Study On Process And Mechanism Of Zn-Mg Coatings Deposited By Vacuum Evaporation

With the development of the steel industry and the promotion of car body lightweight,the application of advanced high-strength steel（AHSS）has become more and more extensive.However,AHSS is easy to corrode in service and needs to be coated with zinc or zinc alloy coatings onto the surface.Currently,the major coating processes are hot-dip galvanizing and electrogalvanizing.If hot-dip galvanizing was adopted,due to the fact that AHSS has been alloyed with more and more elements such as Mn,Si,Cr or Al in order to increase the strength,these alloying elements are prone to selective oxidation and generate more surface oxides during the annealing process,which leads to poor wettability and surface defects such as bare spots of coatings.If electrogalvanizing was adopted,the risk of hydrogen embrittlement of the coated steel sheet is significantly increased although the defects of bare spot can be reduced by electroplating.Moreover,the electrogalvanizing process not only consumes a large amount of energy,but also increases the cost of treating waste water.Therefore,physical vapor deposition（PVD）technology,which is environmentally friendly and flexible,has emerged and attracted more and more attention.In addition,due to the shortage of zinc mineral resources in the world,it is necessary to add the alloying element of Mg into the zinc-based coatings to obtain a novel corrosion-resistant coating better than the current coatings.Both Zn and Mg are metals with lower melting points,in order to improve efficiency and save costs,vacuum evaporation technique has been adopted to prepare zinc-magnesium alloy coatings in this paper.The influence of the main process parameters（substrate temperature,vacuum degree and deposition rate）involved in the vacuum evaporation process was studied.The structure zone model（SZM）for the coatings deposited at different substrate temperatures by thermal resistance evaporation method was established.Properly increasing the substrate temperature was beneficial to obtain a coating with good crystallization and dense structure.The nucleation and growth of evaporated atoms after being deposited on the substrate were analyzed,and the preferred orientation of the c-axis of the evaporated atoms was determined.The alloy phases in the zinc-magnesium coatings were analyzed by simulation calculation,and the forming speed of Mg Zn₂ phase was the highest.Due to the poor adhesion of the direct vapor-deposited Zn Mg layer and the surface oxidation caused by high content of Mg,the zinc-magnesium alloy coating was designed as a multilayer structure of Zn/Zn Mg/Zn to achieve the best comprehensive performance.In order to better understand the structure and performance of each layer in the multilayer structure Zn/Zn Mg/Zn coatings,the microstructure,adhesion and corrosion mechanism of the deposited pure Zn coating,Zn Mg coating,Zn/Zn Mg coating and Zn/Zn Mg/Zn coating were studied respectively.Under the same conditions,the adhesion of the Zn/Zn Mg coating and Zn/Zn Mg/Zn coating was significantly better than that of the single-layer Zn Mg coating.The surface layer of Zn can effectively reduce the oxidation.After comparative analysis,when the substrate temperature was 200℃,the corrosion current of the deposited zinc-magnesium alloy coating was smaller.The corrosion products of the prepared zinc-magnesium coatings were mainly composed of dense Zn₅（OH）₈Cl₂·H₂O,which is more conducive to the improvement of corrosion resistance of coatings.And the corrosion product of Zn O only appeared in the pure Zn coatings.