April 2025
Special Focus: Maintenance, Reliability and Inspection
Improve plant maintenance programs by keeping CUI in check
If not adequately addressed, corrosion under insulation (CUI) has dangerous and costly consequences, including a greater risk of heat losses, increased energy usage, unplanned downtime, leaks and spills.
A plant's maintenance crews must keep energy-intensive processes in refineries and petrochemical plants running at peak efficiency. One critical focus area is the lengthy and complex network of high-temperature piping and equipment insulated against thermal losses and excessive noise.
While many different insulation types exist, they are all susceptible to water from various sources, including rain, saltwater mist, condensation, temperature cycling, washdown water, and process leaks or spills. No matter how well-jacketed piping insulation might be, some water will inevitably find its way inside, migrate through the insulation and reach the pipe’s metal surface to cause aggressive corrosion under insulation (CUI).
If not adequately addressed, CUI has dangerous and costly consequences, including a greater risk of heat losses, increased energy usage, unplanned downtime, leaks and spills. These risks are more pronounced in plant operations that cycle between cold and hot temperatures. At the colder temperatures of the cycle, both liquid water and vapor tend to enter the insulation more readily. As process temperatures rise through the water’s dewpoint, the corrosion rate increases as well.
Keeping up with CUI damage is costly and complicated for maintenance crews. By some estimates, CUI accounts for 40%–60% of pipeline maintenance costs, and 10% of a plant’s total maintenance budget is spent repairing CUI-related damage.1 Leaks caused by CUI pose hazards for plant personnel while damaging the plant’s reputation as a safe, environmentally responsible operation.
Setting a new standard in CUI mitigation. Regardless of the location in the plant or the exact operating conditions, every CUI scenario shares three common components: unprotected metal, oxygen and water. Most CUI mitigation solutions focus on protecting the metal to minimize its contact with water. The selection of the proper insulation can help this process. Stone wool insulation, for example, has an open-cell structure that permits water vapor to move more freely through the insulation. Given sufficient time, the stone wool will dry out, which helps from a corrosion mitigation stance. As stated in NACE Standard RP0198-2017, “Because CUI is a product of water metal exposure duration, the insulation system that holds the least amount of water and dries most quickly should result in the least amount of corrosion damage to equipment.”2
In recent years, the authors’ company has made several design changes to its stone wool insulation products to improve water repellency and corrosion mitigation. For example, stone wool insulation that withstands high-temperature environments has been developed to help minimize the amount of water that the metallic substrate may be exposed to—even at temperatures in the range of 80°C–175°C (176°F–347°F), where the risk of CUI increases.2
Additive technologies have also been developed, including a low-chloride, water-repellent binder that coats individual insulation fibers to minimize water absorption. Stone wool insulation treated with this water-repellency binder has kept critical plant systems drier since 2017.
The latest innovation in stone wool insulation is a proprietary corrosion inhibitor embedded into the inner layer of insulation covering process-critical piping. The inhibitor is placed where the insulation contacts the metal surface of the pipe. Upon contact with water, the inhibitor activates to form a thin protective layer that blocks the pipe from contact with corrosive fluids. The inhibitor also has a buffering effect that alters the chemistry of the water to make it less acidic and, therefore, less corrosive.
Testing to stringent industry standards. A series of industry-standard laboratory tests were commissioned to confirm that the inhibitor-infused insulation would provide the necessary water repellency, corrosion mitigation and durability under various scenarios mimicking real-world conditions.
Performance testing in the presence of chlorides. As water migrates through insulation, chlorides can leach out of the insulation and into solution. Because chlorides are corrosion accelerants, a modified version of test method ASTM C1617-19 was used to evaluate the insulation in the presence of chlorides.3
This method measures the relative corrosion rates of steel in contact with solutions extracted from thermal insulation compared to plain distilled water and solutions containing various concentrations of chlorides. In addition to solutions extracted from inhibitor-treated mineral wool, solutions were extracted from other commonly used insulation—specifically, calcium silicate, expanded perlite and aerogel.
