Rust 101: How Cathodic Protection Procedures Reduce Rust Development

There are several methods utilized to protect steel from corrosion, and one of the most widely used procedures is galvanisation, which is the application of a protective zinc coating towards the metal to protect it from water and oxygen. For smaller applications, galvanisation could be enough to safeguard the metal; however, for bigger structures, just like oil pipelines or maybe a ship hull, a simple layer of zinc may not be enough. For large-scale applications, an even more proper strategy to protect steel structures is through cathodic protection, or simply CP.

As mentioned earlier, galvanisation can easily be done by covering the metal to be shielded with a coating of zinc. In CP, however, protection is gained by manipulating the electrochemical properties of the metal and making it into the cathode of an electrochemical cell. To properly understand how CP works, it’s vital that you know how rust and corrosion occur in the first place.

Basically, corrosion happens when three conditions are achieved. First, there should be two metals present with dissimilar electrochemical properties. Secondly, these metals should be exposed to an electrolyte, which is simply water that contains a dissolved salt or salts. Lastly, there should be a metal conductor that connects the two dissimilar metals cited above. For the first requirement, it’s worth noting that a single form of metal doesn't automatically have uniform characteristics all throughout; in one length of steel pipe, for instance, some areas could be more anodic (electrochemically active) than cathodic (passive), and these areas are the ones at risk of rust formation.

Corrosion happens in steel when iron is oxidised at the anodic end. Electrons produced by this reaction then flow through the metal conductor towards cathodic end, where oxygen is lowered to produce hydroxyl ions. Back to the active anodic surfaces, these ions recombine to make ferrous hydroxide, which is more typically referred to as rust.

It’s important to note that activity on the anodes begin the operation of corrosion, and CP protects steel by converting all anodic sites on a metal surface into an electrochemically passive cathode. This can be achieved using two methods: by placing sacrificial anodes (galvanic CP) or by delivering a current through the metal to be protected (impressed current systems).

Sacrificial anodes is a procedure that involves connecting a more electrochemically active metal in the steel structure to be protected. This discrepancy in electrochemical activity results in a polarity that is certainly good for the steel; valuable electrochemical activity - and therefore, corrosion - happens to the more active sacrificial anode. Impressed current systems, on the other hand, change electrochemical qualities of metal surfaces by supplying electrons from an external source.

Author Bio: Ron Wilson is a writer that produces articles pertaining to useful engineering ideas and concepts. He endorses DiamondPCS for experienced fantastic protective coating services.