Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. This type of damage typically produces oxide(s) or salt(s) of the original metal and results in a distinctive orange coloration. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

During corrosion at a particular spot on the surface of the object made of iron, oxidation takes place and that spot behaves as an anode. The electrons released at this anodic spot move through the metal and go to another spot on the metal and reduce oxygen at that spot in presence of H+ (which is believed to be available from H2CO3 formed due to dissolution of carbon dioxide from air into water in moist air condition of atmosphere. Hydrogen ion in water may also be available due to dissolution of other acidic oxides from the atmosphere). This spot behaves as a cathode. There are five general types of corrosion.

Galvanic corrosion is common and occurs when two metals with different electrochemical charges are linked via a conductive path. In this case, a corrosion resistant coating would be applied to prevent either the transfer of ions or the condition that causes it. Galvanic corrosion can also occur when one impure metal is present. If a metal contains a combination of alloys that possess different charges, one of the metals can become corroded. The anodized metal is the weaker, less resistant one, and loses ions to the stronger, positively charged cathodic metal.

Stress-corrosion cracking can seriously damage a component beyond the point of repair. When subjected to extreme tensile stress, a metal component can experience SCC along the grain boundary—cracks form, which are then targets for further corrosion. There are multiple causes of SCC, including stress caused by cold work, welding, and thermal treatment. These factors, combined with exposure to an environment that often increases and intensifies stress-cracking, can mean a part goes from suffering minor stress-corrosion to experiencing failure or irreparable damage.

General corrosion occurs as a result of rust. When metal, specifically steel, is exposed to water, the surface is oxidized, and a thin layer of rust appears. Like galvanic corrosion, general corrosion is also electrochemical. In order to prevent oxidation, a preventative coating must interfere with the reaction.

Localized corrosion occurs when a small part of a component experiences corrosion or comes in contact with specific corrosion-causing stresses. Because the small “local” area corrodes at a much fast rate than the rest of the component, and the corrosion works alongside other processes such as stress and fatigue, the result is much worse than the result of stress or fatigue alone. 

Caustic agent corrosion occurs when impure gas, liquids, or solids wear a material down. Although most impure gases do not damage metal in dry form, when exposed to moisture they dissolve to form harmful corrosive droplets. Hydrogen sulfide is an example of one such caustic agent.