ABSTRACT

Under-deposit corrosion of carbon steel is a major issue in many industries, leading to pipeline failure. In environments with low fluid flow rates, inert matter settles and accumulates on the bottom of pipelines, leading to localized corrosion. Large pit-like features can be detected under the deposits but limited understanding exists surrounding the use of chemical corrosion inhibitors in stifling their growth. In this study, a bespoke test setup has been used to evaluate the performance of different inhibitor chemistries in stifling the corrosion of deep pit-like features under thick (< 8 mm) inert deposit layers. Potentiodynamic polarization and electrochemical impedance spectroscopy measurements were used to assess the corrosion profile of X65 carbon steel at recession depths of 0 mm and 9 mm in 2 wt.% NaCl brine, under CO2-saturated conditions, at 50 °C. Galvanic corrosion in the presence of corrosion inhibitors was measured via a zero-resistance ammeter. 2-mercaptoethanol was able to reduce corrosion rates in the presence of inert silicone dioxide deposit layers. In contrast, an imidazoline derivative was considerably less effective under the same conditions. Scanning electron microscopy was used for visual assessment of surface damage after 20 hours of corrosion.

INTRODUCTION

Corrosion of carbon steel is the most prominent reason for pipeline failure in a range of industries, from oil and gas transportation to water treatment facilities and nuclear waste storage. Under-deposit corrosion occurs in low fluid flow rate environments, when particulate matter (such as sand and clay) settles on the bottom of transportation pipelines. The presence of deposits results in a diffusion barrier, which significantly alters interfacial solution chemistry compared with that of the bulk.1, 2 This difference in local environments can facilitate accelerated localized corrosion under the deposit, resulting in premature pipeline degradation.1 The occurrence of localized corrosion and under-deposit corrosion are detrimental individually; the combination is often difficult to identify in the pipeline, leading to catastrophic failure and severe consequences. Extensive research exists on under-deposit corrosion and localized corrosion individually, but little research addresses the combined issue, which forms the basis of this work.

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