Study On Fluxes Of Hot-dip Galfan Alloy And Its Interfacial Reaction

Hot dip Galfan alloy coatings are widely used in the field of metal surface protection for its good uniformity and corrosion resistance.However,the high Al content has strict requirements for the steel surface before immersion plating,which restricts the application of Galfan alloy coatings in solvent-based hot dipping process.The fluxes have a direct effect on the surface state of steel substrate,thus affecting the hot dip coating interface reaction and coating quality.It is of great significance to study the influence of fluxes on the process of hot-dip Galfan alloy for improving its coating quality and realizing solvent production.In this paper,five kinds of fluxes were screened out,of which the film-forming state on steel was analyzed.Effects of fluxes on the reaction wetting between molten alloy and steel substrate were studied by improved titration.In the process of hot-dip Galfan alloy,effects of fluxes on the coating microstructure and interfacial reaction were obtained by the short-time immersion plating method.Results show that there is no obvious oxide on the surface of steel with pretreatment of fluxes,which avoids the secondary oxidation.The steel surfaces treated by five fluxes are smoother than that of unplated ones,and the roughness can be reduced from 0.7?m to 0.1?m.The coating quality increases with the decrease of the surface roughness.The wetting processes of molten Galfan alloy and steel substrate are divided into the initial contact stage,the inoculation stage and the interfacial reaction stage.Futhermore,the initial contact angle and inoculation stage are affected by the type of flux films on steel substrate.Reactions occur when the plated steel substrate is in contact with molten alloy,and ZnO and Al₂O₃ on the alloy surface are reduced by fluxes.The flux with the best plating effect makes the initial contact angle between bath and steel matrix is reduced from122.3°to 99.7°,and the incubation period is shortened from 332 s to 220 s.The interface layer of hot-dip Galfan alloy is composed of irregular blocky Fe₂Al₅ and a small amount of lamellar FeAl₃,of which the grain size is close to 1?m.The FeAl₃ phase is preferentially formed on steel surface by the interfacial reaction of Galfan alloy.As the Al content decreases,Fe₂Al₅ phase is formed at the interface between FeAl₃ and the steel substrate.Finally,FeAl₃ phase is distributed far away from steel substrate,and Fe₂Al₅ phase is close to steel surface.With the improvement of plating effect,the thickness of Zn-Al alloy layer increases,but the thickness of Fe-Al interface layer has no obvious change.In hot dipping process,if the flux is easy to decompose,it will float into the zinc ash or evaporate.Conversely,it will remain in molten alloy or steel surface,thus forming small holes or impurities during the interface reaction between steel substrate and molten alloy.