Investigation On Phase Equilibria And Interfacial Reactions Of The Relevant Alloy Systems In Hot Dip Galvanizing

Hot dip galvanizing(HDG)is an economic and effective anti-corrosion technology for steel.HDG has the advantages of simple process,good anti-corrosion performance and wide usage,but still suffers from two major problems:1)the silicon addition increases the thickness and brittleness of the coating,leading to an inferior adhesion.Hence,several alloying elements(Al,Co,Zr,Cu,Ni,Ti,etc.)are added into the zinc bath to control the harmful influence of Si,P in steel,but its mechanism is not clear;2)the corrosion resistance in the Zn-Al melt is unsatisfied.Co-base super alloy has excellent performance of corrosion resistant in Zn-Al melt.However,the interfacial reaction between Zn-Al and Co needs a further research.In this work,the interfacial reactions and phase equilibria of the relevant quaternary alloy systems Zn-Fe-Al-Zr,Zn-Fe-Al-Sn,Zn-Fe-Co-Cu and Zn-Co-Al-Fe in the hot dip galvanizing are researched.The equilibrated alloy,diffusion couple and phase diagram calculation method are combined to study the Al-Zn-Zr ternary system.The 600? and 450? isothermal sections of the Zn-Co-Cu ternary system,the 320?,700? and 1000? isothermal sections of the Al-Fe-Sn ternary system and the 450? isothermal section of the Zn-Co-Al-Fe(93.0 at.%Zn)quaternary system are experimentally researched by equilibrated samples.The interfacial reaction of Zn-Al/Co and the corrosion resistant mechanism of the Co-base super alloy in A1 melt are investigated.Thirteen three-phase regions have been experimentally determined in the 800?isothermal section of the Al-Zn-Zr system.The ternary compound T was confirmed to be stable at 800?,which has a large composition range of 16.84?55.1 at.%Zn and 18.02?56.3 at.%Al.The ternary compound can be in equilibrium with all binary compounds.The maximum solubility of Zn in Zr3Al,Zr2Al,Zr3Al2,Zr4Al3,ZrAl,Zr2Al3,ZrAl2 and ZrAl3 is 7.5,0.84,0.33,0.89,0.91,1.12,0.64 and 3.8 at.%,respectively.The maximum solubility of Al in Zr3Zr,Zn2Zr and ZnZr is 1.6,1.3 and 13.6 at.%,respectively.The Al-Zn-Zr ternary system is evaluated by CALPHAD and the results agree well with the experimental results.The 600? and 450 ? isothermal sections of the Zn-Co-Cu ternary system have 5 and 6 three-phase regions,respectively,whereas no ternary compound is found.The ?-Co5Zn21 and ?-Cu5Zng phases form a continuous solid solution with the cell type of cubic I4 3m(217).The solubility of Co in ? and 8 is 8.8 and 1.8 at.%respectively,and the maximum solubility of Cu in ?1 and ?1 is 7.9 and 14.7 at.%at 600? respectively.At 450?,the maximum solubility of Co in ?'and s is 6.7 and 8.5 at.%respectively,and the solubility of Cu in ?1,?1 and ?2 is up to 11.2,14.4 and 12.9 at.%respectively.The 320,700 and 1000? isothermal sections of the Al-Fe-Sn ternary system have 6,5 and 3 three-phase regions,respectively,whereas no ternary compound is found.The liquid L phase can be in equilibrium with four kinds of Fe-Al intermetallic compounds.The maximum solubility of Al and Fe in L-Sn at 320?reaches 5.1 and 0.6 at.%,respectively.The 450? isothermal section of the Zn-Co-Al-Fe(93.0 at.%Zn)quaternary system has been experimentally determined.The ?-FeZn13 and CoZn13,CoAl and FeAl can form a continuous solid solution phase,but Co4Al13 and FeAl3(or Fe4Al13)cannot.FeAl3 phase can co-exist with CoAl,Co2Al5,Co4Al13,Fe2Al5 or L-Zn;The CoAl phase can co-exist with Co2Al5,T,?,?,Fe2Al5,FeAl3 or L-Zn.The maximum solubility of Fe in CoAl is 27.1 at.%;the maximum solubility of Zn in FeAl3,Fe2Al5,Co4Al13 and Co2Al5 is 13.4,14.6,9.7 and 16.0 at.%,respectively.The interfacial reaction of Zn-Al/Co has been investigated.The addition of Al is able to control the interfacial reaction of Zn/Co.The Gibbs free energy of CoAl phase is much less than those of Co-Zn compounds,so the Co atoms will diffuse through the ?2 layer into the Al-rich area,and nucleate to form stable CoAl layer in the area,inhibiting the Co-Zn reaction significantly.The effects of different Mo,Cr and Si content on the microstructure and corrosion resistance in liquid Al bath at 700? of CoMoCrSi alloys have been investigated.The higher content of Mo and Si increases,the higher Laves phase ratio.The corrosion resistance of CoMo28.5Cr16.7Six alloy increases with the increase of Si content,the corrosion products are mainly(Co,Mo,Cr)2A19 and(Cr,Mo)Al5.