What is Corrosion Under Insulation (CUI) | Thermal Sprayed Aluminium (TSA)

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 corrosion under insulation or CUI often goes undetected until the damage is significant which may lead to catastrophic failures, e.g. on equipment operating under high pressure. The industries which are highly impacted by CUI are

  • Refineries
  • Chemical industries
  • Petrochemical industries
  • Offshore industries and
  • Marine and Maritime industries

Materials prone to CUI

Corrosion Under Insulation may occur on:

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 Corrosion under insulation or CUl

In case, corrosion under insulation is not detected in proper time, it can lead to failures like an explosion, leaks, downtime, equipment failure, etc.

Corrosion Under Insulation Temperature Range

General temperature ranges in which the risk of Corrosion Under Insulation is prevalent is

  • 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 be present in the insulation which greatly promotes corrosion of the underlying surface in the presence of moisture.

READ  Stress or Strain: Which Comes First?

Factors affecting Corrosion Under Insulation

The major factors that affect the corrosion process under insulation (CUI) damage are:

  • Frequency and duration of the moisture exposure
  • Service temperature.
  • Type of insulation and protective barrier condition.
  • Cyclic operation modes.
  • Corrosivity of the environment
  • Type of climate where the system is installed.
  • Proximity to humidity-causing equipment like cooling towers, etc

Corrosion under Insulation 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 for CUI

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

CUI of carbon steel

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

Corrosion under the insulation 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 in the following ways:
    • 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?

By Avoiding that moisture from entering the insulation material and the steel surface; corrosion under insulation can be prevented. These can be achieved by

  • 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; which will result in corrosion under insulation.

Carbon steel:

The following steps are followed to prevent corrosion under insulation for carbon steel piping/equipment materials.

  • Organic coatings
  • Thermal-sprayed aluminum (TSA)
  • Personnel protecting cages
READ  Material Selection & Quality Management for a Cross Country Pipeline

Stainless steel (austenitic or duplex)

To prevent the CUI of stainless steel materials, the below-mentioned steps are taken:

  • 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 CUI protection

The following image (Fig. 4) shows the methods to prevent various types of corrosion under insulation.

Methods of protection
Fig. 4: Methods of protection

Thermal Sprayed Aluminium (TSA) to prevent corrosion under insulation

  • 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.

Advantages of TSA in CUI prevention:

  • 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”)

Disadvantages of TSA:

  • 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

Corrosion Under Insulation 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.

READ  Shear Modulus or Modulus of Rigidity: Definition, Significance, Example

Corrosion Under Insulation Inspection

Various codes and standards provide guidelines for corrosion under insulation inspection methods as listed below:

  • API 510 covers the CUI inspection in clause 5.5.6 of the standard that says inspection for CUI must be considered for externally insulated vessels and those that are in intermittent service.
  • API 570, Piping Inspection Code provides guidance to determine the piping systems susceptible to CUI in clause 5.2.1 and clause 5.4.2.
  • API RP 574, in clause 6.3.3 gives a reference to corrosion under insulation.
  • API RP 583 provides a detailed overview of CUI.
  • ASTM STP 880, provides information on corrosion problems on thermally insulated plant equipment and piping components.
  • NACE SP0198 provides detailed guidance on the Control of Corrosion Under Thermal Insulation and Fireproofing Materials.

Corrosion under insulation is detected by any of the following inspection methods:

  • Visual Inspection
  • X-Rays or Radiography
    • Real-Time Radiography
    • Computed Radiography
  • Digital Detector Array
  • Ultrasonic Thickness Measurement
  • Pulsed Eddy Current

Susceptible Locations for Corrosion under insulation

The areas that are more susceptible to CUI are:

  • Damaged insulation
  • Areas that tend to trap moisture
  • Equipment nozzles and manways.
  • Pressure safety valve flanges
  • Areas exposed to steam vents, deluge systems, process spills, and acid vapors.
  • Areas where steam tracing is leaking
  • Systems operating intermittently above 120 Deg C

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

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.

6 thoughts on “What is Corrosion Under Insulation (CUI) | Thermal Sprayed Aluminium (TSA)

  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.

  4. Most complicated engineering becomes the best.. if it is explained simply… ANup you have explained CUI very well… I have small query … does leaching of Chloride from insulation into the metal surface affects CUI mechanism in SS & CS piping with same gravity or differently…??

Leave a Reply

Your email address will not be published. Required fields are marked *

Recent Posts