Preparation Of Corrosion Product LDH Of Zn-Al Alloy For Hot-dip Galvanizing And Its Anti-corrosion Mechanism

Steel materials are one of the most widely used materials in modern industry.However,steel materials are easily corroded in the use environment(such as ocean,soil,atmosphere,etc.),and the global annual steel corrosion loss amounts to hundreds of billions of dollars.Hot-dip galvanizing is most used technology for steel corrosion protection:the galvanized coating has the advantages of metal coating and sacrificial anode protection at the same time.Zinc-aluminum alloy is one of the most widely used zinc-based coatings due to its excellent corrosion resistance and low cost.According to the theory and model of corrosion mechanism,it is considered that its superior corrosion performance is due to the protective film formed by corrosion products.In order to effectively improve the corrosion resistance and service time of galvanized coatings,it is necessary to systematically explore the phase composition and properties of the corrosion products of zinc coating.At present,many studies believe that the excellent protective performance of Zn Al coating corrosion product film is related to the content and morphology of the layered double hydroxides(Zn₆Al₂(OH)₁₆CO₃,LDH)and simonkolleite(Zn₅(OH)₈Cl₂,ZHC)deposited on the surface during the corrosion process.However,in most investigations on the corrosion behavior of zinc alloy coatings,the phase composition information of the corrosion products is only used as supplementary data for analysis,and there is still a lack of precise explanations for the protection mechanism of LDH and ZHC in the corrosion products on zinc alloy corrosion.Therefore,this work focuses on ZHC and LDH as the research objectives,and prepared on the surface of zinc alloy.The effects of LDH and ZHC on the corrosion behavior of zinc alloys are comparatively analyzed;then the growth and formation behavior of LDH on the surface of zinc aluminum alloys and its influence mechanism on corrosion protection are discussed.The main conclusions are as follows:(1)LDH and ZHC on pure Zn substrate have been prepared by in-situ growth method.It was observed that the morphology of LDH nanosheets is denser than that of ZHC,and the bonding ability of LDH with zinc substrate was better than that of ZHC.The electrochemical analysis showed that both of LDH and ZHC could inhbit the anodic dissolution of Zn substrate and improve the protective property of corrosion product film,thus effectively hindering the corrosion process,and the effect of LDH was more obvious.(2)The growth behavior of LDH on the surface of Zn-5Al and Zn-55Al alloys were investigated.It was found that aluminum content had an important effect on the growth morphology of LDH.The LDHs nanosheets on the Zn/Al eutectoid areas were denser than that on the Zn/Al eutectic areas of Zn-5Al alloy,and nanosheets on the Al-rich dendritic areas were denser than that of Zn-rich interdendritic areas of Zn-55Al.Meanwhile,the existence of Al is necessary to grow the LDH,Al₂O₃ on the Al-rich areas will transform into Al(OH)₃,and LDH nucleation forms on the Al(OH)₃.Therefore,the higher the Al content,the denser the morphology and smaller the size of LDH nanosheets.(3)By comparing the corrosion behavior of Zn-5Al and LDH/Zn-5Al,the influence of LDH on the corrosion resistance of Zn-5Al was discussed,and the morphology,composition and performance of the corrosion products of zinc alloys were analyzed as a function of immersion time.Tafel analysis shows that the LDH film can effectively suppress the cathodic oxygen reduction reaction and improve the corrosion potential of the Zn-5Al alloy.The LDH film can enhance the protection performance of the corrosion product film against the corrosion of Zn-5Al substrate.Compared with the Zn-5Al alloys,the as-prepared LDH nanosheets of LDHs/Zn-5Al serve as the skeleton of the corrosion product film,and the hydrozincite(Zn₅(CO₃)₂(OH)₆,HZ)deposit in gaps of the nanosheets,which will reduce the generation of cracks in the corrosion product film,and enhance the isolation and protection performance of the corrosion product film.(4)By comparing the corrosion behavior of Zn-55Al and LDH/Zn-55Al,the influence of LDH on the corrosion resistance of Zn-55Al was discussed.During the immersion process of Zn-55Al alloy,the Zinc-rich interdendritic areas were preferentially corroded to form corrosion products of ZHC,HZ and a small amount of Zn O.As corrosion progresses,a large amount of LDH was deposited on the surface of Zn-55Al.The preparation of Zn Al-LDHs-CO₃reduced the deposition of amorphous products(HZ),making the products mainly consist of ZHC and LDH,thereby forming a tight corrosion product film on the alloy surface and increasing the shielding effect of the corrosion products film.The electrochemical analysis showed that the LDH film can effectively suppress the cathodic oxygen reduction reaction and improve the corrosion product film resistance,so that it can effectively restrain the corrosion of Zn-55Al alloy substrate.