CUI (corrosion under insulation) is reported as a driver behind the majority of failures in thermally insulated process piping and poses significant maintenance expenditures and service interruptions. Small-bore pipes are more prone to failure from CUI due to their lower wall thickness and lower surface area in comparison to larger diameter pipes. This research work simulates the CUI behavior of small-bore piping over 12 months period in the outdoor settings to mimic the out-of-service conditions in an industrial field setting. For this, two sets of assemblies were made which comprised fibrous stone wool insulations applied over the carbon steel coupons with and without stand-off membranes and low-point drain. Both assemblies were pre-soaked via submerging in the water and tested in the outdoor conditions for 12 months followed by the insulation removal and detailed characterizations. Corrosion behaviors of steel coupons were studied using weight loss, pit depth measurement, surface profile topography, and scanning electron microscopy. Whereas chemical compositions of the corrosion products were investigated using x-ray diffraction. Corrosion rates derived from mass loss data were compared with those calculated using the semi-quantitative RBI (risk-based inspection) method. The kinetics behind the formation of various corrosion products are also discussed. The stand-off membranes and low-point drain resulted in the reduced time of wetness (i.e., moisture exposure time) that in turn resulted in the domination of lepidocrocite (γ- FeOOH) along with reduced uniform metal loss rate and reduced pit depth in comparison to conventional closed-contacting insulation system.

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