Cathodic Protection Rectifiers

Everyone has seen the large-scale results of metal corrosion. The formation of rust or the cracking and flaking caused by brittleness are visible results of corrosion that can ruin property and require costly replacements. These changes in metal result from its interaction with substances in the immediate environment. Though water and salt are the most obvious culprits, corrosion can also be caused by industrial reactants such as acids and chlorides.

On the molecular level, the mechanism of corrosion is the switching of electrons from atoms of one substance to those of another. Oxygen steals electrons from other substances very easily, forming oxides that often have very different properties from the original substance. For instance, when iron is exposed to oxygen, it gives up electrons to it. The reaction is actually burning slowed to a pace where the heat and light generated are not visible. The result of this super-slow burning is a crumbly, brown substance commonly known as rust.

As this engineering service company shows, salt corrosions cause microscopic pitting of metal eventually resulting in a slow wasting of the entire piece. Hydrogen corrosion causes the metal to become brittle and crack, requiring replacement of metal parts in extreme cases.



Corrosion as an Electrochemical Process

Corrosion as an Electrochemical Process





All these activities happen because of a movement of electrons through metal. This movement forms a current between a region of the metal called the anode and a nearby region called the cathode. Oxidation and the resultant corrosion happens at the anode, causing the current flowing to the cathode, where the electrons are eliminated in another type of chemical reaction. This flow of electricity is the reason metal objects will sometimes cause a startling shock when touched. This is often erroneously called “static electricity,” though it is really in constant motion through the metal.

The important thing to remember about this process is that corrosion only occurs at the anode. This raises the possibility of limiting corrosion of a piece of metal by attaching a sacrificial piece of another metal that oxidizes more readily. This metal becomes the anode and focus of the corrosion, whereas the piece to be protected becomes the cathode and only corrodes very slowly, if at all. Since the part protected from corrosion is the cathode, this method is called cathodic protection. The anode eventually becomes unusable because of corrosion and must be replaced.




On a small scale, nature alone provides the energy to make this process happen. On larger objects where natural electrical activity is insufficient for full protection, more electricity must be added to give the anode enough power for large-scale anodization.

Anodes take many forms depending on their application. On the undersides of ships, they are rounded bars attached to the hull. On power cables, they are sometimes metal rectangles placed periodically along the cable length. Regardless of shape, they all serve the same purpose: They are sacrificed to corrosion so something else will escape it. A consultation with an engineering service can determine which type of cathodic protection is appropriate for each setting.