The ASTM C1617 standard calls for diluting 275 ml of the extracted solutions with distilled water to reach a total volume of 3,000 ml. The standard test is performed alongside a control solution of distilled water (0 ppm chloride) and solutions containing 1-ppm and 5-ppm chloride. To examine the effect of elevated chloride concentrations in the extracted solutions on corrosion rates, the standard test was modified to dose the insulation extract with 100-ppm, 300-ppm and 600-ppm chloride. The 600-ppm chloride was the upper limit, as this level of chlorides would be unacceptable per the ASTM C795 standard for stainless steel.4
The extracted solution is dripped onto a heated rectangular steel coupon over four days, and the solution is contained within a short PVC pipe on the steel coupon. The steel coupon's weight is measured before and after the test period, and a corrosion rate in millimeters per year (mm/yr) is calculated.
Corrosion testing in simulated 15-yr rain trials. ASTM G189 is unique among corrosion tests in that it is the only ASTM test for CUI that incorporates a full-scale insulated pipe evaluated under simulated field conditions. This standard is also highly adaptable and can evaluate numerous conditions that elicit CUI.
The test conditions were selected to study an insulation’s CUI mitigation performance upon long-term (simulated 15 yr) exposure to rain. Test conditions mimicked annual rain totals in the U.S. Gulf Coast region, an area with a significant and sizable refining and petrochemical presence and with high incidents of CUI. Finding a universally accepted infiltration percentage through industrial pipe cladding was challenging. Therefore, the amount of rain bypassing the cladding was based on the ASHRAE 160 standard for infiltration rate through metal building cladding. For comparison, three other insulation types were evaluated alongside mineral wool with inhibitors: perlite, calcium silicate and aerogel.
The general test setup followed ASTM G189-21, along with the following options and modifications:
- Test equipment and coupons were clamped using a spring compression system to counter the thermal expansion of the system.
- Ring-formed test coupons were 14.25-mm wide, compared to the 6.35-mm width in ASTM G189-21.
- The insulation samples tested were installed tightly to the pipe without a 1-in. gap, which simulated the majority of actual field installations.
FIG. 1 graphically depicts the test set-up and equipment. The insulation material was sealed to the test pipe using silicone, creating a 25-cm (10-in.) long enclosure. The insulation was secured tightly to the pipe surface using stainless-steel wire. The outer aluminum jacket was secured around the insulation using hose clamp bands and sealed longitudinally and to the flange ends using silicone. This limited inlet and exit points for water.
FIG. 1. A schematic of the CUI test set-up with insulation and cladding installed. The water enters through the top and leaves the insulation system via a drain hole at the bottom. Source: ROCKWOOL.
Deionized water was injected through separate 6-mm holes drilled down to the pipe surface at the 12 o’clock position. This allowed the solution to be introduced at the interface between the insulation and pipe surface, and a single drain was placed at the 6 o’clock position. The test used a total of 7.9 l of water and ran for 30 d.
Test results in the presence of chlorides. In the modified ASTM C1617 testing, the results show that mineral wool with corrosion inhibitors actively protects steel with the addition of chlorides at concentrations up to 600 ppm (FIG. 2). The mineral wool with inhibitors performed to the same level as perlite and calcium silicate at all additional chloride concentrations.
FIG. 2. Corrosion test results with 100-ppm and 600-ppm chloride added to test solutions.
However, the aerogel material with 100-ppm chlorides generated a corrosion rate of of > 1 mm/yr [> 40 mils per year (mpy)], which puts it in the 5-ppm chlorides level according to ASTM C1617. The high probability of aggressive corrosion with aerogel may lead to leaking under the PVC containers and subsequent contamination of adjacent samples, thus compromising the entire test. Therefore, aerogel was not included in subsequent tests with 300-ppm and 600-ppm chloride.
In testing with 600-ppm chloride (FIG. 1), the metal coupons exposed to mineral wool with corrosion inhibitors plus 600-ppm chloride show a lower average mass loss corrosion rate at 0.1 mm/yr (3.87 mpy) than the 0-ppm chloride reference solution at 0.16 mm/yr (6.17 mpy).
