Organizational Structure And Properties Of Electroless Nickel And Uranium On The Pulse

Uranium is chemically active, which is extremely apt to be corroded in many environments. Nickel deposits which had fine crystal grain, lower porosity and excellent corrosion resistant performance were prepared on uranium with pulse-plating technology, the disadvantages of direct current (DC) plated deposit were overcome. Microstructure and performance of pulse-plated nickel deposits were studied by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), optical microscope (OM), electrochemical measurement technology, scratchier and tribometer. The mechanisms of crystal grain fine and corrosion of pulse-plating nickel deposit were discussed.The crystallographic structure of the nickel deposit pulse-plated on uranium is face centered cubic and has a highly preferred orientation of (200).The relative preferred orientation of (200) of pulse-plated nickel is higher than DC plated nickel deposit. When the pulse peak current density is greater than 19A/dm2, the relative preferred orientation of (200) of pulse-plated nickel increases with the accretion of the pulse peak current density.The grain size of pulse-plated nickel is smaller than DC nickel deposit, and reaches to nano-scale. The smallest crystal grain size of these deposits is 45.5nm.With the increase of pulse peak current density, the overpotential of pulse electro-crystallization raises, and the driving force of electro-crystallization is strengthen, then the rate of nucleation is higher than the growth of grain. On the other hand, the adorption of chemical species in the bath during the off-time interrupts the the growth of grain. As a result, the grain size is refined.Without inclusion, the boundary between crystal grains of pulse-plated nickel is clear. The pulse-plated nickel has multi-crystal nature. Grown periodically, it has lamellar structure. The pulse-plated nickel has fine grain, and has the ability to cover uranium substrate fully. The pulse-plating can effectively lower the porosity and decrease the thickness of nickel deposit on uranium.In 50ug/g Cl- potassium chloride aqueous solution and 75C 95 %RH moisture atmosphere, the corrosion rates of nickel pulse plated uranium are accelerated with elapsed time, but they are lower than nickel DC plated uranium. The former has higher corrosion potential, greater polarization resistance, lower corrosion current and greater magnitude of impedance than the latter. The electrochemical corrosion characteristics and resistant performance of pulse plated nickel deposit for uranium substrate is better than DC deposit.In potassium chloride aqueous solution, the corrosion of nickel pulse plated uranium at thepore in deposit is faster than that of other area. With the process of corrosion, the corrosion pit deepens gradually. For uranium substrate, nickel deposit is a cathodic coating, and then the corrosion resistance is based on the physical barrier. To increase the integrality and decrease the porosity of nickel deposit can improve the corrosion resistant performance for uranium substrate.All nickel deposit on uranium which was etched by ferric chloride has good adhesive strength. Wearing pulse plated nickel on uranium, without flaking off, the evident plastic deformation takes place on the surface. It's wear behavior is adhesive.It can be concluded that the pulse-plating can effectively fine crystal grain of nickel deposit on uranium.In comparison with DC plated plating, the pulse-plating can lower the porosity of nickel deposit and improve the corrosion resistance performance for uranium. The pulse plated nickel deposit prepared at the on-time of 0.5ms and the off-time of 5ms has relatively lower porosity and better corrosion resistant performance for uranium substrate.