart are in contact with each other, the the one nearest the top will sacrificially corrode for the one below it which is less reactive.

The Application

With regards to your boat and its protection, which i assume is why you are reading this, the different metals in your boat which are in contact are all vulnerable to corrosion in this manner.

Because your engine is probably made of cast iron, with various cast aluminium parts, while your  propellor shaft made with a certain grade of stainless steel and your propellor itself is made with bronze.....etc. You can see that there are numerous potential cases of  sacrificial corrosion possibilities! One way to protect your boat is to connect up a more reactive metal than is currently on the boat( the anode) , another is to isolate as many of the dissimilar metals as possible.

The Anodes

The principle of Galvanic Corrosion can be used to our benefit to slow down the corrosion of metals on our boat.

Metals on our keel, rudder,skeg, skin fittings and various other components, are all subject to corrosion. By connecting a metal of higher reactivity to these components, the metals can be protected. This is where the commercial anodes come into play. The metal which corrodes first is called the anode. West Coast Anodes sell Zinc and Aluminium anodes.

Now something which is commonly misunderstood is that Zinc anodes are more sacrificial than Aluminium ones.

It is true that zinc as a metal is more reactive than aluminium in most of its alloy states, however, when the metals are made into commercial anodes such as US Mil Spec- A-1800k type anodes(the most common standard of zinc boat anodes), the specific mixture of metals( the alloy) means that in fact the aluminium alloy is more reactive than the zinc alloy. As can be seen below, the open circuit potential of Aluminium Alloy is more than that of the zinc. (-1100mV compared to -1050mV). This is why in brackish water, which is lower in ion concentration than sea water, aluminium anodes are recommended as they are slightly more reactive and less likely to form a crust in the brackish water.

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