Why are anodes gradually worn away

Aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis. Even though aluminium is more abundant than iron in the Earth's crust , aluminium is more expensive than iron. This is mainly because of the large amounts of electrical energy used in the extraction process.

Aluminium ore is treated to produce pure aluminium oxide. The electrolytes used in electrolysis are ionic compounds :. Aluminium oxide is insoluble in water, so it must be molten to act as an electrolyte. However, the melting point of aluminium oxide is high. A lot of energy must be transferred to break its strong ionic bonds , and this is expensive.

To reduce costs, powdered aluminium oxide is dissolved in molten cryolite. The use of molten cryolite as a solvent reduces some of the energy costs involved in extracting aluminium. The diagram shows an aluminium oxide electrolysis cell. Both the negative electrode cathode and positive electrode anode are made of graphite, a form of carbon.

Aluminium ions receive electrons at the negative electrode and are reduced to aluminium atoms :. The molten aluminium sinks to the bottom of the cell, where it is tapped off.

In other words, this form of sacrificial anodes cathodic protection uses the naturally occurring electrochemical potential difference between different metallic elements to provide unrivalled, long-lasting protection for the primary metal structure. This form of cathodic protection is tried and tested across many industries for preventing rust on submerged and underground metallic structures.

The most popular metal alloys for making these anodes are magnesium , aluminium and zinc. What is the difference between using each of these for the rods? Well, each alloy actually has a very specific application. Magnesium anodes are the most active.

This means that they are the only ones that work really well in the low conductivity of fresh water due to their high negative electrode potential. These kinds of metal alloys are both suitable for use in salt water where resistivity is generally lower.

Aluminium is highly resistant to hard water making it perfect for these types of applications and in older tanks. It also does carry a better environmental footprint than zinc anodes as they do not contain cadmium, which is harmful to marine ecologies. Aluminium possesses a lower voltage rate than its magnesium counterpart and also offers a far slower consumption rate than either magnesium or zinc. Typical uses are for the hulls of ships and boats , offshore pipelines and production platforms, in salt-water-cooled marine engines, as well as small boat propellers and rudders.

On the other hand, Zinc-based sacrificial anodes are also suitable for salt water applications and are deployed to protect marine assets. Zinc is not suitable for use at high temperatures though, as it tends to passivate. These types of anodes can be produced in rather complex geometry, as opposed to aluminium, which can be useful in more streamlined designs e.