17.6 Corrosion
Corrosion can be viewed as the process of returning metals to their natural state-the ores from which they were originally obtained. Corrosion is considered to be the breakdown of materials due to chemical reactions.
It is usually oxidation with air molecules and often in the presence of water. Corrosion also occurs when an acidic/basic corrosive substance touches another material. When a material corrodes, its physical properties change. Problems with corrosion are mostly with metal, though other materials can corrode.
Metals corrode because they oxidized easily. Corrosion Involves the oxidation of a metal. Since corroded metal often loses its structural integrity and attractiveness, this Spontaneous process has great economic impact. Approximately one-fifth of the iron and steel produced annually is used to replace rusted metal due to the corrosion.
Corrosion is a form of erosion. Some materials, such as stainless steel, are highly resistant to corrosion. Metals that may corrode can be protected by plating, painting, and other means.
Perhaps the most familiar example of corrosion is the formation of rust on iron. Iron will rust when it is exposed to oxygen and water. Rust formation involves the creation of a galvanic cell at an iron surface, as illustrated in the Figure below.
The above Figure shows Corrosion can occur when a painted iron or steel surface is exposed to the environment by a scratch through the paint. A galvanic cell results that may be approximated by the simplified cell schematic Fe(s) | Fe2+(aq) ||O2(aq), H2O(l) | Fe(s).
The relevant redox reactions are described by the following equations:
Further reaction of the iron(II) product in humid air results in the production of an iron(III) oxide hydrate known as rust:
The stoichiometry of the hydrate varies, as indicated by the use of x in the compound formula. Unlike the patina on copper, the formation of rust does not create a protective layer and so corrosion of the iron continues as the rust flakes off and exposes fresh iron to the atmosphere.
Since steel is the main structural material for bridges, and automobiles, controlling its corrosion is extremely important. To do this, we must understand the corrosion mechanism. Instead of being a direct oxidation process as we might expect, the corrosion of iron is an electrochemical reaction.
Steel has a nonuniform surface because the chemical composition is not completely homogeneous. Also physical strains leave stress points in the metal. These nonuniformities cause areas where the ion is more easily oxidized (anodic region) than it is at others (cathodic regions). In the anodic regions each iron atom gives up two electrons to form Fe2+ ion.
Because of the migration of ions and electrons, rust often forms at sites that are remote from those where the iron dissolved to form pits in the steel. The degree of hydration of iron oxide affects the color of the rust, which may vary from black to yellow to the familiar reddish brown.
The electrochemical nature of the rusting of iron explains the importance of moisture in the corrosion process. Moisture must be present to act as a kind of salt bridge between anodic and cathodic regions.
Steel does not rust in dry air, a fact that explains why cars last much longer in the arid Southwest than in the relatively humid Midwest.
The following Figure shows Electrochemical Corrosion of Iron:
One way to keep iron from corroding is to keep it painted. The layer of paint prevents the water and oxygen necessary for rust formation from coming into contact with the iron. As long as the paint remains intact, the iron is protected from corrosion.
Other strategies include alloying the iron with other metals. For example, stainless steel is mostly iron with a bit of chromium. The chromium tends to collect near the surface, where it forms an oxide layer that protects the iron.
Iron and other metals may also be protected from corrosion by galvanization, a process in which the metal to be protected is coated with a layer of a more readily oxidized metal, usually zinc. When the zinc layer is intact, it prevents air from contacting the underlying iron and thus prevents corrosion. If the zinc layer is breached by either corrosion or mechanical abrasion, the iron may still be protected from corrosion by a cathodic protection process, which is described in the next paragraph.
Another important way to protect metal is to make it the cathode in a galvanic cell. This is the Cathodic protection whichis a method most often used to protect steel in buried fuel tanks and pipelines.
The above Figure is a Cathodic Protection of an Underground Pipe.
Reference: https://www.youtube.com/watch?v=q0CAfXV-YdY
Reference: https://www.youtube.com/watch?v=voKRXIZjzvQ
Reference: https://www.youtube.com/watch?v=jQoE_9x37mQ
SUMMERY AND KEY CONCEPTS
Spontaneous oxidation of metals by natural electrochemical processes is called corrosion, familiar examples including the rusting of iron and the tarnishing of silver. Corrosion process involve the creation of a galvanic cell in which different sites on the metal object function as anode and cathode, with the corrosion taking place at the anodic site. Approaches to preventing corrosion of metals include use of a protective coating of zinc (galvanization) and the use of sacrificial anodes connected to the metal object (cathodic protection).