You must have often come across corroded metal components, but have you ever tried thinking what corrosion is and what does it lead to? Usually seen in the case of metals, when the essential properties of a metal begin to deteriorate due to a chain of chemical reactions between the electrons of the given metal and air or water molecules, it results in corrosion. For instance, when a tin roof is under constant exposure of wind and rain, it results in rusting of the roof. This is a form of corrosion. One of the many ways in which corrosion can be avoided is by coating the metallic surface with a protective layer, so that it does not come in direct contact with water and oxygen. You must have noticed a polymer coating applied on automobiles to prevent them from corrosion. Other methods, such as plating and cathodic protection, are also used to minimize the level of corrosion. This article provides a brief insight of the various types of corrosion usually seen.
Different Kinds Of Corrosion
Uniform or General Corrosion
This form of corrosion is uniformly distributed over the entire surface area or a large section of the total area. Uniform corrosion usually results due to the breakdown of protective coating systems on structures. Oxidation of steel, dulling of a polished surface or etching of a surface due to acid cleaners are few examples of uniform corrosion. If this form of corrosion is allowed to continue, the surface may eventually become rough and result in more destructive types of corrosion. Uniform corrosion is easily predicted and can be controlled effectively through cathode protection and enamel or any other form of coating. In few cases, uniform corrosion adds further appeal to a surface. Patina is created by naturally tarnishing the copper roofs, while rust hues are formed by weathering steels.
This is a localized form of corrosion, which results in cavities produced on the surface material. This is a more damaging form of corrosion with respect to uniform corrosion, since even a small pit, with minimal overall metal loss leads to a complete failure of the entire machine. The pits can be either hemispherical or cup-shaped and can be with their mouth open or covered with a semi-permeable membrane of several corrosion products. Pitting corrosion can result due to localized chemical or mechanical damage to the protective oxide film. This may happen due to low dissolved oxygen concentrations or high concentrations of chloride. This form of corrosion can also result due to damage or careless application of a protective coating or even presence of non-uniformities in the metallic structure of the component.
When two dissimilar materials are coupled together in a corrosive electrolyte, this form of corrosion takes place. Due to the formation of a galvanic couple, one of the metals which become the anode corrodes faster than it otherwise would, if it were alone. The cathode metal on the other hand corrodes slowly. Galvanic corrosion occurs only when two electrochemically dissimilar metals are in electrical contact and are exposed to an electrolyte.
This is a localized form of corrosion and is associated with a stagnant solution in a micro-environmental level. It generally occurs in crevices or shielded areas like gaskets, washers, lap joints, clamps and fastener heads. It is initiated due to the depletion of inhibitor or oxygen in the crevice, a shift in acidic conditions in the crevice, or a gradual piling up of ion species like chloride in it. For instance, when the oxygen diffusion in the crevice is restricted, the cathodic oxygen reduction reaction in the area cannot take place and the concentration cell in turn becomes anodic in character. This imbalance leads to the formation of highly corrosive micro-environmental condition in the crevice and leads to metallic dissolution.