Electroless Nickel Plating of Material Surfaces for Ultracold Neutron Storage Traps

Electroless nickel plating of various alloys is one of the most technologically mature methods for producing functional coatings with high corrosion resistance, wear resistance, and stable service performance. In the field of ultracold neutron (UCN) physics, nickel-phosphorus coatings based on the ⁵⁸Ni isotope are of particular interest. Its exceptionally high neutron optical potential, approximately 342 neV, is substantially higher than that of natural nickel, which is approximately 245.5 neV. At the same time, ⁵⁸Ni and natural nickel have essentially identical chemical properties and differ only in their nuclear characteristics; therefore, process development and optimization of the deposition procedure can be performed using the more readily available natural nickel. The incorporation of phosphorus decreases the total neutron optical potential to values in the range of 190-220 neV, but makes it possible to modify the magnetic properties of the nickel coating in order to reduce magnetic losses of UCN upon reflection from the surface. Although extensive studies have been devoted to the deposition of Ni-P coatings on aluminum and steel substrates, the formation of high-quality coatings on beryllium- and zirconium-based alloys remains insufficiently investigated. These materials are of particular interest for chambers of superfluid-helium-based UCN sources, since they can reduce activation-related heat loads and ensure efficient cooling of the source to the required temperatures of approximately 1 K under high-radiation-flux conditions. In this work, electroless Ni-P coatings deposited on the surfaces of the aluminum-magnesium alloy AMg6, the aluminum-beryllium alloy AM162H, and the zirconium alloy Zircaloy-4 were systematically investigated, with particular emphasis on AM162H and Zircaloy-4 as promising structural materials for UCN-source chambers. The deposited coatings were comprehensively characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma optical emission spectrometry (ICP-OES), surface profilometry, instrumented nanoindentation, scratch testing, and electrochemical polarization measurements followed by Tafel analysis. These complementary techniques provided information on the phase-structural state, surface morphology, continuity, thickness, elemental composition, roughness, hardness, elastic modulus, indentation stiffness, adhesion behavior, and corrosion resistance of the coatings. The ICP-OES measurements independently confirmed the compositional trend established by EDS. The coatings deposited on AM162H and Zircaloy-4 exhibited higher hardness and substantially better corrosion resistance than Ni-P/AMg6. Scratch testing indicated satisfactory interfacial adhesion for the coatings on AMg6 and AM162H, whereas the Ni-P/Zircaloy-4 coating exhibited comparatively weaker adhesion to the substrate under the applied test conditions. Particular attention was paid to coating continuity, composition, and thickness because through-thickness defects would expose the substrate, which has a lower neutron optical potential, impurity elements could increase UCN absorption and reduce the neutron optical potential of the coating, and an appropriately selected coating thickness would enable the rational use of the expensive ⁵⁸Ni isotope. The results provide an experimental basis for further optimization of electroless Ni-P coatings for components of UCN production and storage systems.