51313-02533-Dynamic Pitting Analysis in Liquid and Wet Gas Petroleum Pipelines

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Product Number: 51313-02533-SG

ISBN: 02533 2013 CP

Author: Zhenjin Zhu

Publication Date: 2013

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This paper first proposes a theoretical model to investigate the transition from metastable pitting to stable pitting within liquid and gas petroleum pipelines. Based on the Tafel’s law and the Nernst equation autocatalytic pitting corrosion takes place once ferrous ions inside pits are accumulated to the level which makes the anodic potential reach the critical pitting potential. Hence accumulation of Fe2+ ions in pits is crucial for the transition from metastable pitting to stable pitting. In this model the transient Fe2+ ion concentration was determined from the balance of the generation rate of Fe2+ ions due to iron oxidation effusion rate of Fe2+ into the bulk solution due to fluid convection passing over incipient pit initiation sites as well as the precipitation rate of Fe2+ scales or corrosion product formation. Specifically effusion of Fe2+ ions from the pit bottom into the bulk solution is always governed by mass transport which encounters the following diffusion barriers: (1) from the pit bottom to the pit mouth then (2) from the inner scale surface to the outer scale surface and (3) from the outer scale surface to the bulk solution. Overall thickness and porosity of formed scales were approximated in terms of difference of Fe2+ ion concentration at the pit mouth and the saturation level of corrosion products. Based on the developed model accumulation of Fe2+ cations inside the pits and variation of anodic potential with respect to pit geometry and operating parameters could be quantified. As well criteria of transition from metastable pitting to stable pitting for those key parameters such as flow velocity solids deposition level and chloride ion concentration could be obtained. Through a comparison with field data measured from In-line Inspection (ILI) and ultrasonic testing (UT) the obtained criteria can provide operators with practical assistance to enhance integrity and reliability of both gas and liquid petroleum pipelines.