Gradual decay of metal by electrochemical process or by chemical is corrosion. Galvanic corrosion is a generic form. An anode, cathode and an electrolyte are necessary to form galvanic corrosion. This combination is known as a galvanic cell. It is formed when two dissimilar metals are electrically connected by an aqueous solution that causes electron transfer.
Chemical reduction forms when the current enters the electrode from the electrolyte. Electrical potential difference occurs when anode and cathode are separated in a conductive electrolyte. The charged cat-ions flow from anode to cathode via a conductive electrolyte. An electrical circuit is formed by this action, and corrosion occurs at the anode. The cathode may corrode to a lesser extent.
Oxidation happens when anode loses electrons, which causes a positively charged metal surface. Cat-ions attract negative anions in electrolyte forming a new compound. It loses its former metal properties forming rust or iron oxide. Reduction refers to the gain of electrons at the cathode. Thus the cathode retains its metallic properties. The occurrence and magnitude of corrosion depends on the potential difference between anode and cathode.
Metals of highest potentials generally appear at the anodic end of the galvanic series and those with lowest potentials are at the cathodic end of the galvanic series. As a general rule, metals at the farther end of the galvanic series are more susceptible to corrosion when put together in a solution.
Galvanic corrosion is invariably due to an electrochemical process wherein incompatible metals are connected to an electrical field through an electrolyte. Non-compatible metals – examples are aluminum and copper and aluminum and iron. Aluminum has a high affinity with oxygen. It instantly forms a tough oxide film which retards further oxidation.
Aluminum and steel components are protected by powder coating. They come with attractive colors and a thick coating gives long lasting protection from corrosion. If the coating is scratched, corrosion starts gradually peeling the coating.