THE EFFECT OF CURRENT DENSITY ON THE SURFACE MORPHOLOGY IN THE THIN-COATING PROCESS OF NICKEL ON Zr-2 USING THE ELECTROPLATING METHOD

Abstract

After the Fukushima-Daiichi reactor accident in 2011, one of the research and development focuses of nuclear fuel worldwide has been on coatings for Enhanced Accident Tolerant Fuels (EATF). Chromium (Cr) coatings are considered suitable due to their high oxidation resistance; however, Cr has limitations, particularly its poor diffusion on certain materials such as zirconium (Zr). Nickel coatings are therefore used as an interlayer to overcome the diffusion problem of chromium on zirconium substrates. Several surface coating methods are available, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), high velocity oxygen fuel (HVOF), detonation gun (D-Gun), and electroplating. Electroplating was chosen in this study because of its high productivity, simple equipment, and low cost. The purpose of this research was to investigate the effect of current density on the surface morphology in the electroplating of Zr-2 with nickel. Electroplating is a process in which metal ions in an electrolyte solution are driven by an electric field to deposit onto a material. The electrolyte solution used in this study consisted of 200 g/L NiSO₄·6H₂O as the nickel source, 35 g/L NiCl₂·6H₂O as the activator, and 30 g/L H₃BO₃ as the pH buffer. The current densities applied were 0.015 A/cm², 0.025 A/cm², and 0.05 A/cm². After deposition, hardness tests were conducted, and the surface morphology was examined using SEM and EDS. The results showed that increasing the current density led to larger average nickel grain sizes, namely 4.68 µm, 6.19 µm, and 6.84 µm, as well as larger pore areas on the surface, namely 6.5 µm², 20.85 µm², and 27.98 µm². Micro-Vickers hardness tests indicated that higher current density increased hardness values, measured at 163.48 HV, 178 HV, and 234.25 HV, respectively. Cross-sectional SEM analysis revealed that the coating produced at a current density of 0.015 A/cm² showed better quality compared to higher current densities. This was evidenced by smaller pore areas, thinner coating thickness (7.44 µm compared to 19.17 µm and 8.42 µm), and the absence of defects at the coating–substrate interface, which were observed at 0.025 and 0.05 A/cm². To achieve thickness values closer to calculations, uniform spacing between the cathode and anode as well as the use of a fresh electrolyte solution are recommended. The use of a nickel interlayer can be a promising option to improve the surface performance of Zr-2.