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Corrosion

bridge
Corrosion is an important mechanism of materials failure, that occurs when a metal interacts with the surrounding environment, and degrades as a result of electrochemical reaction. For example, in 2009, corrosion of an eyebar on the San Francisco Bay Bridge reduced its strength, leading to failure under normal service conditions.
corrosion
In the basic corrosion process, material is removed from the metal in the form of ions, in this case, atoms that have lost electrons. This step is called the oxidizing or anodic reaction. The lost electrons are taken up in a reducing, or cathodic, reaction. The corrosion creates a defect in the material, which may affect the strength and lead to unexpected failure. In the Bay Bridge failure, the corrosion lead to the development of a dangerous crack in the eyebar.
failure chart
When a cracked material is subjected to cyclic loading, such as on the Bay Bridge, if the load exceeds the upper limit of the fracture strength, the material will fail. If the load stays below the lower limit, the crack will grow to a stable size and stop.