Research On Zn-Ni Coating And Zn-Al Plating Process On Automotive Fasteners Steel And Galvanic Corrosion

Automotive fasteners contact with different metal parts during the service.As a result,galvanic corrosion occur and affects driving safety.Currently,electroplating and dacromet coating technologies which are mainly used in traditional fasteners are limited by the poor corrosion resistance and environmental pollution issues,therefore it is difficult to meet the application requirements in the automotive field.However,Zinc-nickel coating has better corrosion resistance than pure zinc and Chromium-free zinc-aluminum coating uses environmentally friendly passivation agent and binder which keeps the same corrosion resistance performance in the meantime.Therefore,the study of chromium-free zinc-aluminum coating and zinc-nickel coating have remarkable application value in the automotive field.In this research,the process parameters of chrome-free zinc-aluminum coating and zinc-nickel coating on the surface of automotive fastners steel are studied.Based on the coating preparation process,two coatings are prepared on the surface of the plate and bolt;the durability and safety of the two coatings on the surface of the automotive fastener material were evaluated by the galvanic corrosion of the metal material after coupling,thereby the galvanic corrosion between the fastener and the other metal is reduced.This technique will provide great help to the design and optimization of vehicle anti-corrosion.With higher content of the alloy in the zinc-aluminum coating,the corrosion resistance of the coating is stronger.The addition of the chromic anhydride can enhance the adhesion of the interface,and the preparation process of microstructure uniform and dense zinc-aluminum coating process is determined as follows:zinc alloy powder is 80Zn-20 Al,powder content is?20³0?%.Sodium molybdate is used as passivating agent.Baking temperature is?80⁹0??,?10¹5?min.the curing temperature is 300?,30 min,and the time is 30 minutes.During the electroplating process of zinc-nickel alloy,with temperature increasing,content of Zn²⁺/Ni²⁺decreases and the nickel content in the coating decreases.The optimum process parameters for the preparation of zinc-nickel alloy are alkaline plating nickel,with the current density of 4A/dm²,the bath temperature?40⁴5??,the p H is 8⁸.5,and the Zn²⁺/Ni²⁺ratio is about 0.6.The corrosion behavior of ST12 steel and hot rolled steel SS400 galvanized with zinc-aluminum coating and zinc-nickel alloy coating indicates that the chromium-free zinc-aluminum coating and zinc-nickel coating are both anodic coating and the corrosion process is accelerated.However,during the corrosion process,when the galvanic capacitance of ST12 and SS400 treated with zinc-nickel coating is corrected,the galvanic current is smaller.The galvanic corrosion resistance is better than zinc-aluminum coating,and the corrosion rate is significantly smaller than the zinc-aluminum coating.Combining environmental corrosion test and electrochemical test results analysis,zinc-nickel coating is more suitable for the treatment of the fastener surface of hot rolled steel SS400 parts.This research introduces a new way to help optimize coatings for bolts that connect parts with different surface materials:combination of environmental corrosion analysis and electrochemical tests.