Corrosion Under Insulation

Corrosion Under Insulation (CUI) is a term for various types of corrosion mechanisms, but it is always caused by the presence of electrolyte containing chlorides. The main reasons are:

  1. – Rainwater or heavy mist
  2. – Leaked or spilled liquids from process
  3. – Condensation

All equipment and pipelines are in danger if they are operating between -5oC and +180oC. Also out of range components are in danger too if cyclic operation or dead/live process is on the table.

Insulation System’s Effect


All kind of equipment and pipeline having hot insulation cover are in danger of CUI, as insulation material creates an ideal combination for water and steel surface contact in long term.

Certainly, insulation’s role becomes more or less effective for CUI according to the engineering of the system. This is a multi-disciplinary phase that includes the mechanical design of the equipment/pipeline and its harmony with insulation as it is part of the whole system. The cladding and insulation material choice and their installation specifications become the most effective parameters of CUI mitigation. And yes, “mitigation” is the optimum strategy during the war against CUI. In order to do so, the items below should be considered during the design phase:

1. Insulation Material Choice

Most hot insulation materials have a lot of open space or porosity in them. Actually, insulation materials can be defined as open-cell or closed-cell according to their permeability. If water leaks into the insulation material and the operating temperature is not hot enough to quickly get rid of it, then any unprotected steel surface will be affected by CUI because of this very ideal ambience.

Choosing optimum insulation material is the critical decision at design phase. Alternative options can be found at:

2. Cladding or Jacketing

The purpose and practicality of the cladding system, defines the parameters at design phase. There could be some common purposes like mechanical resistance and weather protection, or some other purposes like chemical resistance, need for vapor barrier, accessibility, etc.

Cladding can be subdivided into metal and non-metal, each of which has specific characteristics and a scope of applications. Alternative options can be found at:

Also, flexible insulation jackets are remarkable solutions for insulation and maintenance purposes. Alternative options can be found at:

Even the cladding is metal or non-metal or flexible cover, the supporting accessories like drain plugs or inspection ports could be very helpful to mitigate CUI. Alternative options can be found at:

Besides, there are still some other design options such as non-contact systems which is based on creating a gap between insulation and hot surface or between insulation and cladding. In order to do so, spacers could be used and extra chance for aeration and drainage could be given to the water trapped inside.

Frankly, this is a pre-design phase issue which is very important as whole insulation system design would be based on it. It is always useful to choose an international standard (such as CINI) which is constantly updated to determine insulation specifications without relying on questionable procedures. Such standards include design and implementation features based on scientific facts and are constantly updated, usually on an annual basis.

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QA/QC during construction is often left to the insulation contractors’ organization sometimes without any inspection and test plan (ITP). According to the complexity of the project, it is usually useful to invest in independent autonomous QA/QC party that is also drawing up inspection and test programs (ITP), in which critical “hold” and “witness” points are checked. These steps are vital when commissioning and setting up an inspection and maintenance strategy.

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4. Maintenance and Inspection Needs During Operation

Accessibility for maintenance and inspection purposes during opertaion should be considered at design phase. For example, making it possible to easily change gaskets or check for leakage can determine insulation design for valves. Removable insulation covers or inspection ports over cladding are best instruments for the purpose.

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Return on Investment (ROI)


As expected, CUI mitigation and/or optimum insulation approaches come with a surcharge comparing with conventional insulation systems. But it is very obvious that an optimum insulation system has a great cost advantage than the traditional ones.

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Also, CUI mitigation and a maintenance policy according to this have a direct link to overall facility efficiency. And since investors talk in terms of money, these figures become important those causes remarkable results in ROI calculations.



Remarkable know-how about CUI has been accumulated in the last decades. Unfortunately, plenty of them has been disappeared according to senior managements’ “process-first” approaches. Since there is no academic department for “industrial insulation”, this knowledge accumulation has been kept by insulation professionals, manufacturers, and contractors. Therefore, facility owners should keep multi-disciplinary approach to the whole system starting from design, continues at construction and whole life cycle of the components of the facility. Then we can come to a significant point for a continuous approach for CUI mitigation and insulation awareness.



A proper insulation system has many advantages such as reducing costs, mitigating environmental effects, helping the whole process system not to be worked overloaded, etc.

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CUI mitigation is a multi-system approach including metallurgic design, mechanical design, surface protection, insulation, etc. For an optimum and proper insulation system all relevant disciplines should be combined within engineering aspect which can directly contribute the efficiency of the facility only by itself.

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