Cathodic Protection 101

 

Cruising one’s vessel into the horizon is no doubt fun but those who love boating admit that they derive pleasure from maintaining their boat too. This, in a sense, is no different than a car enthusiast spending hours detailing his car. Maintaining a boat requires a lot of boxes being checked off, whether it be washing the boat, changing the oil or checking the propeller for debris. It’s crucial to spend some time periodically to ensure your boat has everything it needs to function safely and effectively on the water. One such item is attaching sacrificial anodes to underwater parts so as to prevent corrosion of the metal. Luckily, you only have to worry about this once a year so long as the right anode is chosen in relation to the boat size and water type.

What is Cathodic Protection?

Whenever two different metals are immersed in water and are connected to each other, they form a battery of flowing current. The current or electrons that flow between the two metals is essentially one of the metals giving up bits of itself.

This process of wearing away of the metal is known as galvanic corrosion. Considering your boat is continuously in contact with water, it’s highly susceptible to this type of corrosion. Cathodic protection prevents corrosion by converting all of the anodic areas on the metal surface to cathodic areas by supplying electrical current from an alternate source such as a galvanic anode. In other words, cathodic protection occurs when an additional metal that has more electrochemical potential (i.e. easily corrodes) such as aluminum is introduced so that it sacrifices itself in preference to the steel structure.

History of Cathodic Protection

The science behind cathodic protection was first brought to light by British scientist and inventor Sir Humphry Davy in 1824. Davy started his experiments with zinc and iron as sacrificial anodes in response to the British royal navy’s need to prevent corrosion in naval ships.

Over a hundred years later, cathodic protection was further developed in the US to meet the demands of the rapidly expanding oil and natural gas industry that needed corrosion protection for underground and underwater pipelines.

Choosing the right alloy

Sacrificial anodes are made from active metals such as magnesium, zinc or aluminum. These three alloys have a more negative electrolytic potential than steel in the galvanic series which make them an ideal choice for cathodic protection of a less negative metal such as steel (see Table 1).

Metal  Electrolytic Potential
Carbon +0.3
Lead  −0.5
Cast iron −0.5
Mild steel −0.2 to −0.5
Aluminium −1.05
Zinc −1.1
Magnesium −1.6

Table 1: Thegalvanic series helps to determine which alloy should be used for cathodic protection. E.g. To protect a steel structure (-0.5), a more negative alloy such as Aluminum (-1.05) is chosen so that it corrodes faster thereby preferentially sacrificing itself (Source: Wikipedia)

Magnesium anodes are used in fresh water, whereas zinc is used in salt water. Aluminum can be used not only in both salt and brackish water, but is also more efficient and longer lasting than its zinc counterparts which results in increased time between replacements. What’s more, aluminum anodes are less toxic compared to zinc anodes. This is because zinc contains cadmium, an element known for its high toxicity to plants and other marine life.

If you're interested to learn why we should do away with zinc anodes, read this article. 

Application of sacrificial anode

For optimum efficiency, the size of the anode must be directly proportional to the size of the structure being protected, 1% being a good ratio. Anodes are relatively easy to install. Simply ensure electrical continuity between the steel structure and the anode using a low conductor such as lead. When the anode is half gone, it’s time to be replaced.

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