Asset Intelligence Report

A Primer on Corrosion Under Insulation (CUI)

February 13, 2017

It's one of the most well-known damage mechanisms in our industry, yet corrosion under insulation (CUI) still makes up a very large percentage of plant maintenance expenditures.

This Asset Intelligence Report on Corrosion Under Insulation (CUI) serves as a primer to increase your familiarity with this widely-researched topic. The report contains the following sections:

  • An overview of the mechanism
  • History of CUI
  • Causes of CUI
  • Detection methods and techniques
  • Prevention and mitigation
  • Codes, standards, and best practices
  • Further reading

Intro from the AIR. Click below to read the full version.

Important note: Corrosion under insulation (CUI) can often be very difficult to predict and even more challenging to locate without removing the insulation. The purpose of this report is to serve as an introductory primer. For additional information and resources on CUI, we recommend performing your due diligence. The references provided in this report are an excellent place to start, especially API RP 583.


Corrosion under insulation (CUI) is one of the most well known phenomena in the process industries, and yet it still makes up an inordinately large percentage of global maintenance expenditures. CUI is a subject that is well researched and understood; extensive studies have been commissioned to determine the causes, effects, prevention, and mitigation of CUI.

In the simplest terms, CUI is any type of corrosion that occurs due to moisture buildup on the external surface of insulated equipment. The buildup can be caused by one of multiple factors that are detailed below. The corrosion itself is most commonly galvanic, chloride, acidic, or alkaline corrosion. If undetected, the results of CUI can lead to the shutdown of a process unit or an entire facility, and in rare cases it may lead to a process safety incident.

History of CUI

Corrosion under insulation has been around since insulation started being put on pipes. However, CUI was not generally understood until the release of ASTM STP 880 “Corrosion of Metals Under Thermal Insulation” in 1985. This led to the funding of a study by the US Materials Technology Institute to determine the effectiveness of nondestructive evaluation (NDE) methods in dealing with CUI. Not one single NDE technique was identified as being best, but multiple techniques used together were seen to increase confidence levels for detecting CUI. Fortunately, NDE technology and techniques have improved significantly since that early study.

In 1998, NACE published RP 0198-98, The Control of Corrosion Under Thermal Insulation and Fireproofing Materials - A Systems Approach. When published, RP 0198-98 was the only standard specifically directed at combating CUI that was available to the public. This recommended practice suggested using protective coatings to prevent CUI.

Significant strides have been made since the release of the RP 0198-98, and there are now several ways in which to detect and prevent CUI.

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A Primer on Corrosion Under Insulation (CUI)

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