Corrosion under insulation (CUI) poses a significant avoidable challenge for the hydrocarbon processing industry (HPI) today. It primarily affects refineries, impacting steel piping, storage tanks, container vessels, and other process equipment subjected to extreme temperature fluctuations. Insulating materials like calcium silicate or mineral-wool can help mitigate thermal cycling effects, but gaps and seams in the insulation layer make it susceptible to moisture infiltration from external sources or the process environment itself.
Understanding the mechanisms behind CUI, it occurs when water and contaminants penetrate insulated systems with specific water retention, permeability, and wetability characteristics. Possible sources of water include rainfall, cooling tower drift, steam discharge, washdowns, and condensation due to insulation not being vapor tight. Factors like breaks in waterproofing, inadequate system design, incorrect installation, or poor maintenance practices contribute to water entry.
Once the insulation system becomes wet, the trapped water, along with corrosive substances like chlorides and sulfates, accumulates in the annular space or crevice between the insulation and equipment surface. This corrosion process affects substrates made of carbon steel (CS) or austenitic and duplex stainless steel (SS). CS is susceptible to CUI when exposed to moisture under any insulation within a temperature range of 25°F to 350°F (-4°C to 175°C), with the corrosion rate influenced by specific contaminants and steel surface temperature. Austenitic and duplex SS can experience external stress corrosion cracking (ESCC) at higher temperatures, typically ranging from 120°F to 350°F (50°C to 175°C), with the presence of waterborne chlorides concentrating on the hot SS surface.
CUI remains a hidden problem as the progress of degradation is concealed by insulation and cladding, making it difficult to detect until insulation is partially removed for maintenance or repairs. Typically, insulation is only removed every 15–20 years, making timely identification of deterioration less likely. However, it is crucial to promptly identify lurking corrosion on these substrates, especially considering the changing conditions of coatings, insulation, and refinery operations over time.
CUI has become a significant concern for reliability engineers due to the operational disruptions caused by major equipment outages, whether for inspection, maintenance, or catastrophic failures. Therefore, the use of protective coatings is recommended to combat CUI effectively. In the petrochemical and refining industries, shop-applied inorganic zinc (IOZ) coatings are commonly used as primers on new CS piping, but they require topcoating to extend their service life, particularly when used under thermal insulation. Immersion-grade protective coatings are considered the best defense against CUI in both CS and austenitic/duplex SS, as they provide corrosion resistance in the presence of trapped water.
NACE International, The Corrosion Society, provides industry guidance and standards, such as SP0198-2010, which reflects the latest insights and approaches for CUI prevention and mitigation in the oil and gas industry. Research and development efforts are essential to address the evolving challenges posed by CUI, as modern facilities operate at higher temperatures than in the past. Protective coatings suitable for refinery applications must be able to withstand accelerated and real-world testing protocols, including the boiling water test, which evaluates resistance to thermal cycling. These coatings should also offer enhanced properties for construction, transportation, and worker safety while reducing the total cost of ownership and extending the service life of equipment under insulation.
By meeting these requirements and passing rigorous testing standards, second-generation CUI coatings can effectively protect against corrosion, even in demanding high-heat environments.
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