Study On The Alloy Phase Growth Of Hot-dip Coatings Under Different Zinc Bath Characteristics

The hot dipping process as the one of the most extensive corrosion process was studied by Analysis of micro structure and anticorrosion properties of coating to determine the parameters of the zinc bath composition,the immersion temperature and the immersion time,instead,the state of zinc bath,which has an important influence on the coating as the source of the process,is ignored.Based on the characteristics of zinc bath,this paper analyzes the influence of zinc bath surface tension,zinc bath fluidity,A1 and RE of zinc bath on the coating,and provides a new thinking route and basic theory for the design of hot dip galvanizing and the pre control of coating quality.In this paper the dipping temperature and the dipping time series was set,the workpiece respectively was dipped plating in Zn,Zn-0.05%Al,Zn-0.1%Al,Zn-0.2%A1,Zn-0.3%Al,Zn-0.5%Al,Zn-0.2%Al-0.2%RE,Zn-0.2%Al-0.3%RE based on previous measurement and analysis of characteristic parameters of the zinc bath by project group.The microstructure and phase of the deposited coating were observed and analyzed by scanning electron microscopy(SEM),energy spectrum analysis(EDS)and X ray diffraction analysis(XRD),the influence of zinc bath surface tension,zinc bath fluidity,A1 and RE of zinc bath on the coating was studied,obtained the following conclusions:The surface tension and fluidity of zinc bath have little influence on the surface quality of hot dipped zinc alloy and zinc aluminum alloy,and have great influence on the apparent quality of zinc aluminum alloy.The higher surface tension leads to the increase of the leakage rate of the hot dip galvanized zinc aluminum alloy.The better fluidity of zinc bath leads to the decrease of the surface adhesion of the hot dip coating.The decrease of the surface tension of zinc bath has a significant effect on the growth of the alloy phase in the hot dip galvanized coating at the early stage of the immersion plating,and it also promotes the growth of the alloy phase in the hot dip coating of Zn-Al alloy and the hot dip coating of the Zn-Al-RE alloy,but the promotion effect is relatively small.At the same time,the higher surface tension will lead to uneven growth of the microstructure of the hot dip galvanized alloy.The improvement of the fluidity of zinc bath can promote the thinning of the thickness of the ? layer,but the fluidity of the zinc bath is stronger and the thickness of the ? layer in the coating is more influenced by the operation of the plating process when the hot dipping of pure zinc and the hot dipping of Zn-0.lAl alloy above 460 C and the hot dipping of Zn-0.2A1-0.2RE alloy above the temperature of the plating bath are more than 450.Meanwhile,the improvement of zinc bath fluidity can refine the microstructure of Zn-Al alloy hot-dip coating and Zn-Al-RE alloy hot dip coating.The aluminum in the zinc bath leads that the growth of the alloy phase in the early stage of hot dip plating is mainly controlled by the diffusion speed of the zinc element form the interface reaction speed.The elements inhibit the growth of ? phase during the hot dip plating process and promote the formation of the ? phase.With the increase of the immersion temperature,the effect of the aluminum element on the growth of the alloy phase is weakened,the grain refinement is achieved.The content of aluminum affects the formation order of alloy phase in the coating.The alloy phase formed first by hot dipping of pure zinc and Zn-0.05A1 hot dip coating is ? phase.The order of the coating is ???+???+?+?,Zn-0.1Al hot dipping and Zn-0.2A1 hot dipping are first formed by 8 phase,and the sequence of the coating is???+???+?+?n.RE does not directly participate in various reactions in hot dip plating,but it has high activity in itself.It is biased and diffused in the coating tissue,that promotes the diffusion of the zinc liquid iron matrix,thus the incubation period of the immersion plating process is shortened,the growth rate of the alloy phase in the coating is accelerated,and the grain size of the alloy phase in the coating is refine.Among them,the influence of ? phase layer is particularly obvious.The increase of RE content makes the bonding between the ? phase layer and the 8 phase layer more closely.However,with the increase of immersion temperature,the effect of RE content on the microstructure of zinc bath is weakened.In the bath of zinc aluminum alloy,the order of alloy layer is RE:???+???p+?k+???p+?k+?+?.