To one degree or another, nearly all engineering materials experience some type of interaction with a large number of diverse environments, which often impair a material’s usefulness as a result of the deterioration of its mechanical properties (e.g., strength), physical properties, or appearance.
Corrosion is defined as the unintentional and destructive degradation of a metal due to an electrochemical reaction. Familiar examples include the rusting of automotive body panels and radiator and exhaust components. Deterioration by physical causes is not called corrosion but is described as erosion or wear. In some instances, corrosion accompanies physical deterioration, though. Nonmetals are not included in this definition of corrosion. Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may leach away, but the term corrosion is restricted to the electrochemical attack of metals.
A rusted (corroded) carbon steel chain
What is Rust?
The word “rusting” applies to the corrosion of iron and plain carbon steel. Rust is a hydrated ferric oxide that appears in the familiar color of red or dark brown. Therefore, carbon steel rusts and also corrodes, but non-ferrous metals such as aluminum, copper, and zinc corrode but do not rust.
Like other natural hazards such as severe weather disturbances, corrosion can cause expensive and catastrophic damage to vehicles, home appliances, water and wastewater systems, pipelines, bridges, and public buildings. The problem of corrosion is one of the significant proportions; in economic terms, it has been estimated that approximately 5% of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. The consequences of corrosion are all too common. The first step in effective corrosion control is to have an in-depth knowledge of the various forms of corrosion, the mechanisms involved, how to detect them, and how and why they occur.
With an understanding of the mechanisms and causes of corrosion of metals and alloys, it is possible to take proactive measures to prevent them or reduce the rate of degradation. For example, we may select a more resistant material or change the nature of the environment.
Corrosion as a Result of Electrochemical Reactions
All forms of corrosion, except for some types of high-temperature corrosion, occur through electrochemical reactions, in which there is the transfer of electrons from one chemical species to another and requires an anode, a cathode, an electric and ionic path between the anode and cathode. Metal atoms characteristically lose or give up electrons in what is called an oxidation reaction. The site at which oxidation takes place (i.e., metal) is called the anode. Oxidation is sometimes called an anodic reaction. The electrons generated from each metal atom that is oxidized must be transferred to and become a part of another chemical species in what is termed a reduction reaction. That location at which reduction occurs is called the cathode. The reduction reaction is also known as the cathodic reaction.
A schematic of a simple corrosion cell
Uniform and Localized Corrosion
In general, corrosion attacks can be divided into two main categories of uniform and localized corrosion. Further, it is convenient to classify corrosion attacks based on the appearance of the corroded metal. Each corrosion form can be identified by visual observation, where in most cases the naked eye is sufficient, but sometimes magnification is helpful or required. Fontana and Greene have classified corrosion attacks into eight forms, which have gained the widest acceptance with subtle variations between corrosion scientists. These eight forms are (1) uniform (or general corrosion), (2) galvanic (two dissimilar metal corrosion) (3) crevice corrosion, (4) pitting, (5) intergranular corrosion, (6) erosion Corrosion, (7) selective leaching, and (8) stress corrosion.
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