~~Under-deposit Corrosion (UDC) is a form of localised corrosion caused by the concentration of dissolved chemicals (e.g. CO2 Hydrochloric and Sulphuric acids) scales and sand deposits. This localized corrosion phenomenon creates a region that is chemically and physically different to the rest of the uncovered steel. The effect of sand settling on the surface of fluid transportation pipelines has been a major source of concern and the corrosion control and monitoring (especially) of localised corrosion failures in the oil and gas production facilities has not been thoroughly investigated.This paper will review the current understanding of the mechanisms of under-deposit corrosion of pipelines due to significant accumulation of solids in the presence of CO2. A set of experiments were conducted under different conditions in order to investigate UDC it examined the barriers that influence inhibition of under deposit corrosion and the mechanisms used in the field to mitigate UDC.The investigation focussed on the effect of sand deposit (5 and 10mm sand deposit) as the dominant under-deposit corrosion mechanism in determining the corrosion rates of carbon steel (X65) at different temperatures (25 and 80°C). An electrochemical linear polarisation resistance (LPR) measurement technique was adopted in static corrosion tests in CO2 saturated conditions. The experiments assessed the potential of a corrosion inhibitor to adequately control the material degradation caused by sand deposit and hence inhibit the localised corrosion in the covered regions.The LPR results for direct corrosion measurements for the non-sand deposited (Non-UDC) or bare metal samples were higher compared to the sand deposited samples under the same environmental conditions. However the surface analysis revealed severe pitting in the under-deposit samples. The experimental tests recorded a lower corrosion rate when the sand was saturated with corrosion inhibitor through the solution. A bonding of the sand onto the surface of the carbon steel surface especially at higher temperature was observed; this was enhanced by the formation of iron carbonate scale and these combines to form a barrier or a layer on the steel surface.Localised corrosion monitoring and analysis were conducted using standard electrochemical corrosion monitoring apparatus and the characterisation of corroded surfaces using SEM EDX optical Microscopy and Interferometer.Keywords: CO2 corrosion UDC Corrosion inhibitor Scaling.
