Application of Magnetic Flux Leakage (MFL) method to non-destructively characterize the microstructure and corrosion behaviour of API X65 grade steel

Abstract

The goal of this research is to investigate the ability of the Magnetic Flux Leakage (MFL) method to characterize the microstructural features and corrosion performance of heat-treated API X65 grade steel in a non-destructive way. Various microstructures were created by austenitizing the specimens of API X65 steel at 950 °C for 45 min, followed by cooling in the furnace, air, oil, and water. SEM observations indicated that applying various heat-treating cycles led to three major microstructural changes in the ferrite phase, including volume fraction, grain size, and morphology. It was also found that an increase in cooling rate, increases the corrosion current density (icorr) obtained by the Potentiodynamic Polarization (PDP) test and decreases the charge transfer resistance (Rct) measured by Electrochemical Impedance Spectroscopy (EIS) technique. Furthermore, surface analysis of corroded samples showed that the higher ferrite (anode) to cementite (cathode) ratios in the furnace/air-cooled samples boost the corrosion performance, while uniform and localized corrosion have taken place on the oil and water quenched samples due to the higher density of dislocation and their acicular structure. Non-destructive evaluations indicated that microstructural changes as a result of various heat-treating cycles dramatically affect the magnetic permeability, which considerably influences the magnetic flux leaked from the surface and detected by the Hall effect sensor in the MFL method. Two MFL parameters, peak-to-peak amplitude, and slopes of the raw curves of the Hall sensor outputs at zero-crossing points were studied for the specimens. The results revealed good correlations between MFL parameters and values of icorr and Rct. Additionally, in this study, an “optimal range” has been proposed for the MFL outputs, which can be adjusted according to the required range of mechanical properties and corrosion behavior in the inspection process of API X65 steel specimens after heat treatment.