While the test results at 300-ppm chloride are not presented in FIG. 2, the results showed that the mineral wool with inhibitors yielded an average corrosion rate of 0.09 mm/yr (3.47 mpy), putting it in the same range as the corrosion rates for calcium silicate [0.19 mm/yr (7.38 mpy)] and expanded perlite [0.17 mm/yr (6.74 mpy)].
Test results under simulated 15-yr rainfall levels. The ASTM G189-21 test results show that mineral wool with corrosion inhibitors performed quite well after exposure to a simulated 15-yr’s worth of rain (FIG. 3). The pipe's top and bottom sections show low uniform corrosion rates. The aerogel insulation, by contrast, exhibited significantly more corrosion on the bottom than on the top of the pipe. This higher corrosion rate on the bottom is possibly a consequence of the water concentrating at the bottom due to gravity. Water may not have exited the system as quickly as at the top, which provided a greater opportunity for CUI to develop.
FIG. 3. Corrosion test results, 15-yr simulated rainfall levels.
Like the inhibited mineral wool, the corrosion levels for perlite appear to be uniformly distributed, with slightly more corrosion on the bottom. In terms of an average uniform corrosion rate (the corrosion rate of the entire coupon), the pipe insulated with mineral wool with inhibitors showed a rate of 0.00853 mm/yr. Pipe insulated with aerogel had a rate of 0.14314 mm/yr, while the pipe insulated with perlite had a rate of 0.05145 mm/yr.
Protecting pipes and maintaining superior insulation performance. The stringent corrosion testing showed that stone wool insulation containing inhibitor provides superior water repellency and corrosion mitigation and maintains durable performance at significant simulated rainfall levels. These results suggest that this insulation effectively mitigates the effects of CUI while improving the safety of people working in refineries and petrochemical plants.
However, plant operators also need assurances that the treated stone wool will retain its insulating properties. To that end, the authors’ company commissioned a series of third-party tests to study the inhibitor-infused stone wool’s thermal and acoustic insulation performance. The results confirmed that the insulation maintains its thermal insulation performance to minimize heat losses in hot pipes, leading to gains in plant efficiency while reducing energy consumption and greenhouse gas emissions. The insulation also delivers superior acoustics insulation performance to protect plant personnel from inherently noisy plant operations, meeting all insertion loss classification levels of ISO 15665.
The insulation is manufactured in lightweight mandrel wound sections that provides additional transportation and installation benefits. The stone wool insulation is delivered to plant facilities in split-and-hinged pipe sections that apply easily to pipes with less downtime. In addition to lower installation costs, mandrel wound sections reduce logistics, handling challenges and material costs.
From a plant maintenance perspective, because the inhibitor-infused insulation absorbs less water and keeps corrosion rates low, it also lowers the likelihood of unintended energy losses that could interrupt operations. This keeps plant processes running at optimal levels while reducing downtime and total maintenance costs.
LITERATURE CITED
1 Bavarian, B., B. Samimi, Y. Ikder, L. Reiner and B. Miksic, “Protection effectiveness of vapor corrosion inhibitor for corrosion under insulation,” Department of Manufacturing Systems Engineering & Management College of Engineering and Computer Science, California State University, Northridge (U.S.), and FNACE Cortec Corp., online: http://www.cortecvci.com/Publications/Papers/5448_CUI_bavarian_F13.pdf
2 NACE Standard SP0198-2017, “Control of corrosion under thermal insulation and fireproofing materials—A systems approach,” 2017, online: https://store.ampp.org/sp0198-2016
3 ASTM C 1617, “Standard practice for quantitative accelerated laboratory evaluation of extraction solutions containing ions leached from thermal insulation on aqueous corrosion of metals,” 2019.
4 ASTM C 795 (2018), “Standard specification for thermal insulation for use in contact with austenitic stainless steel,” 2018.
The Authors
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