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As you know, Dyplast periodically sends out Technical Bulletins related to mechanical (pipe) insulation in applications where temperatures are between low-temperature steam and cryogenic (such as LNG).

Corrosion under insulation

Technical Bulletin 0920 is focused on the following topics related to Corrosion Under Insulation (CUI):

In Technical Bulletin 0920, we also discuss how corrosion differs between the two common types of steel used in such piping systems. We define carbon steel as having less than 10.5% chromium (yes, that’s a generalization), and stainless steel as a steel with a minimum of 10.5% chromium content by weight. There are, of course, many types and grades of both carbon and stainless steel, with varying degrees of susceptibility to different corrosion processes. Yet there are also several specialized types of CUI such as formicary corrosion of copper, which in general are not associated with lower temperature applications and therefore are not addressed herein. Metals vary considerably in their susceptibility to various types of corrosion.


  1. CUI can occur under any insulant - some more likely than others.
    • Insulants with higher water absorption and water vapor transmission can exacerbate CUI.
    • Insulants with higher leachable chlorides (or fluorides) can exacerbate CUI (yet such ions must be leachable at the actual temperature present, and the temperature must be above 32°F).
    • CUI is time and temperature-dependent.
  2. CUI can be prevented from occurring by following basic guidelines:
    • Pre-design identification of environmental factors that could exacerbate CUI, such as rain, acid rain, humidity, “salt air,” wash-down water/chemicals, sprinkler systems, fire control systems, etc.
    • Pipe/component design using appropriate metallurgy for the conditions, and control of metallurgy during construction (e.g. welding; pickling/heat treating; avoidance of dissimilar metals in contact, etc.).
    • Proper specification and application of any pipe coatings.
    • Proper selection and installation of insulation system materials (insulant, vapor barriers, jackets, mastics, etc.).
      • This includes compliance with the guidelines of the manufacturers of the various insulation system components as well as the procedures established by a qualified insulation engineer, including for example, keeping the insulation and pipe surfaces dry during the installation process.
      • Zero-perm vapor barriers can eliminate moisture penetration from the environment.
    • Operational diligence, including such actions as avoidance of excessive impingement of water on pipe during wash-downs, periodic inspection for corrosion, and proper replacement of the insulation and insulation components after mechanical repairs or inspection

Dyplast is not a piping system designer, so a qualified insulation engineer should provide specific advice based on the application.

To read the full Technical Bulletin, click here.