CUI (corrosion under insulation) is among the key concerns for the integrity of process equipment and pipelines. Various measures to detect and fix the damages from CUI pose significant maintenance expenditures in hydrocarbons processing facilities. The key reason behind CUI is the limitation of thermal insulations to absorb the moisture and soak the underneath metal from wicking action. Other than CUI, trapped moisture in the soaked thermal insulations causes heat loss from process systems, thereby posing the risk of additional damage mechanisms and increased operating expenditures. This study addresses the impact of robust drain openings and insulation stand-offs on the CUI rate of carbon steel under four different testing conditions namely isothermal wet, isothermal wet-dry, cyclic wet, and cyclic wet-dry, respectively. Corroded specimens were further characterized using surface topography and scanning electron microscope. The impacts of temperature and moisture cycling on the corrosion attributes were also characterized using the linear polarization resistance method followed by an investigation of corrosion modes via optical microscopy. Insulation stand-offs in conjunction with robust drain opening resulted in the lowest corrosion rate. With insulation stand-offs and drain openings, the cyclic temperature conditions caused higher metal loss than that in isothermal conditions.
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.
Solid particle erosion (SPE) and dents (from contact loads) are among numerous surface degradations in the hydrocarbon industry that can in turn compromise the longevity of protective coatings. Both these degradation mechanisms can induce cracks that allow the corrosive solutions to seep through those cracks and corrode the underlying metal, thereby defeating the purpose of surface protection. Nickel-phosphorus (Ni-P) coatings have been known for decades for their corrosion resistance, but their applications in hydrocarbon industries are impeded by their tribological limitations, namely low wear resistance. In the current research work, graphene nanoplatelets were introduced to an Ni-P electroless plating bath in various concentrations (30 mg/L, 60 mg/L, and 100 mg/L) to achieve three different compositions of ternary Ni-P–graphene coatings, namely Ni-P-30 mg G, Ni-P-60 mg G, and Ni-P-100 mg G, respectively. Surface roughness was characterized via topography employing a laser confocal microscope. Coating hardness was characterized using Vickers hardness and the composition analyses were carried out via energy dispersive spectroscopy. SPE was conducted via Tungsten carbide (WC) erodent ball at three different impact angles and two different particle velocities. Finally, Hertzian indentation was performed under two different loads to characterize the denting behavior of coatings. Eroded and dented coatings were further visualized via an optical microscope. The highest concentration of graphene (by 18 vol.%) in Ni-P-30 mg G coating improved the hardness, leading to the smallest size of indents during both SPE and Hertzian indentation. Also, Ni-P-30 mg G exhibited no evidence of cracking under normal impact angle and particle velocities of 35 ms−1 and 52 ms−1.
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