Research On Protection Mechanism Of Zn And Zn-Al Liquid By Thermal Spraying Composite Material And Composite Structure Coating

Molten metal corrosion is one of the most common forms of materialfailure.Especially molten zinc?Zn?and aluminum?Al?are strongly corrosive,could react with most metals or alloys and form Fe-Zn alloy and Fe-Al alloy compounds,which constantlyconsumes Fe,resulting in the metal materialfailure.Hot-dipping technology is one of the effective methods for preventing metal corrosion.Under the hot-dip galvanizing and hot-dip aluminizing conditions,sinking roller and stabilizing roller in the zinc pot are corroded and worn by molten metal,which leads to shorten the service life and directly affects the product quality and work efficiency.The preparation of metal,ceramic or composite coating using thermal spraying method is a possible way to improve the service life of these components.There are also some application reports in China and abroad,while most of them are focus on the applications and methods,the attention on failure mechanism is relatively less.In this paper,the corrosive process of carbide ceramic and oxide ceramic coatings in Zinc liquid and Zn-Al solution were studied,and the structure and composition of the coating material were improved according to the experiment results,which hoped to improve the corrosive resistance of coatings.The results showed that the diffusion corrosion was the main failure form of WC ceramic material in zinc solution at 460?.Zn and bonding phase Co formed CoZn₁₃??2?hard brittle phase.The elastic modulus of this phase was far different from other component in the coating.So the internal stress in the coating was produced by thermal stress,which leads to fracture and failure of coatings.The failure form of Cr₃C₂-NiCr ceramic material was the existence of tiny holes in the coating surface which produced porous and loose NiZn₈???phases,they will make the ceramic phases peel off from the coating surface.The addition of Cr,Mo,W and other high melting point metals made the corrosion diffusion of Zn and Al elements slow,which improved the corrosive resistance of optimized WC ceramic materials.The results showed that Cr,Mo and W particles were spread on the surface of bonding phases,forming a“shielding impact”with ceramic phases,effectively blocking the infiltration of Zn and Al liquid,reducing the rate of bonding phase reaction for producing brittle compounds.The failure mechanism of plasma spraying Al₂O₃ and ZrO₂ ceramic coatings in Zn and Zn-Al solution was that a certain number of pores exist in ceramic coatings,the ceramic coating was brittleness and largely different from the thermal expansion coefficient of substrate.So with the increase of corrosive time,some finer cracks were formed firstly in the coating surface,and gradually expanded inside the coating,but the cracks were not perpendicular to the coating,since these cracks were not create the channel for Zn solution.As the crack continues to expanded,the coating appeared layered peeled off,especially the bond layer peeling off and the crack propagation,was the main reason leading to coating failure.Thus,a single Al₂O₃ and ZrO₂ coatings were hard to present good service life;it needs to improve the bonding strength between ceramic coating and bonding layer,and the toughness of ceramic phase.Sine the improved NiCoCrAlY/Al₂O₃ and Mo/Al₂O₃ gradient coatings reduced the change of thermal expansion coefficients and the stress concentration in the interface,which slowed down the crack propagation and prevented the cracks of the coatings,the anti-cracking property of the coating wasthen enhanced.The toughness of metal bonding layer was better,and the resistance for nucleation of micro-crack and expansion were larger.During the thermal shock processthe thermal stability of coating was obviously higher than that of single Al₂O₃coating,showed that the functional gradient coating can reduce the difference of the thermal expansion coefficient between coating and substrate,and improve the coating fatigue life.In the corrosive process of Zn and Zn-Al liquid,the failure type of NiCoCrAlY/Al₂O₃gradient coating was still peeling failure.The stress concentration was caused by the pores in the ceramic coating,which formed the crack source and expanded rapidly.The Zn solution entered into the coating through these pores and cracks,which weakened the adhesion of layers,resulting in coating failure.The main failure reason of Mo/Al₂O₃ gradient coating was the existence of pores and crack propagation.The oxidation of Mo occurred in the weak part of the interface,which reduced the cohesion and caused crack expanding and eventual failure.