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.
For high temperature equipment, water entering an insulation material and diffusing inward will eventually reach a region of dryout at the hot pipe or equipment wall. Next to this dryout region is a zone in which the pores of the insulation are filled with a saturated salt solution. When a shutdown or process change occurs and the metal-wall temperature falls, the zone of saturated salt solution moves into the metal wall.
Upon reheating, the wall will temporarily be in contact with the saturated solution, and stress-corrosion cracking may begin. The drying/wetting cycles in CUI associated problems are a strong accelerator of corrosion damage since they provoke the formation of an increasingly aggressive chemistry that can lead to the worst corrosion problems possible, e.g. stress corrosion cracking, and premature catastrophic equipment failures.
Types of Corrosion Under Insulation
By understanding the types of corrosion that can occur under insulation, the proper materials and construction can be employed to prevent them. Intruding water is the key problem in CUI. Special care must be taken during design not to promote corrosion by permitting water to enter a system either directly or indirectly by capillary action. Moisture may be external or may be present in insulation.
What is the Mechanism of Corrosion Under Insulation?
The mechanism of corrosion under insulation involves three requirements
- Availability of oxygen.
- High temperature.
- Concentration of dissolved species.
Normally, as the temperature increases, the amount of oxygen dissolved in solution decreases as the boiling point is reached resulting in reduced corrosion rates. However, on the surface covered by insulation, a poultice effect is created which holds in the moisture which essentially makes it s closed system. In fact the measured corrosion rates associated with corrosion under insulation follow trends to higher corrosion rates commonly associated with only pressurized systems. Furthermore, in cases where precipitation becomes trapped on the metal surface by insulation, corrosive atmospheric constituents such as chlorides and sulfuric acid can concentration to also accelerate corrosion. In some cases, chlorides are present in the insulation which greatly promotes corrosion of the underlying surface which it becomes laden with moisture.
How do I Inspect for Corrosion of Metals Under Insulation?
The most common and straightforward way to inspect for corrosion under insulation is to cut plugs in the insulation that can be removed to allow for ultrasonic testing. However, many times such plugs can be the source of moisture leakage. The main problem with this technique is that corrosion under insulation tends to be localized and unless the inspection plug is positions in the right spot the sites of corrosion can be missed. Other techniques that are available include special eddy current techniques, x-ray, remote TV monitoring and electro-magnetic devices.
How do I Prevent Corrosion Under Insulation?
The most serious problem is the system already in service with a know corrosion under insulation problem. Inhibitors have been tried with varying success since repeated wet / dry cycles may make inhibitors ineffective. This is an area of opportunity. However, long terms performance and efficacy must be proven. Water proofing to prevent the ingress of water from outside sources is another method. However, it has been shown that sometimes these techniques tend to lock in moisture which can also increase corrosivity. Careful selection of insulation materials to prevent those that contain high levels of corrosive impurities such as chlorides is critical to reducing corrosion under insulation.
One of the best but most expensive options to prevent corrosion under insulation is the use of protective coating systems. Unfortunately, in most cases, coatings that have been successful for atmospheric service are used under insulation with disastrous results. In it often a surprise that under-insulation service is a more severe condition than straight atmospheric service. Special coating system must be utilized that have proven performance. In some applications inorganic zinc has worked well, but not in others. Anticorrosion and inhibitive coatings are being are also being proposed or considered for longer term performance.