Chemicals, water, bacteria – all can lead to the “corrosion” of materials and shortened lifetimes or catastrophic failures. Understanding how corrosion occurs can be very complex because it is an electrochemical process.
Corrosion is as complex as it is common. Look around you and see what we all refer to as “rust.” Of course, that is the simplest situation that might exist. Corrosion involves chemistry, materials, and the environment in which the material is placed. Take, for example, that simple case of common steel coming into contact with water and the air. Oxygen and iron exist – forming iron oxide – rust. Corrosion does not just attack metals, even though that is what we think of most often. It can occur for polymeric materials, ceramics, dissimilar metals, or even due to the fretting of materials.
Corrosion can occur in a number of ways:
- Over the complete surface of a material.
- It can be localized occurring only at specific sites. It can occur in pits, crevices, or even under paint if a small hole forms in the paint allowing contaminants onto the surface under the paint.
- It can be the result of dissimilar metals, which is referred to as galvanic corrosion. The two metals must have an electrical contact, be electrochemically dissimilar, and exposed to an electrolyte.
- Environmental cracking can occur as well. Terminology you might have heard before includes stress corrosion cracking, hydrogen-induced cracking, or liquid metal embrittlement
- Flow assisted corrosion can occur when there is high-speed flow, impinging flow, or cavitation.
- Dealloying is also a type of corrosion. This occurs when one element in an alloy is attacked in a metal. This has occurred in plumbing fittings, resulting initially in pinhole leaks that cause large amounts of water damage in homes. These fittings are made of brass and when one of the elements is lost it is referred to as dezincification.