The Electrodeposition Of Nickel And Nickel Cobalt Alloy From Dense CO₂/Surfactant/Electroplating Solution Emulsion

Metallic functional membrane materials have significant physical and chemical properties in terms of electricity, magnetism, mechanics. They have a board application in the high and new technology areas, such as fuel cells, sensors, printed circuit boards, semiconductor integrated circuit. At present, the use of electrochemical deposition technique in material surface to obtain metal functional membrane layer is one of the important technology of modern industrial production. Although, the technology that electroplating metal in aqueous, has been underway for more complete and systematic research. Many disadvantages exist in the system. Such as,（1） Electrochemical window of water is narrower, that limits the electrodeposition of certain metals.（2）Hydrogen bubbles formed in the process of electroplating. It will causes pinhole in the coating surface so that the thickness of the film has to been increased to reduce the defect rate. Not only the film degree is restricted, but also we have to spend a lot of precious metal resources. Therefore, the study to establish a new electrodeposition reaction system is not only the challenge electrochemical industry faced with, but also to protect the limited metal resources of our country. The system will be help to obtain high quality metal functional membrane layer and to realize harmless production process at the same time. It is an important research topic of social sustained economic development. Since 2003, the Japanese scholar Sone and Korean scholars Kim team carried out supercritical nano plating（SNP）. It is according to the characteristics that dense CO₂ and plating solution can be built into the electrical conductivity of emulsion system without surfactant. Compared with traditional methods, this technology can not only reduce the use of the plating solution, greatly saves the metal consumption, but also can improve the coating performance to a certain extent, for the development of electroplating technology provides a new way of thinking. but also can improve the coating performance to some extent. It provides a new way for the development of electroplating technology.The research of this paper aims to establish a high conductivity dense CO₂/surfactant/electroplating solution emulsion system based on the screening and compounding of surfactants by SNP technology. And then, using SNP technology realizes the electroplating of nickel and nickel cobalt alloy on the brass substrate surface. The research will provide certain reference data for SNP technology to broaden the application of SNP technology in the field of alloy electroplating.The research of this paper mainly includes the following three parts:1. Screen or compound of surfactants, and establish an electroplating system of SNP technology. Observe and compare foaming ability and foam stability of dozens kinds of surfactants by a high pressure reactor with windows. Screen the suitable surfactants and their mixture ratio, and form a high conductivity dense CO₂/surfactant/electroplating solution emulsion system to establish SNP distribute the both according to certain proportion, the dense CO₂/surfactant/electroplating solution emulsion system with high volume expansion can be formed. On the basis of the traditional Watts bath plating solution, 0.24 vol% of surfactant B15 with respect to the electroplating solution are used. The volume ratio of plating solution is 45 vol% to the total volume of reaction chamber. The temperature is 323 K and the pressure is 10 MPa with the stirring speed at 1200 rpm. The solution can swell to the cathode the overall degree of immersion under the above conditions. When current density was 2 A/cm2, we can get a smooth and bright complete nickel film on the brass substrate.2. Performance characterization of bright nickel film electroplated by SNP: Contrast the nickel film electroplated through SNP with the one electroplated through conventional method in microscopic morphology, grain texture, microhardness and corrosion resistance. Compared with electroplating in Watts bath, the nickel film electroplated through SNP is smoother and more refined. The most preferential crystal growth direction of electroplated nickel is（111）. The microhardness of film electroplated through SNP is high to 740 HV and the corrosion resistance is better.3. The nickel cobalt alloy electroplated through SNP: We established establish a SNP system electroplating nickel cobalt alloy, which is based on adding cobaltous sulfate in SNP system electroplating nickel. The possibility of electroplate nickel cobalt alloy through SNP is discussed in the research. And we examined the influence from pressure, current density, volume fraction and stirring speed on the coating performance. It turned out that, density, viscosity of system changes because of cobalt sulfate’s adding. For getting the complete alloy plating on the brass substrate, the pressure has to be increase over 12 MPa to make the SNP system fully expand. The microhardness of alloy coating is influenced by the grain size and the cobalt content in coating. Although increasing the pressure helps to grain refinement, it lead to lower cobalt content in coating. As a result, the microhardness will decrease to 700 HV to 740 HV. With volume fraction increasing, the most preferential crystal growth direction of electroplated nickel changes from（111） to（220） and the grain size grows. As a result, the microhardness and corrosion resistance of deposition decrease. With the current density increasing, the most preferential crystal growth direction in（111） turned stronger and the grain size grows. When the current density is 2A/cm2, the microhardness is the highest up to 740 HV. When the stirring rate is 1000 rpm, the deposition is the smoothest and the most refined, the microhardness and corrosion resistance of deposition is best.