Study on the effect of the crack closing of AlCoCrFeMnNi high entropy alloy electro-spark deposited coating by plasma nitriding on the corrosion resistance

The formation of cracks and porosities in the hard coatings is inevitable within the coating applied through the electro-spark deposition (ESD) method. However, applying post-treatment surface modification to alter and control surface defects is an effective method. In this study, the AlCoCrFeMnNi high entropy coating was applied to the structural steel using the electro-spark deposition method, followed by plasma nitriding at 550 °C. The results of the microstructural studies conducted using field emission scanning electron microscopy, showed that closing the cracks and porosities leaded to smoother surface. To evaluate the corrosion resistance, polarization and electrochemical impedance spectroscopy (EIS) were conducted in a 3.5 wt% NaCl solution at room temperature. Results showed that applying a plasma nitriding alters the crack network on the surface of the AlCoCrFeNiMn hard coating, which significantly increased the corrosion resistance of HEA coating. Additionally, the formation of the nitride phases on the surface of the coated layers caused the formation of the galvanic cells of nitride/oxides and reduced the corrosion rate of the samples after plasma nitriding; however, localized corrosion especially pitting corrosion occurred. Repairing the cracks network, formation of nitride phases and increasing the lattice distortion are three main outcomes of the diffusion of nitrogen that contribute to improve the corrosion resistance.