CUI (corrosion under insulation) is among the leading damage mechanisms active in oil refining and hydrocarbon facilities. Reportedly, 10% of the total maintenance budget of a typical refinery is spent on inspecting and fixing CUI damages. The mechanism of CUI is complex and influenced by many factors namely the design of the insulated system, insulation type, ambient conditions, condition of coatings and insulation materials, as well as operating parameters. This study addresses the quantified influence of cyclic temperatures on CUI in comparison with constant temperatures for various insulation designs namely the closed-contacting and contact-free with low-point drainage. It also investigates the CUI behaviors under moisture cycling (i.e., wet-dry conditions) of candidate insulation designs in comparison to a fully wet environment. To simulate this, corrosion rates were determined under Isothermal wet, isothermal wet-dry, cyclic wet, and cyclic wet-dry conditions using weight loss measurement as per applicable ASTM G189-07 standard. The influence of cyclic temperatures and wet-dry conditions were also studied using the linear polarization resistance method in the system with contact-free insulation. The corroded coupons were then characterized using a microscope and surface topography. The wet-dry conditions under both cyclic and constant temperatures caused more corrosion rates than those under wet conditions. Also, cyclic temperatures caused more corrosion rates than constant temperatures under both wet and wet-dry conditions.


Corrosion under thermal insulations namely CUI (Corrosion under insulation) is among the key degradations posing integrity risks to the hydrocarbon processing, chemical, and petrochemical facilities. 1 CUI is reportedly known as the reason behind 40-60% of failures in the piping for the oil refineries. Whereas the small-bore piping (i.e., NPS< 4") is more prone to this damage mechanism where reportedly where up to 81% of failure result from CUI. 2Monetary spending to inspect and fix CUI-related failures cost 10% of overall maintenance budget in a typical medium-sized oil refinery.2CUI risk is influenced by numerous operational and environmental factors which challenges its assessment and control in a typical AIM (Asset integrity management) program. The key driving factor behind CUI is the aerated moisture on the metals that comes from the moisture-saturated thermal insulations. 3 The type and condition of insulation materials have significant impact on their moisture absorption tendency and subsequent CUI rate(s). Other than triggering CUI, the absorbed moisture degrades thermal insulations via increasing the thermal conductance. Reportedly, moisture content of 5% within stone wool insulation causes a 25% increase in thermal loss(es). 4

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