Corrosion under insulation (CUI) refers to the external corrosion on the metallic pipe/ equipment body subjected to thermal insulations. CUI manifests localized corrosion (mainly) and has always been a driver behind failures on thermally insulated pipelines. Despite the advent of numerous measures namely protective coatings, and hydrophobic insulations, the issue of CUI remains an inevitable reality for the pipelines especially those which undergo submerging conditions from the rainwater in the culverts, being buried under the snow piles, water flooding, etc. All these moisture intrusion events result in the soaking of insulation thereby exploiting the CUI risks. This study addresses the ambient temperature CUI behavior of a thermally insulated carbon steel pipe to mimic the out-of-service (normally happens during maintenance shutdowns, mothballing, etc.) behavior of thermally carbon steel pipeline(s). The insulated pipe assembly was submerged under the water for a two day's period followed by exposure to outdoor conditions for one year. The insulated assembly was checked for corrosion behaviors using confocal laser microscopy, and x-ray diffraction; followed by the interpretation of corrosion modes and kinetics.


Corrosion under insulation (CUI) is among the key damage mechanisms in the hydrocarbon industry which mainly manifests localized corrosion on thermally insulated equipment, tanks, piping, and pipelines.1 CUI is a highly frequent degradation in the oil refining facilities where reportedly 40-60% of failures in the piping result from CUI. In addition, small bore piping (i.e., nominal pipe size, NPS < 4 inch. or 100 mm) are even more prone to CUI and reportedly up to 81% of failures in these piping caused by CUI. Expenditures towards inspection and fixing of CUI damages constitute 10% of the overall maintenance spendings in a typical oil refinery.2

The moisture-soaked thermal insulations are the leading cause of CUI. Moreover, the design of insulated system influences the extent of moisture absorption, trapping, and consequential CUI damage.3 Other than triggering CUI, the absorbed moisture degrades thermal insulations via increasing the thermal conductance. Reportedly, moisture content of 5% within fibrous stone wool insulation causes a 25% increase in thermal loss(es). 4 In a typical closed-contacting insulation design, the CUI manifests localized corrosion via macro-galvanic cells as has been reported in the previous studies by Rana et. al. and Pojtanabuntoeng et. al. 5-6 Industry standard regard the CUI risk to be the highest up to a temperature of 175°C and decline with increasing temperature, as the moisture is perceived to boil-off from the pipe skin and permeate outwardly through the insulation as vapor-drive.1

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