CATHODIC


FOR SMALL CRAFT PROTECTION


for cladding the hulls of naval vessels. Davy found that he could preserve copper in sea water by the attachment of small quantities of iron or zinc; the copper became, as Davy put it, “cathodically protected”.


BY ELLIOTT BERRY MIIMS


The first practical use of cathodic protection is generally credited to Sir Humphrey Davy in the 1820s. Davy’s advice was sought by the Royal Navy in investigating the corrosion of copper sheeting used


To understand cathodic protection, it is necessary first to understand how corrosion is caused. For corrosion to occur, three things must be present:


1. Two dissimilar metals 2. An electrolyte (water with any type of salt or salts dissolved in it)


3. A metal (conducting) path between the dissimilar metals


Basic Cell Corrosion


Metallic Connection Electrical Current


Cathode Corrosion


Electrochemical Reaction


Anode


The two dissimilar metals may be totally different alloys – such as steel and aluminium – but are more likely to be microscopic or macroscopic metallurgical differences on the surface of a single piece of steel.


In this case,


we will consider freely corroding steel, which is non-uniform.


If the above conditions exist, the following reaction takes place at the more active sites: (two iron ions plus four free electrons).


2Fe => 2Fe++ + 4e-


The free electrons travel through the metal path to the less active sites, where the following reaction takes place: (oxygen gas is converted to oxygen ion - by combining with the four free electrons - which combines with water to form hydroxyl ions).


O2 + 4e- + 2H2 O => 4 OH-


Recombination of these ions at the active surface produce the following reaction, which yields the iron corrosion product: (iron combining with oxygen and water to form ferrous hydroxide).


2Fe + O2 + 2H2 O => 2Fe (OH)2 Electrolyte 36 | The Report • March 2017 • Issue 79


This reaction is more commonly described as ‘current flow through the water from the anode (more active site) to the cathode (less active site).’


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