A Study On Processes And Fundamentals Of Zn-Ni And Zn-Ni-P Alloys Electroplating

Zinc deposits have been widely used to protect steel from corrosion due to their low costs. However, the corrosion resistance of the zinc coatings can cope with automobile etc demands in company with the fast development of modem industry and technology, so, many methods have been proposed to improve the corrosion resistance of the zinc coatings. Among these, electrodeposited zinc alloys (such as zinc-nickel, zinc-iron, zinc-cobalt) have been found to possess various beneficial properties, include excellent corrosion resistance, good ductibility, low hydrogen embrittlement and small internal stress. Zn alloys are being considered as substitutes for zinc, cadmium and chromium plating widely used in automobile, aviation, shipbuilding, construction, means of transportation and electronic industry. In China, however; this new technique is virtually at a stage of lab research. This dissertation, based on current research, tries to study the electroplating process of weak acid chloride based Zn-Ni and Zn-Ni-P and the fundamentals, aiming to shorten the gap between China and the world concerning the zinc alloys electroplating technique.The main subjects of the dissertation are as follows:For the first time, oil of vitriol is used to sulfonate OP, and the optimum conditions of sulfonating reaction are ascertained. By making sulfonated OP as carrying agent brighter and co-operating with appropriate amount of main brighter and assistant brighter, a zinc alloy electroplating additive, which has a high turbidity point and wide brighter current density region, is obtained.The processes of Zn-Ni alloy electroplating from faintly acid chloride bath and Zn-Ni-P alloy electroplating from uncomplexed acid chloride bath have been studied. Through the analysis of the electrolyte ingredients, the process variables and their relationship with the Ni and/or P content in the coatings, the optimum processes have been obtained. The corrosion tests show that the corrosion resistance of the Zn-Ni coating with the content of I 3wt0/o Ni is six times as strong as that of the Zn coating of the same thickness with white chromate film, and the Zn-Ni-P coating with the content of 1 5wt% Ni and O.9wt% P is eight times as strong as that of the Zn coating.The optimum processes of fomiing a thick anticorrosive black chromate film on a Zn-Ni coating has been obtained. In this process, the phosphatization is incorporated to increase the thickness of the film, and the transition element salt is used as blacken agent to take the placeIIof soluble silver or cOPper salt. The Zn-Ni coating with the black chromate film is eigh to ninetimes as StrOng as tha of the Zn coating.The eleCtrOchendcal behaviors of the Zn-Ni electrolyte and the Zn-Ni-P eleCtrOlyte havebeen nded resPeCtively by linear POtential sweep method. It has been discovered that Niconient in the Zn-Ni coating is influenced by the comPOsihon of the electroIyte, by theadditives and by tempeha. It also has been discovered tha Ni and P in the An-Ni-P platingprocess show cooPefative effect.Qulck corrosive exPeriment weak polariZation and altemating cimnt hoPedance methodsare used to examind the excelleni cormsive resisate of Zn-Ni co8tirig. The mechtrisms ofthe cormsion of ZnNi and Zn coatings are studied by using altemaing cwrent impedancemethods and the theories of energy band theory and quantUm electrochemisnySevetal kinds of eleCtrchendcal methods are used tO explore the mechanism ofphosPhona incoopration during the electredeposihon of Znoui-P alloy The ldnatics equationsof the P reducing reaction are derived bom the mechbosm and the klnetic parameters areobtained bo the experiments. The cooPerative effect of Ni and P in the Zn-Ni-P platingprocess can be explained by the me-sm.