Corrosion Resistance Of Niobium Carbide Coating On 5CrNiMo By TD In Molten Aluminium Alloy

In this paper,niobium carbide coatings were prepared on the surface of 5CrNiMo steel by thermal diffusion?TD?technology.The 5CrNiMo steel substrate and the niobium carbide coating were subjected to hot dip aluminum experiments.The weight loss analysis was used to evaluate the advantages and disadvantages of the corrosion resistance of the steel substrate and niobium carbide coating in molten aluminum alloy.The microstructure and phase structure of steel substrate and niobium carbide coating after the immersion test were observed by SEM,EDS and XRD,and the 5CrNiMo steel substrate and niobium carbide coating were analyzed from the perspective of thermodynamics and dynamics.The main conclusions are as follows:?1?The cross-sectional structure of 5CrNiMo steel after the immersion test is divided into three layers:surface layer,diffusion layer and substrate.The surface layer is aluminium alloy,the diffusion layer is Fe-Al-Si ternary compound layer,and the substrate is 5CrNiMo steel.There is a transition layer at the interface between the diffusion layer and the substrate after 750°C immersion test for 2 h.The transition layer is divided into two continuous layers.The layer near the substrate is Fe₂Al₅,and the layer near the diffusion layer is?5?Fe19Si10Al71?.?2?The weight loss per unit area of 5CrNiMo steel increases slowly at the initial stage of 700°C immersion test,and then the melting loss increases rapidly,while the weight loss per unit area increases approximately linearly for 750°C and 800°C immersion test.As time goes on,the effect of temperature on the weight loss is increasing.?3?The thickness of the diffusion layer formed on the surface of 5CrNiMo steel in molten aluminium alloy increases with the increase of temperature and time,which conforms to the Arrhenius relationship.?4?The corrosion rate of steel substrate in molten aluminium alloy varies with corrosion time,which can be roughly divided into two stages:reaction layer formation and reaction layer advancement.?5?Single phase NbC coating prepared by TD at 900°C for 6 h was observed on5CrNiMo steel.The coating is uniform and dense with a thickness of about 8?m.The surface hardness of the niobium carbide coating is remarkably improved,and the hardness value is about 2000 HV_0.05.The niobium carbide coating is an ultra-high hardness coating.?6?Niobium carbide coating effectively separates molten aluminium alloy from steel substrate,prevents the corrosion of molten aluminium to steel substrate,and remarkably improves the resistance of steel substrate to molten aluminium alloy.The failure mode of niobium carbide coating in molten aluminium alloy is mainly due to the reaction between the coating and molten aluminium alloy resulting in the spalling of the coating.Al₄C₃,Al₂O₃and Al?NbO₄?compounds are mainly formed on the coating surface after the immersion test.