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Corrosion Under Insulation (CUI)

Corrosion Under Insulation (CUI)

The porosity of the steel increases with the heat and enables an excellent bond to be achieved. Belzona has therefore developed heat activated products which become less viscous with temperature and penetrate hot surfaces.


What makes us different?

Cost Effectiveness

  • One of the key features of Belzona products for CUI is their heat activated curing properties, which allows them to be applied directly to hot surfaces, minimising downtime.
  • Belzona products exhibit high adhesion and bond to manually prepared surfaces, overcoming the traditional challenges of surface preparations during the application.

Ease of Use and Safety

  • Belzona products are solvent free and cold applied, eliminating hot work, and have been designed to be easy to use and as safe as possible.This solution differs from the thermo-spray systems as these require being heated in order to work, entailing a need for special safety permits; for instance: aluminium linings applied by thermo-spray (TSA). Belzona offers a solution which does not require preheating and therefore avoids the associated hazards and problems.


  • Belzona products are resistant to a wide range of chemicals and heat and in addition provide excellent corrosion resistance. As well Belzona can offer a long-lasting weather and water resistant insulation cover, preventing corrosion occuring below the insulation.

Why does it work?

The porosity of the steel increases with the heat and enables an excellent bond to be achieved. Belzona has therefore developed heat activated products which become less viscous with temperature and penetrate hot surfaces. This leads to products wetting out the surface and achieving better bonding performance even to manually prepared surfaces.


Pipework and vessels, which operate at high or very low temperatures, are predominantly covered by insulation to prevent heat and energy loss and for health and safety issues. The problem of Corrosion under Insulation (CUI) however can occur in both carbon steel and stainless steel. This is mainly as a result of water ingress into thermal insulation on pipework and vessels leading to rapid corrosion.

Some of the causes for the presence of water may be;

  • Water trapped in the insulation during construction, e.g. storage in places where moisture exists, application carried out during poor weather.
  • Joints in traditional claddings as they can form an inherent design weakness
  • Mechanical damage of traditional cladding
  • Spillage of liquid in low lying areas
  • Condensation if there are cyclic temperature fluctuations
  • Leaks in the trace heating system
  • Poor fabrication standard of the protective cladding

Other factors that affect and accelerate the onset of CUI are ions such as chlorides and sulphides. Under certain circumstances these ions can accelerate the corrosion and can lead to different forms of corrosion such as pitting corrosion or stress corrosion cracking. These ions can either be sourced from the insulation material itself, or from the local environment e.g. acid rain and decomposition of organic matter such as dead vegetation or spillages.

Temperature fluctuations also play a major part in the severity of the corrosion taking place. In general all chemical reactions (of which corrosion is a form) are accelerated by increases in temperature.

The worst form of attack is a combination of cyclic temperature and the presence of ions as this can create a concentration mechanism whereby the repeated condensing and evaporation of liquid deposits increases more and more the concentration of the ion species at the point of corrosion, massively accelerating the rate of corrosion.

These forms of corrosion are of particular concern as they are often unforeseen and hidden from view until premature and potentially catastrophic equipment failure occurs.

Environmental Damage

Health and Safety Hazards

Hot outer surfaces are generally lagged / insulated for health and safety reasons. The places where CUI occurs therefore represent a physical hazard to anyone working nearby. Where CUI is very serious, it could also eventually cause catastrophic failure, such as collapse, explosion or fire where oxygen is present in oil-saturated insulation. These are very serious concerns to employers and should be immediately addressed to avoid any personal injuries.

Loss of Energy

Insulation is widely used not only for health and safety reasons but also for energy conservation. Unlagged or uninsulated equipment can therefore be very energy-inefficient as heat energy is readily lost to the atmosphere, creating an unnecessary financial cost.

Environmental Damage

The CUI problem can eventually cause a loss of containment from the vessel or pipeline, which pollutes the local environment and may eventually cause contamination of ground water.

It is therefore clear that CUI potentially causes significant problems and plant owners have a huge responsibility to ensure that CUI problems are not left unaddressed.

Traditional Maintenance Challenges

Conventional repairs which may include traditional coatings and re-strengthening or replacement of equipment are not popular as this work can only be done in a costly shutdown period. In-situ repairs which can be applied outside the shutdown period would be the first choice, but the problem with CUI is that such repairs are traditionally difficult to complete due to the following reasons;

  • Surfaces are too hot to directly apply products
  • There is normally the requirement to bond to manually prepared surfaces
  • Hot work is normally not allowed

It is therefore essential for any solution provider that they address all of these challenges so that this work can be applied in-situ and outside the planned shutdown periods.


With over 55 years of experience, Belzona's systems and techniques have enabled us to develop repair composites which can address CUI problems in-situ.

Many organisations across a wide range of industries worldwide have benefited from Belzona's Corrosion under Insulation solutions, in addressing problems with;

  • Pipework
  • Vessels
  • Nozzles
  • Scrubbers
  • Absorbers
  • Evaporators
  • Flare Drums
  • Risers
  • Insulation
  • Columns
  • Steam lines
  • Exhaust Ducting
  • Boilers

The effectiveness of Belzona solutions for CUI has been proven through these applications, resulting in avoidance of replacement costs and minimising of interruptions, as well as prolonged equipment life.