Corrosion Under Insulation (CUI)

Last update: Jan 13, 2017

Corrosion Under Insulation (CUI) is one of the most well-known phenomena in the process industries, and yet it still makes up an inordinately large percentage of global maintenance expenditures. CUI is a subject that is well-researched and understood; extensive studies have been commissioned to determine the causes, effects, prevention, and mitigation of CUI. 

In the simplest terms, CUI is any type of corrosion that occurs due to moisture present on the external surface of insulated equipment. The damage/attack can be caused by one of the multiple factors, and can occur in equipment operating at ambient, low, and heated services, depending upon conditions. Moreover, CUI can occur in equipment that is in service, out of service, or in cyclic service.

The corrosion itself is most commonly galvanic, chloride, acidic, or alkaline corrosion. If undetected, the results of CUI can lead to leaks and the shutdown of a process unit or an entire facility.

Codes, Standards, and Best Practices

  • API 510, Pressure Vessel Inspector Program is an inspection code that covers the in-service inspection, repair, alteration, and rerating activities for pressure vessels and the pressure relieving devices protecting these vessels. It applies to most refining and chemical process vessels that have been placed into service. CUI inspection is covered in section 5.5.6 of the standard (Tenth Edition released April, 2014).
     
  • API 570, Piping Inspection Code - Inspection, Repair, Alteration and Rerating of In-Service Piping Systems provides guidance on how to determine which piping systems are most susceptible to CUI (section 5.2.1), as well as some of the most common locations to find CUI (section 5.4.2) on those systems that are determined to be susceptible to CUI (Third Edition released November, 2009).
     
  • API RP 574, Inspection Practices for Piping System Components discusses inspection practices for piping, tubing, valves (other than control valves), and fittings used in petroleum refineries and chemical plants. In order to aid inspectors in fulfilling their role implementing API 570, this document describes common piping components, valve types, pipe joining methods, inspection planning processes, inspection intervals and techniques, and types of records. CUI is covered in section 6.3.3 (Third Edition released November, 2009).
     
  • API RP 583, Corrosion Under Insulation and Fireproofing covers design, maintenance, inspection, and mitigation practices to address external CUI as it applies to pressure vessels, piping, storage tanks and spheres. It examines the factors that affect the damage mechanisms, and provides guidelines for preventing external corrosion or cracking under insulation, maintenance practices to avoid damage, inspection practices to detect and assess damage, and guidelines for conducting risk assessments onequipment or structural steel subject to CUI (First Edition released May, 2014).
     
  • ASTM STP 880, Corrosion of Metals Under Thermal Insulation provides information on corrosion problems that can occur on thermally insulated plant equipment and piping components if its insulation becomes wet (First Edition released 1985).
     
  • NACE SP0198-2010, Control of Corrosion Under Thermal Insulation and Fireproofing Materials – A Systems Approach (Published July, 2010). This standard is a replacement for NACE RP0198-08 (March, 2004). 

 

Recommend changes or revisions to this definition.

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May/June 2004 Inspectioneering Journal
By John Reynolds at Intertek

CUI may be the most well known and widespread corrosion phenomena in our industry. It’s also one of the most difficult to prevent because by and large no matter what precautions we take, water eventually gets into the insulation and begins to do it’s dirty work, sometimes sight unseen until process leakage occurs. And it’s not isolated to just insulation. Corrosion under fire-proofing (CUF) is also prevalent in our industry and requires the same type of inspection planning, design prevention, and mitigation that is required for CUI.

March/April 2001 Inspectioneering Journal
By Deal Moore at NDE Seals, Inc.

Determination of the mechanical integrity of pipe and equipment in US process industries has evolved from day to day business to federal mandate (OSHA 1901.119) to a part of industry best practice. Insulated pipe and equipment pose specific challenges for the examination effort. This article is an attempt to review the regulations that are driving these efforts and the innovations designed to address these challenges. Understanding both will enhance the value added services offered by the insulation industry.

September/October 2000 Inspectioneering Journal
By John Reynolds at Intertek, and John Reynolds at Intertek

This article continues to outline the 101 essential elements that need to be in place, and functioning well, to effectively and efficiently, preserve and protect the reliability and integrity of pressure equipment (vessels, exchangers, furnaces, boilers, piping, tanks, relief systems) in the refining and petrochemical industry.

January/February 1999 Inspectioneering Journal
By Joseph E. Pascente at Lixi, Inc.

One of the greatest challenges facing many of refining, fossil power, and pulp and paper industries is: How to effectively examine their insulated piping?

Partner Content

Offshore platforms are exposed to some of the roughest conditions on earth and require regular attention to ensure they are structurally sound and safe for continued operation. With so many components and major joints at elevated locations, it is clear why a well-trained rope access technician can be an invaluable resource for offshore operators.

November/December 1996 Inspectioneering Journal
By Michael Twomey at CONAM Inspection Inc.

Corrosion under insulation (CUI) is a real threat to the onstream reliability of many of today's plants. This type of corrosion can cause failures in areas that are not normally of a primary concern to an inspection program. The failures are often the result of localized corrosion and not general wasting over a large area. These failures can be catastrophic in nature, or at least, have an adverse economic effect in terms of downtime and repairs.

July/August 1996 Inspectioneering Journal
By Hegeon Kwun at Southwest Research Institute, and Richard L. Lopushanksy at Southwest Research Institute

Engineers and scientists at Southwest Research Institute (SwRI) in San Antonio, Texas, may have found a cost-effective and practical method of detecting ID and OD corrosion of insulated piping systems.

September/October 1995 Inspectioneering Journal
By Dr. Nand K. Gupta at Omega International Technology, Inc.

In May 1995, Omega International Technology, Inc., began testing a new system to measure pipe wall thicknesses using digital radiography (RT) scanning. This new system has the potential for being faster, less labor intensive, and shown improved accuracy over traditional ultrasound testing, and at a lower cost. Perhaps best of all, scanning can be performed while the pipe is in service, insulation in place.

May/June 1995 Inspectioneering Journal

Corrosion is one of those "equal opportunity" hazards that affects all industries indiscriminately, to the tune of billions of dollars annually in repair and replacement costs. Some types of corrosion are readily apparent, such as rusting of unprotected plain carbon steel tanks and piping.