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Dates: February 6, 2023 - February 10, 2023
In the simplest terms, Corrosion Under Insulation (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 on thermally insulated equipment that is in service, out of service, or in cyclic service. Because these surfaces are not generally available/accessible for visual examination, the onset of corrosion cannot be easily identified, and in extreme cases, severe corrosion with consequential loss of system integrity can occur.
CUI is a prevalent industry problem affecting thermally insulated equipment in the onshore and offshore oil and gas industries, as well as the petrochemical, specialty chemical, fertilizer, and related industries. It is one of the most well-researched and understood damage mechanisms in the refining and chemical process industries, and yet it still represents an inordinately large percentage of global plant maintenance expenditures. For this reason, exercising good engineering practices during the design phase and throughout the entire lifecycle of equipment requiring thermal insulation is crucial. The design engineer concerned about CUI should specify a quality immersion grade metal coating, a durable water repellent thermal insulation installed with a complete well-sealed weather barrier/jacketing system with drainage features.
The corrosion itself can manifest in several different forms, the most common of which are galvanic, acidic or alkaline, and chloride. According to the National Board of Boiler and Pressure Vessel Inspectors:
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, most coatings will break down eventually and allow chloride-laden moisture to contact the surface of the stainless steel. Low chloride, durable, water repellent insulation and well-applied weather barriers will also help avoid the onset of ECSCC. The ASTM C795 Standard Specification for Thermal Insulation for Use in Contact with Austenitic Stainless-Steel measures an insulation’s fitness for use with austenitic stainless-steel piping and equipment. Current versions of the standard also measure fluoride ion concentrations combining the two into an overall halide measurement of concern. 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 hydro process stainless steel piping operating well above 600°F (315°C).
For the most part, 300 series stainless steels are 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, and 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.
There are several common methods used for detecting CUI, including “brute forcing” (i.e., removing insulation, inspecting, mitigating, and re-insulating), conventional and unconventional radiography, pulsed eddy current, guided-wave ultrasonics, and ultrasonic thickness measurements from the internal surface of the equipment. Some operating facilities apply risk and/or criticality analyses to prioritize pressure vessels and piping for CUI inspection, versus “brute forcing.” Unfortunately, there is no NDT “silver bullet” for CUI yet.
Prior to selecting one or more inspection methods, one should understand what is or is not likely to be found based on the limitations of the methods considered, and the impact on decisions that will be made about the anticipated reliability and suitability for service of the component(s) in question (i.e., what risk or probability of failure remains). As with any inspection strategy, it is typically most effective to utilize several complementary approaches.
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