Analysis of Microstructural Transformations Induced by Thermal Aging of Co-Cr-Ni-W-C Alloy Deposits and Their Impact on Corrosion Behavior in a 3.5% NaCl Medium

This paper presents a study on the microstructural evolution of deposits based on the stellite F alloy during temperature aging and consequently its effect on corrosion resistance in 3.5% NaCl medium. The deposits were made by transferred arc plasma (PTA) spraying on an austenitic steel substrate. Heat aging treatments were carried out at 950 °C for durations ranging from 50 to 500 h. The microstructural study was performed using scanning electron microscopy coupled with energy dispersive spectrometry and the x-ray diffraction technique allowed the identification of the phases. Quantitative chemical analyses of each phase were performed using the CAMECA X50 microprobe and the electron energy loss spectrometer on a transmission electron microscope. The electrochemical behavior was achieved by monitoring the open-circuit potential and plotting the potentiodynamic and cyclic polarization curves. The electrochemical parameters of the aged bare and coated samples, in aqueous solution of NaCl 3.5% at 298 K, are derived from the polarization curves. The results of the microstructural study show a significant evolution of the microstructure of the deposits during heat treatments, marked by the decomposition of M₇C₃ carbides and the precipitation of M₂₃C₆ and M₆C secondary carbides in the matrix. The presence of these carbides and the enrichment of the matrix in Cr improve the protective properties of the passive film formed, and consequently, the corrosion resistance of stellite alloy deposits.