Recently we discussed the basics of corrosion under insulation. CUI is a blanket term that refers to several different types of corrosion processes. Based on how a part is used and what conditions it’s exposed to, the kind of corrosion that develops may differ.
Listed below are five common types of corrosion
General Attack Corrosion
This is the most common type of corrosion and is caused by a chemical or electrochemical reaction, that results in the deterioration of the entire exposed surface of a metal. General attack corrosion is considered as a safe form of corrosion, due to the fact that it is predictable, manageable and often preventable.
Galvanic Corrosion
Galvanic corrosion is also known as bimetallic corrosion and dissimilar metal corrosion. It occurs when two different metals are located together in a corrosive electrolyte. The most severe attack occurs at the joint between the two dissimilar metals. Three conditions must exist for galvanic corrosion to occur:
- Electrochemically dissimilar metals must be present
- The metals must be in electrical contact
- The metals must be exposed to an electrolyte
Fretting Corrosion
Occurs when there is repeated wearing, weight and or vibration on an uneven, rough surface. Resulting in pits and grooves on the surface. This type of corrosion is typically found in rotation and impact machinery, bolted assemblies, bearings, and surfaces exposed to vibration during transportation.
High-temperature Corrosion
Takes place in gas turbines, diesel engines, furnaces and other machinery. It is a chemical attack from gases, solid or molten salts, or molten metals, typically at temperatures above 750°F. High-temperature corrosion is also known as dry corrosion or scaling.
Flow-Assisted Corrosion
Results when a protective layer of oxide on a metal surface is dissolved or removed by wind or water, exposing the underlying metal to further corrode and deteriorate.
CUI is a common problem across many industries, costing facilities millions of dollars each year in repair and replacement costs. Protecting piping from CUI should be considered always when designing a robust system. Knowing what can lead to CUI and how to protect the piping can lead to years of worry free maintenance.
GLT Products fabricates various mechanical piping insulation products that can help in reducing CUI. Download our Fabricated Pipe Insulation Guide as a starter.
