Effect Of Nickel On Microstructure And Properties Of Hot-dip Galvanized Coating On High Strength Dual Phase Steel

As a typical representative of Advanced High-Strength steel,Dual-Phase steel is widely used in modern industrial fields due to its excellent mechanical properties,such as safety parts and structural parts on automobile bodies.Complex service environmental factors,such as Silt,oil and pollutants in the air,can easily cause corrosion damages on the steel body.As a result,Hot-dip galvanizing is an effective means of protection.However,the microstructure and mechanical properties of the resulting coating and the substrate are different,and it is easy to cause defects such as cracking and falling off during processing.Due to the addition of many alloying elements,especially Si,to the dual-phase steel,its existence will cause silicon reactivity during the immersion plating process,causing problems such as ultra-thick coating,dullness and poor adhesion,which will make it resistant to corrosion.Performance and processing performance deteriorated.For this reason,referring to the solution to similar problems in hot-dip galvanizing such as killed steel,consider adding alloying elements to the zinc bath to suppress silicon reactivity and improve the quality of the coating.In this paper,two dual-phase steels with different silicon contents are used as galvanized substrates,and Ni is used as an additive element.The phase composition and microstructure of Zn-x Ni(x=0.00%,0.03%,0.06%,0.10%,0.12%)alloy coatings are studied by means of metallurgical microscope,SEM,EDS,etc.The growth kinetics of each coating is studied by changing the immersion time.In addition,the corrosion resistance and wear resistance were tested and compared with the immersion corrosion,electrochemical corrosion and friction test machines.Studies have shown that the addition of nickel has a significant inhibitory effect on silicon reactivity for dual-phase steel with a silicon content of 0.17%.With the increase of Ni content,the thickness of the coating increases first and then decreases.Among them,adding 0.10%of Ni in the zinc bath is the most suitable.At this time,the thickness of the coating is the thinnest and the structure is the most ideal—the?phase has good brightness,the?phase has no over-thickness phenomenon and is in a regular columnar distribution,and the proportion of the?phase increases.Moreover,the cracks in the?phase are reduced and there is no instability.In addition,there are Ni-rich regions and Zn-Fe-Ni ternary compounds in the middle of the coating,which are the reasons for the inhibition phenomenon.In contrast,it has no obvious effect on high-silicon dual-phase steel.A comparative analysis of the corrosion morphology and corrosion rate,electrochemical polarization curve and impedance spectra of the coatings in 5%Na Cl solution revealed that the thickness of the coating and the density of the structure have a greater impact on the corrosion resistance.The thicker the coating,the denser the structure The better the corrosion resistance.When the thickness of the coating reaches a certain level,the addition of nickel shows a significant optimization effect on its corrosion resistance.The higher the Ni content,the better the corrosion resistance of the coating.Using X-ray diffractometer to analysis the corrosion products of different coatings,it was found that both of them are the same.Corrosion products mainly are consist of ZnO,ZnCO₃,FeCO₃,FeO(OH),ZnCl₂·4Zn(OH)₂ and a small amount of Fe₃O₄,Fe₂O₃,in which Zn O and Zn Cl₂·4Zn(OH)₂ form a passivation film on the coating surface to slow down the corrosion process.The comparison of the surface hardness and roughness of the coatings with different nickel content shows that the hardness of the Zn-0.10Ni coating is the highest,and the surface of the Zn-0.12Ni alloy coating is the smoothest.With the increase of nickel content,due to the comprehensive influence of many factors such as the surface hardness and roughness of the coating,the?also increases first and then decreases,and the friction coefficient is the smallest when the nickel content is0.12%.And when the nickel content in the zinc bath increases from 0.03%to 0.12%,the wear scar width,height,and volumetric wear rate of the coating decrease by27.81%,14.04%,and 41.33%,respectively,indicating that the higher the nickel content of the coating,the more resistant it is.The better the grinding performance.