Corrosion Under Insulation (CUI)

A simple definition of CUI is external corrosion of a steel structure, such as pipes, vessels, or tanks fabricated from carbon, low alloy, and austenitic stainless steel, that occurs underneath insulation due to the permeation of moisture/water (both).

CUI primarily occurs due to one, two, or all three of the following:

1. Faulty design
2. Faulty installation of the insulation
3. Physical damage to the insulation

Faulty design includes specifying incorrect insulation material and the initial design of the structure (wrong grade of material, corrosion/water traps, galvanic corrosion, etc)

Faulty installation of the insulation includes wrong overlap, joint banding, adhesive, etc.

Physical damage to the insulation includes mechanical damage, impact from scaffolding, etc.

Inadequate inspection and maintenance must be included in the above, as the detection of CUI is usually an indication that the management of corrosion is not effective.

Mechanisms of Corrosion

Factors influencing CUI –

CUI occurs when water/moisture and oxygen are present on the steel surface.

Other factors include –

• The temperature of the structure
• Temperature cycling
• Type and grade of the structure
• Concentration on the structure of ions contributing to CUI, such as chlorides

Note: The particular feature of corrosion is that once initiated, it will progress as an electrochemical reaction.

Corrosion Under Insulation (CUI)

CUI is a well known and widespread corrosion phenomenon in the process industries, specifically oil, gas, and petrochemical, which use many insulated structures subject to variable corrosive environments.

Virtually every process plant with insulated structures will have CUI to some extent.

The extent of the CUI will depend upon the management and inspection control and the repair and rectification process of the plant during operation and shutdown.

Corrosion Under Insulation

CUI represents all types of corrosion that may occur on the structure (pipes, vessels, etc.) beneath an insulation system and can be triggered by multiple factors or conditions.

CUI is insidious; the processes occur hidden beneath the insulation and cladding and are often only discovered much later when the damage caused is extensive.

CUI is difficult to detect and can lead to plant shutdowns and, in extreme cases, may cause catastrophic failures.

Corrosion Under Insulation

Many statistics have been produced to demonstrate how damaging CUI can be to asset owners and operators.

According to a study presented to the European Federation of Corrosion:

• The highest incidence of leaks in refining and chemical industries are due to CUI and not process corrosion
• Over 80% of most piping leaks occur in diameters smaller than 4”
• Between 40-60% of piping maintenance costs are related to CUI (NACE, 1992)
• CUI is associated with the most lengthy refinery downtimes

CUI has serious financial implications for asset owners and operators.

Correct CUI management can reduce maintenance costs considerably.

Insulation

Insulation has been used for many years.

CUI only became a recognised problem in the early 1960s.

Literature from 1965 first noted Chloride-External Stress Corrosion Cracking (Cl-ESCC) in Austenitic Stainless Steels, but it was not until the early 1980’s that CUI in carbon steel was more recognised.

Major (and very expensive) plant shutdowns have occurred due to extensive CUI in the oil, gas, and petrochemical industries.

Corrosion Under Insulation and Fire Protection

Due to the requirement for fire protection on vessels and structures on oil and gas platforms, petrochemical plants, etc., fireproofing materials are widely used, predominantly with cementitious type materials.

Similar to insulation, fireproofing materials become damaged, allowing water to penetrate under the fireproofing material and corrosion occurs, referred to as corrosion under insulation (CUI).

Lack of regular inspection and maintenance can exacerbate the situation.

The fireproofing material often cracks and falls away from the substrate. Fire protection is no longer afforded, and in the event of a fire, the designed fire protection (30, 60 mins, etc.) will not be achieved.

Corrosion under fireproofing (CUF) can lead to plant shutdowns and major repairs or refurbishment.

Corrosion Under Insulation

CUI is a major issue for oil and gas (both onshore and offshore), chemical processes, and other related industries. It is estimated that 10 percent of these industries' total annual maintenance costs are dedicated to repairing damage from CUI.

Severe cases of CUI may put personnel, environment, and industry reputation at risk.

CUI is difficult to detect, and mitigation is not easy.

CUI is often detected when it’s too late to repair, and full refurbishment is required.

Corrosion Under Insulation

It is estimated that the corrosion rates of carbon steel (in CUI conditions) can be up to 20 times higher than in naturally aerated atmospheric conditions.

The speed of this corrosion can lead to a serious situation with plant and equipment.

There are global examples of CUI causing fire to plant which has cost many millions of pounds, along with irreversible environmental damage.

Corrosion Under Insulation

Before the 1970s, thermal insulation was generally used only for energy conservation on high-temperature structures (above 150 C). Most of the moisture that penetrated through the insulation would evaporate at these high temperatures, and CUI was not a widescale problem.

Later in the 1970s, thermal insulation was used at lower temperatures to reduce energy costs. Moisture trapped within and under cladding and insulation created CUI.

CUI became a prevalent issue; however, it would be another ten years or so before it was acknowledged as a major problem in the oil, gas, and petrochemical industries.

Corrosion Under Insulation

CUI is often difficult to detect as insulation materials often hide it. In most cases, the only way to determine the extent and severity of CUI is to remove the insulation physically.

CUI is often critical to the safe running of process systems, and mitigation (maintenance and repair) is not easy and often very expensive.

Remote Non-destructive evaluation /Non-destructive testing (NDE/NDT) methods such as thermography, radiography and ultrasound are other tools used to indicate CUI severity without removing insulation.

Corrosion Under Insulation

CUI is a widespread problem in many industries.

There has been considerable research by professional bodies, independent bodies, asset and plant operators, and professional individuals on CUI (and CUF).

The cost of CUI repairs is still extremely high and constitutes a high percentage of maintenance costs.

If properly managed (such as integrity programs), long-term reliability (including cost reduction) is possible in most cases.

Correct design and choice of installation are essential. If you can stop moisture/water from penetrating into and under the insulation materials, then you will stop CUI.

Inspecting insulated structures is also essential to identify early problems, such as damage to the insulation (jacketing), which will have long-term benefits and reduce unplanned shut-downs.

Corrosion Under Insulation

There are many publications on different aspects of CUI and CUF.

Further information can be found at:

• The European Federation of Corrosion
• The Energy Institute (UK)
• AMPP (recommended practices)
• API (recommended practices)
• CINI

Publications:

• Corrosion-under-insulation guidelines. (S.Winnik) - As illustrated

Summary

Within this training unit, we have reviewed the basic process for corrosion and CUI.

We discussed the history of CUI and how the industry has only taken action on the problem since the 1980s.

We also discussed the risks to management and plant and equipment owners.