Corrosion under insulation: A Presentation

What is Corrosion Under Insulation (CUI)?

The corrosion under insulation (CUI) war has been fought for many years in the chemical and petrochemical industry. Corrosion Under Insulation is severe localized corrosion damage that is caused by the moisture (intruding water) present on the external surface of insulated equipment. The corrosion processes are well understood but yet CUI often goes undetected until the damage is significant which may lead to catastrophic failures, e.g. on equipment operating under high pressure.

Materials prone to CUI

CUI May occur on:

  • Carbon steel (general/localized corrosion)
  • Stainless steel (localized corrosion/stress corrosion cracking)

Effect of Corrosion Under Insulation

CUI (Corrosion under insulation) cost studies have shown that:

  • 40 to 60 % of pipe maintenance costs are caused by CUI
  • NDE/inspection costs with a high confidence level for detecting CUI are equal to or exceed field painting costs
  • Approximately 10 % of the total maintenance budget is spent repairing damage from CUl

CUI Risk

General temperature ranges in which risk of CUI is present:

  • Carbon steel: -4 °C to + 175 °C: Risk of CUI (highest risk area: +60 °C to +120 °C)
  • Stainless steel: +50 °C to +175 °C
  • Insulation of process equipment is normally implemented when the outer steel temperature exceeds 50 °C (due to the risk of work-related injuries as well as heat loss).

Mechanism of Corrosion Under Insulation?

CUI involves the requirement of the following three elements:

  • Availability of oxygen.
  • High temperature.
  • The concentration of dissolved species.

In general, with an increase in temperature corrosion rate reduces as the amount of dissolved oxygen decreases. But when it is covered by insulation, the moisture makes it a closed system and retains the dissolved oxygen. In some cases, chlorides and acids may present in the insulation which greatly promotes corrosion of the underlying surface in the presence of moisture.

Corrosion of Carbon Steel

Insulated carbon steel corrodes due to:

  • Infiltration of water under insulation (rain, process liquids, firewater, etc.)
  • Condensation water
  • Ingress of external contaminants

The insulation material may also contribute to CUI:

  • Creates a crevice for water retention
  • May absorb water
  • May leach contaminants that increase the corrosion rate

Temperature and oxygen – corrosion rate

Corrosion rate of Steel
Fig. 1: Curve Showing Corrosion rate of Steel with respect to temperature.

CUI of carbon steel

CUI of Carbon Steel
Fig. 2: CUI of Carbon Steel

Corrosion of Stainless Steel

Stainless steel (austenitic or duplex) corrodes due to:

  1. Chlorides (or other halides) in presence of water are transported to the hot surface
  2. The chlorides are concentrated by evaporation of the water
  3. The chloride concentration reaches a critical level causing stress corrosion cracking (SCC)
  4. (The critical chloride concentration depends on Temperature and alloy type) The insulation material may also contribute to CUI:
  • Creates a crevice for water retention
  • May absorb water
  • May leach contaminants (e.g. chlorides) that cause ESCC
Industry Accepted Chloride Stress Corrosion Cracking temperature Limits
Fig. 3: Industry Accepted Chloride Stress Corrosion Cracking temperature Limits

How is CUI avoided?

Avoid that moisture enters the insulation material and the steel surface:

  • Correct selection and design of the insulation material
  • Good design of the item to be insulated – e.g. it is difficult to insulate around flanges, taps, flowmeters, supports, etc.
  • The cover above the insulated item (against rainfall, etc).

Application of corrosion protection

On average, 60% of all insulation in service for more than 10 years will contain corrosion-inducing moisture!

Carbon steel:

  • Organic coatings
  • Thermal-sprayed aluminum (TSA)
  • (Personnel protecting cages)

Stainless steel (austenitic or duplex)

  • Organic coatings
  • Thermal-sprayed aluminum (TSA)
  • Al-foil wrapping (prevents ESCC and pitting)
  • (Personnel protecting cages)

Organic coatings on carbon steel

  • Experience shows that organic protective coatings on piping in CUI service range from 5-13 years
  • Once the protective life is reached, field re-painting is necessary (or periodic NDE in order to monitor CUI).
  • Coatings may be damaged during handling and installation – leaving unprotected areas.
  • Important parameters for coatings used for CUI prevention: – High permeability resistance (barrier protection) – High flexibility (for cyclic temperature service)

Methods of protection

Methods of protection
Fig. 4: Methods of protection

Thermal Sprayed Aluminium (TSA)

  • TSA may provide long-term corrosion protection at significant life cycle cost savings, however at higher initial costs.
  • TSA provides atmospheric corrosion protection for more than 40 years.
  • Has been recorded to provide 25-30 years of maintenance and inspection-free CUI service.
  • Application by electric arc or flame spray.
  • It requires high-quality surface preparation and strict application control.


  • The coating is robust
  • Minor pores/damages are protected by the surrounding coating
  • No under-rusting
  • Unlimited construction sizes
  • No risk of deformations (”cold process”)


  • No treatment in hollow spaces, inside pipes, etc.
  • The treatment prescribes a certain structural design
  • Sharp edges must be chamfered/rounded
  • Often large variations in layer thickness
  • Quality control
  • Price

CUI Prevention Strategy

Data from operating facilities shows that water-free insulation is not practical in aging facilities. Thus a CUI prevention strategy is necessary in order to provide long-term and reliable prevention of CUI:

A choice between strategies:

  1. Organic coatings on carbon steel; Ongoing re-painting; NDE (does not prevent CUI but predicts remaining life).
  2. TSA on Carbon Steel

Stainless steel (+ TSA or Al foil wrapping) Initial, maintenance, and inspection costs need to be assessed for each choice in order to give the lowest total lifetime cost.

Few more useful Resources for you…

An Article on Forms of Corrosion
Corrosion under insulation: A Presentation
Corrosion Protection for Offshore Pipelines
Corrosion Monitoring Techniques & Surveys: A short Presentation
Guide for Coating Selection for External Bolting to Reduce Corrosion
Application of Anti-Corrosive Linings in Oil and Gas Industry
Anti-Corrosive Composites for Oil and Gas Industry
Piping Materials Basics
Piping Stress Analysis Basics
Piping Design and Layout Basics
Few Jobs for you…

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

3 thoughts on “Corrosion under insulation: A Presentation

  1. This is a very informative post, but I’d like to know what sources were used to gather the statistical information in your post.

  2. Very well explained about Corrosion Under Insulation.

    I would like to add several inspection methods that can help determine the presence of CUI without removing the insulation. None are foolproof and most don’t offer a good understanding of the maximum depth of the CUI damage.

    Brute force: Involves stripping the insulation off the equipment to have a look. Time-consuming, fairly expensive work process (especially if the insulation contains asbestos).
    Non-destructive testing: Allows assessing the presence of CUI without stripping the insulation. Includes various flavors of radiography (X-ray), pulsed eddy current (PEC), and UT thickness measurements from inside equipment.
    Other: Neutron backscatter and infrared thermography. Help find moisture under insulation, which may help find CUI. False calls—wet insulation, no CUI; CUI, dry insulation that was clearly very wet.

    Thanks For Sharing!

  3. Nicely explained and the diagrams completely help to understand the statistical information. Sharing your information with friends to give them a better understanding on corrosion insulation. Appreciate you for sharing the information cheers.

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