In industrial plants, insulation is applied to reduce heat loss and to keep process temperatures stable. Beneath that insulation, however, a form of deterioration can develop out of sight. Known in the industry as corrosion under insulation, or CUI, this problem accounts for a significant share of maintenance budgets. What makes it difficult is that the damage usually stays hidden until the insulation is removed.
How It Develops?
The basic condition for corrosion under insulation is moisture reaching the metal surface and staying there. Water enters the system through several routes. Rain and snow can pass through damaged cladding joints. Water used during washdown and steam cleaning can follow the same path. On cold lines, moisture does not come from outside at all; it forms on the surface through condensation.
Once inside, water is held in the pores of the insulation material. If the protective coating on the metal is weak, this continuous contact starts the corrosion process. Because insulation also slows down drying, the surface stays wet for longer and the process accelerates.
Where the Risk Is Highest
Corrosion does not progress at the same rate everywhere. Areas where water collects and cannot drain are affected first. The underside of horizontal pipe runs, the area around support shoes, flange and valve connections, elbows and instrument tapping points all fall into this group.
Temperature is another factor. Lines that run continuously at high temperature stay dry, which reduces the risk. Lines operating close to ambient temperature, running intermittently or frequently started and stopped carry a higher risk, because the wetting and drying cycle repeats. For carbon steel, the range between roughly minus ten and one hundred and seventy five degrees Celsius is widely treated as the critical band.
Intermittent lines therefore deserve particular attention. During shutdown periods the line cools and condensation can form, and this cycle repeats with every restart.
Coating Selection
The first line of defence against corrosion under insulation is not the insulation itself but the protective coating on the metal. Selecting a coating suited to the operating temperature and preparing the surface correctly before application both have a direct effect on service life.
Any shortcut in surface preparation can cause the coating to fail early. Weld seams, sharp edges and connection points are the areas most often overlooked during application, so extra care at these points is worthwhile.
The Role of the Insulation Material
Material choice also affects the risk. Materials with low water absorption and a closed cell structure limit how far moisture can travel towards the metal surface. Foam glass and closed cell elastomeric rubber are among the groups that stand out in this respect.
With fibrous materials, the integrity of the outer cladding becomes even more critical. You can review our material groups and application areas on our industrial insulation page.
Cladding and Detail Work
The overlap direction of metal cladding is planned so that water cannot enter. On vertical runs the upper section overlaps the lower one, and on horizontal runs the overlap is kept on the upper side.
Screw holes, sealed joints and instrument penetrations are the points where water most often gets through. Checking these details individually during installation costs far less than correcting the consequences later.
Drainage points also need to be provided so that any water that does get in can escape. In a system with no outlet, even the best coating will eventually struggle.
Inspection and Planning
Corrosion under insulation cannot be eliminated entirely in most plants, so the approach is built around managing the risk. Lines are grouped by risk level and inspection frequency is set accordingly.
In high risk areas, partial removal of insulation for visual examination remains the most reliable method. Alongside this, staining on the cladding, deformation of the metal sheet and water tracks at joints serve as early warning signs in the field.
Building these inspections into the planned shutdown schedule reduces the overall workload. Our maintenance services cover this planning work, shaped around how your plant actually operates.
Stainless Steel Lines
The picture is somewhat different on stainless steel lines. Here the main concern is chloride induced stress corrosion cracking rather than general corrosion. Chlorides from the insulation material or from coastal air can accumulate on the metal surface together with moisture.
For this reason, material selection requires particular attention in coastal plants and offshore applications. Choosing insulation materials with low chloride content is one of the measures that reduces this risk.
The Cost Side
The real cost of corrosion under insulation comes from downtime rather than from replacement material. Once lost production and unplanned intervention are added to the calculation, the figure rises quickly.
Damage identified through a planned inspection programme, on the other hand, can be scheduled into an existing shutdown. The difference between these two scenarios explains why preventive work is the more economical route.
In Refineries and Petrochemical Plants
The subject becomes even more prominent in plants with long pipe runs and large numbers of flanged connections. In the insulation work we carry out for refinery and petrochemical facilities, application details are planned with this risk in mind.
If you would like Mod Endüstriyel Servisler to carry out an assessment at your plant, you can reach us through our contact page.
Frequently Asked Questions
In which temperature range is corrosion under insulation most common?
For carbon steel lines, the range between roughly minus ten and one hundred and seventy five degrees Celsius is treated as the critical band. Lines running continuously at high temperature stay dry, which lowers the risk.
Can the damage be detected without removing the insulation?
Staining on the cladding, sheet deformation and water tracks at joints can provide early warning. A reliable assessment still requires partial removal of the insulation.
Which insulation material reduces the risk?
Materials with low water absorption and a closed cell structure limit moisture reaching the metal. Material selection is evaluated together with the operating temperature.
Which matters more, the coating or the insulation?
The protective coating on the metal is the first line of defence. Insulation and cladding details support that protection rather than replace it.