Electrode Scratching Technique As a Robust Way to Decouple Erosion-Corrosion Components

Abstract

The substantial number of parameters and their interdependence makes the modelling and prediction of erosion-corrosion rates a rather challenging task, yet owing to increasing computational capacity one that is potentially solvable. To accurately estimate the rate of material loss, universally accepted explanations of the mechanisms of combined erosion and corrosion must be developed. The absence of reliable data can undermine safety and predictability of industrial processes, such as oil and gas transportation. The aim of this work is to demonstrate the application of an electrode scratching technique coupled with microstructural characterisation for improved understanding of synergy in erosion-corrosion. In this work, a rotating disc electrode scratching setup, with well-defined and controlled flow conditions, was developed to study individual contributions to erosion-corrosion. Linear potentiodynamic and potentiostatic polarization techniques were used to reproducibly monitor the kinetics of dissolution and repassivation of API X65 steel electrodes upon scratching. Samples characterized using high-resolution scanning electron microscopy (SEM) and white-light interferometry (WLI), confirm the synergy; the losses due to erosion-corrosion are larger than that of the summation of the separate contributions of erosion and corrosion. Focused Ion Beam (FIB) milling was implemented for in-situ lift-out of lamellae from scratched samples for Transmission Electron Microscopy (TEM) characterisation. We confirm distinct microstructural changes take place in the vicinity of scratches: with nanoscale grain refinement and orientation changes observed (See Figure 1). These results, coupled with electrochemical data and micro-hardness measurements, suggest time-dependent surface-hardening processes affect material loss rates during mechanical-electrochemical coupled corrosion. Figure 1