This piece is second in a series of posts on corrosion control. The first post provided an introduction to corrosion control and gave an overview of some of the methods used. Here, we are going to dive deeper into the world of cathodic protection, one of the more practiced and effective ways of controlling corrosion in process. In a later post, we'll see how the principle of cathodic protection can be used in complex impressed current cathodic protection (ICCP) systems for delicate control over corrosion. First, we need some background to understand how it all works.
Cathodic protection works by using the normally detrimental mechanism of galvanic corrosion as an advantage. Galvanic corrosion is a type of corrosion which occurs when two metals of differing elemental makeup come into contact in the presence of an electrolyte. This electrolyte can simply be a bit of water containing some sodium chloride, a common occurrence even beyond the oceans. These three constituents create an electrical circuit which allows the flow of electrons towards the metal with the higher electric potential, which is called the cathode. As a result of this electron flow, there is an ion flow in the same direction. This means that over time the metal with the lower electric potential, known as the anode, will lose ions, degrading it eventually to the point of not being electrically conductive. The process of galvanic corrosion is the principle which makes batteries work, and the ion migration is the reason batteries eventually stop working.
This observation led scientists to develop what they call the galvanic series. This is simply a list of metals ordered by their ability to win the battle for electrons relative to the other members of the list. Gold and platinum are near the top of this list, with zinc and magnesium on the bottom. This means that given a connection between gold and zinc in an electrolyte, the gold will take electrons from the zinc and also cause the zinc to decay through ion migration. Because gold and zinc have a high difference in electric potential, the current created will be large (at least in galvanic corrosion terms) and the corrosion of the zinc anode will happen quickly.
Even before the theory of galvanic corrosion was fully understood, Sir Humphry Davy cleverly suggested the British Navy actively use the process to protect their ships. The British Navy had been having problems with their copper ships corroding, so they asked Royal Society member Humphry Davy to investigate a solution. Working alongside Michael Faraday, Davy furthered the theory of electrochemical reactions by experimenting with reactions of copper, zinc and iron in a salt solution. As he had suspected, the galvanic corrosion process occurred, and he had begun the list which would ultimately become the galvanic series. So he suggested to the British Navy that they attach a small piece of zinc to the copper ships with the idea that the zinc would act as a sacrificial anode making the hull of the ship its cathode. Thus cathodic protection was born!
Of course cathodic protection has become more sophisticated as the galvanic series has become more clear. Engineers can now design with galvanic corrosion in mind, either avoiding the use of differing metals altogether, or planning a cathodic protection system when different metals must come into contact. These systems can be simply adding a sacrificial anode as is used for underground gas, water or septic tanks, towers, bridges, and smaller ships among others. Larger ships require a more precise control over the corrosion situation, which is where impressed current cathodic protection comes in. We'll leave that for next time.
Learn more about corrosion in this short video.