Failure Analysis & Durability Improvement

Corrosion

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Corrosion is the attack on a component by a chemical reaction with its environment. One of the most common examples is rusting of iron or steel, which is caused by iron chemically reacting with oxygen, and can be significantly increased by the presence of water. Corrosion occurs at the surface, where both the component and its environment come into contact. The simplest way of combatting corrosion is to put a barrier in place to separate the two, such as paint or a coating of zinc. Some metals are more resistant to corrosion than others. Aluminum, like iron, reacts with oxygen to form an oxide. Unlike iron, the oxide layer on aluminum adheres tightly to the surface & inhibits further oxidation, instead of flaking off and allowing corrosion to penetrate deeper into the component.

Since corrosion is a chemical process, it attacks the different constituents of the microstructure at different rates. Corrosion damage will show some microstructural features (Those more resistant to corrosion) remaining while those around it are gone. This is shown in the pit on the left of the photo below, where there are some white ridges remaining while the material around it has been removed. Contrast that with the pit on the right which due to erosion, a mechanical process.

Corrosion pitting (Left) caused by sulfidation & Erosion pitting (Right) in Stainless Steel

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Failure Analysis & Durability Improvement

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Ed Pope
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Ed@Failure-Analysis-Durability.com

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