How Does Corrosion Under Insulation Occur?

by Technical Services

July 21, 2026

Corrosion under insulation (CUI), is a form of corrosion that develops beneath insulation systems on piping, vessels, and other insulated process equipment. Because the damage is hidden by insulation and cladding, it can progress for years before becoming visible. In some of the more severe cases, CUI can thin pipe walls, cause cracking, and ultimately lead to loss of containment. There are some warning signs however that operators can use as early warning signs to bigger insulation related issues. Below is a step by step overview of the entire CUI process and how it leads to failures.

1. The insulation and/or cladding gets damaged

CUI often begins with a small weakness in the exterior cladding system.

Water can enter through:

  • Damaged or missing seals
  • Open seams in the cladding
  • Improperly sealed penetrations
  • Dented or punctured jacketing
  • Poorly repaired inspection points
  • Deteriorated weather or vapor barriers

Some damage is easy to see, but many water-entry points can be well hidden around supports, valves, joints, and be concentrated at low points.

2. Water enters then travels along the insulation

Rain, condensation, steam leaks, and process fluids can all enter a compromised insulation system. The location where water enters may not be where corrosion occurs. Moisture can travel along the pipe or equipment surface before collecting at elbows, supports, nozzles, terminations, and other low points. This is one reason CUI can be difficult to locate through visual inspection alone.

3. Moisture becomes trapped

The way water behaves depends partly on the insulation material and system design. Closed-cell insulation absorbs less water, but moisture can still enter through joints and damaged areas. Once beneath the insulation, it may remain trapped against the equipment surface or collect at low points. Open-cell and fibrous insulation can absorb and retain larger amounts of water, like a sponge. Wet insulation holds moisture in contact with the equipment while also increasing heat transfer through the system, negatively affecting thermal performance of the system/process. If the source of water is not corrected, the equipment may be repeatedly exposed to moisture.

4. Wet and dry cycles create corrosive conditions

As operating temperatures change, trapped water may evaporate, migrate, condense, and wet the surface again. These repeated cycles can:

  • Concentrate salts and contaminants
  • Damage protective coatings
  • Create localized corrosion cells
  • Keep corrosion active over long periods

Wet insulation can also increase heat loss, affect process temperatures, and increase energy consumption which can lower revenue.

5. The protective coating breaks down

The coating on the pipe or equipment surface is one method of defense against CUI but over time, coatings may deteriorate because of age, temperature cycling, poor surface preparation, mechanical damage, prolonged moisture exposure, or chemical contamination. Once water reaches exposed metal, corrosion can begin beneath the insulation and spread beyond the original coating defect.

6. The metal begins to deteriorate

Most process piping is made from either carbon steel or stainless steel and CUI affects them both differently.

Carbon steel

On carbon steel, CUI often causes localized wall thinning and deep pitting. As the remaining wall becomes thinner, it loses its ability to safely contain the internal pressures of the process. Failure may begin as a small corrosion hole or develop into a larger rupture.

Stainless steel

On stainless steel, chlorides beneath the insulation can contribute to pitting and chloride stress corrosion cracking. These cracks and pits create very small leak paths that are difficult to detect. In many cases, the first visible sign is a pinhole leak.

7. Loss of containment occurs

Once corrosion or cracking penetrates the equipment wall, process material can escape is various ways.

Pinhole and corrosion-hole leaks

Liquids may travel beneath the insulation before emerging through a damaged seam or joint. As a result, the visible leak may be located some distance from the actual corrosion site. Gas and vapor can behave similarly, migrating through the insulation system before becoming detectable. Small releases can still create serious hazards, particularly when the process contains flammable, toxic, or high-temperature materials.

Catastrophic ruptures

Pressurized carbon-steel piping can fail suddenly if corrosion removes enough wall thickness. Gas, volatile liquids, and liquefied gases may contain significant amount of energy. A sudden rupture can release large quantities of material, generate debris, and damage nearby piping, equipment, or personnel. If the released material is flammable, it may ignite and escalate into a fire or explosion.

The broader impact of Corrosion Under Insulation:

CUI affects more than the equipment directly involved.

It can lead to:

  • Personnel safety risks
  • Unplanned shutdowns
  • Production losses
  • Higher maintenance and inspection costs
  • Increased energy consumption
  • Environmental releases
  • Regulatory consequences
  • Reputational damage
  • CUI is a chain of events

CUI usually develops through a predictable sequence:

The insulation system is damaged. Water enters. Moisture becomes trapped. Coatings deteriorate. Corrosion attacks the metal. The wall thins or cracks. Containment is lost. Preventing water ingress, maintaining cladding and seals, selecting appropriate coatings and insulation, and inspecting high-risk areas can help interrupt this chain before a leak or rupture occurs.

But water ingress problems are inevitable, that’s why energy industrial facilities and refineries Pyrogel and Cryogel to insulate their critical process at their plants. To learn more about how we’ve helped our customers, check out our Case Studies page.