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Overview of Corrosion Under Insulation (CUI)

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 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.


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External Chloride Stress Corrosion Cracking

External Chloride Stress Corrosion Cracking (ECSCC) is a specific form of CUI that occurs in austenitic stainless steels (300 series SS). Most of those who own solid stainless steel equipment operating in the CUI temperature range are likely to experience ECSCC at some point.

Good coatings, properly selected for the purpose and properly applied will give some protection for a period of time. Unfortunately, the vast majority of coatings will break down eventually and allow chloride laden moisture to contact the surface of the stainless steel. Low chloride insulation and well-applied weather barriers will also help avoid the onset of ECSCC.

The older version of calcium silicate insulation which contained chlorides is especially prone to causing ECSCC. Although the temperature range of 140 °F (60 °C) to 300 °F (150 °C) is likely to be the most active region for ECSCC, there are numerous data points reported outside of that temperature range, both above and below, including severe ECSCC of hydroprocess stainless steel piping operating well above 600 °F (315 °C).

For the most part, 300 series stainless steels are fairly strong, thus it’s likely that equipment made from this material will form a leak before it breaks completely. The leak itself will probably be small as well. Because of this, the potential for a large safety event is small compared to other forms of SCC.

This doesn’t mean catastrophic rupture is impossible. There is always the potential for what is known as “plastic collapse,” and of course, even small leaks can be hazardous or produce an undesirable reliability impact.

When it comes to detecting ECSCC after it’s already occurred, inspection methods for ECSCC are normally relegated to surface techniques such as liquid penetrant (LPT) or specialized eddy current probes. For this reason prevention is usually the best option.

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). 

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      Although CUI is widely recognized as a primary driver for damage in the petrochemical industry, the solutions often lack the sophistication devoted to other damage mechanisms.

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