Numerical Investigations on Localized Corrosion from Single and Multiple Pits in Fuel Tanks

In recent years, the planned replacement of traditional fuels with biofuels is the object of frequent debate and interest, as the behavior and effects of these substances on the structural materials constituting the storage units are still unknown. Moreover, limited information is available on the hazards of the process and the assessment of the risk associated with biofuel production and storage requires improvements. The relevance of this issue needs operators to adopt a control plan for the equipment to ensure safe ageing. Equipment ageing has been identified in many cases as the primary cause of the release of hazardous substances. Atmospheric storage tanks, in particular the bottom, are critical from this viewpoint if adequate monitoring and integrity management plans are not adopted. Bottom corrosion control is carried out by means of thickness measurements, usually every 10 years or more, as part of a complete inspection of the entire tank based on widespread standards. Accurate bottom integrity measurements can only be made during scheduled stops, when the tank is taken out of service, emptied, thoroughly cleaned and visually inspected. In this context, the behavior of innovative biofuels and advantages related to their adoption requires laboratory and modeling expertise to evaluate the corrosion rate. Based on widespread standards, the corrosion rate is usually estimated as the ratio of the thickness reduction and the time interval between two inspections. However, discrete thickness measurements cannot determine exactly the maximum corrosion depth of the bottom of storage tanks, where materials may exhibit localized corrosion in the form of pits. To assure reliability of a given material under a certain environmental condition, it is thus needed to make a quantitative prediction of the local corrosion rate. The goal of the present study is to track the evolution of metal/electrolyte interface to predict the rate of corrosion arising from single and multiple interacting pits. To this purpose, numerical simulations are performed on a simple model that combines electrolyte and electrochemical behavior of the interface rendering a phase-field dependent form of the Nernst-Planck equations. In order to describe the 2D spatio-temporal evolution of the six chemical species involved in the corrosion process (H+, OH- Na+, Cl-, Fe2+, FeOH+) and of the liquid/metal interface as well as the spatial distribution of the potential, we use a consolidated finite-element framework in COMSOL Multiphysics. The analyses allow to characterize the functional dependence of the corrosion rate on several key parameters, such as the electrochemical potential, the number and the mutual distance among pits, as well as the chemical composition of the electrolyte.