Gold – Most ‘noble’ metal Titanium
Stainless steel 316 (passive) Stainless Steel 304 (passive) Silicon bronze
Stainless Steel 316 (active) Monel 400
Phosphor bronze Admiralty brass Cupronickel Brass plating Yellow brass Naval brass 464 Tungsten
Stainless Steel 304 (active) Chromium plating Copper Cast iron Mild Steel Lead Tin
Zinc anode Aluminium anode
Magnesium anode – Least ‘noble’ metal
So, whilst it would be really useful to build our boats out of gold and titanium, they are not the most practical of hull building materials for several reasons. You may be surprised to see plain old mild steel so far down the table and that is probably the most significant observation to make. Mild steel is readily corroded by almost all of the other marine metals, however that is mitigated by what is called the surface area effect, whereby a less noble metal (e.g. a steel hull) can lead to the corrosion of a more noble metal (e.g. a small brass skin fitting) due to the imbalance in their relative surface areas.
This brings us to the final consideration, how effective is any given anode? The choice of anode material is dependent on the medium in which the vessel is predominantly used. Magnesium anodes are best suited to fresh water, aluminium for brackish waters and zinc for salt water. An easy way to remember this is to remember how water flows with this cunning euro-mnemonic “Mountains Am Zee”: M (fresh water – magnesium) A (brackish – aluminium) Z (sea – zinc). Next to consider is the size of the anode and how much metal it can ‘protect’. There is no precise answer as to how effective any individual anode will be as its position, electrical contact and the size and composition of nearby metals all influence its performance. As a rule of thumb however, we should expect a typical anode to be effective to a distance of 6-7 times its own length. So, for a typical 3.5kg magnesium narrowboat
anode that is approximately 300mm long, it should be effective to a distance of about 2 meters in any direction.
A sacrificial anode’s effectiveness is directly proportional to the purity of the metal from which it is cast. Zinc anodes must be more than 99% pure zinc if they are to function effectively and their performance is greatly reduced by the presence of impurities such as lead or iron. So, whilst high quality zinc anodes from a reputable supplier are expensive, you are paying for this guarantee of quality that the anode will do its job. There are plenty of lesser quality anodes available online which come at a greatly reduced price. These are typically cast by scrap merchants melting down old zinc fittings and part worn anodes without the equipment or expertise to refine the resultant metal. As is nearly always the case; you get what you pay for.
Narrowboat anodes can be fitted as either welded or bolt on The Report • September 2020 • Issue 93 | 93
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