Role of KOH and NaNO2 on the structural and corrosion properties of anodic spark electrolysis coatings produced on aluminium

Doping light elements into ceramic coatings on different metal substrates by anodic-spark electrolysis (ASE) to improve their properties, such as wear and corrosion resistance, has recently attracted a lot of attention. In this study, nitrogen-doped Al2O3 composite ceramic coatings had been fabricated in eco-friendly KOH-NaNO2 electrolytes using the anodic-spark electrolysis (ASE) method after 9 min at a fixed applied ASE voltage (75 V higher than the breakdown voltages). To deposit a nitrogen-doped coating with high amounts of oxynitride phases possible, we thoroughly studied the ASE coatings deposited in different total variable salts concentrations (KOH+NaNO2) and NaNO2/KOH ratios of ASE electrolytes. The coating properties were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), and the electrochemical impedance spectroscopy (EIS) tests. The results indicated that the coating produced in the KOH-NaNO2 electrolyte with a low total variable salts concentration (2 gr.L-1) and a high NaNO2/KOH ratio value (3) is optimum in the investigated conditions. It has the highest percentage of nitrogen-doped phases, such as N-doped gamma-Al2O3 and gamma-AlON (gamma-Al2.78O3.65N0.35), and a homogeneous morphology of surface with the smallest average size of pores (<14 mu m2). This coating showed the significantly higher corrosion resistance with a 4.101104 x 106 omega cm2 value compared to the uncoated aluminium substrate with a corrosion resistance value of 0.094195 x 106 omega cm2 after 48 h of immersion in the 3.5 wt% NaCl solution. The approach presented herein provides an attractive way to modify the surface of aluminium alloys to improve corrosion behaviour.