Corrosion Failure Mechanism And Rapid Evaluation Technology Of Typical Industrial Product NdFeB Coated With Epoxy Coating

The electrochemical activities of different phases of NdFeB vary greatly,and the corrosion resistance of the alloy is poor.Therefore,epoxy organic coating is usually used for NdFeB surface protection in industry.In this paper,the corrosion resistance of NdFeB coated with epoxy coating was rapidly evaluated by copper ion solution immersion method;the failure data of NdFeB coated with epoxy coating under natural sun exposure were accumulated,and the failure rules in different simulated environments were compared to obtain the quantitative expression of the total corrosion strength of organic coating;finally,the initiation and expansion rules of local corrosion of NdFeB substrate were studied To explore the influence of environmental media on the corrosion morphology of NdFeB alloy.The main research contents and conclusions are as follows:(1)Accelerated failure method of organic coating immersed in copper ion solution:In this method,copper ions are introduced into NdFeB substrate and deposited to form cathode particles,which induce local galvanic corrosion and intensify local peeling between coating and metal substrate,so as to achieve the purpose of identifying coating defects earlier.The results show that the failure process of the coating accelerated by copper ion is the common result of the electrochemical reaction inside the coating bubble and the mechanical fracture of the coating.Compared with the neutral salt spray test,the evaluation effect of copper salt accelerated test on epoxy coating is 10 times of that of neutral salt spray test when the failure rate of the sample is 80%.(2)The failure behavior of epoxy coated NdFeB samples in coastal natural environment:The surface roughness of the epoxy coating increases and the gloss decreases after exposure to the sun.The coating impedance is close to the failure value of 105 Ω·cm2 after exposure for 450 days.After exposure for 700 days,the coating impedance is lower than 104 Ω·cm2,and it has completely failed.FTIR test showed that the content of C-C and C-O bonds on the surface of the coating decreased significantly after exposure to sunlight,and the coating was seriously aged.The atmospheric corrosion of the coating was correlated with Cu ion immersion test and neutral salt spray test,and the contrast relationship between different environments was established.(3)The mechanism of inducing and expanding local corrosion of sintered NdFeB alloy:Under the thin liquid film,the local corrosion of NdFeB tends to start at the junction of Nd rich phase and main phase,which may be related to the formation of protective oxide film on the surface of Nd rich phase.The results show that the whole corrosion morphology is spatially separated from the cathode and anode regions,which is due to the Fe2+ concentration near the anode region.The higher the concentration,the lower the pH,and the existence of Fe near the anode in the form of soluble salt.The influence of different environmental parameters on the morphology of the cathode region indicates that the cathodic reaction occurs in the annular region far from the anode region,which may be the result of the interaction of ion transport in solution and cathodic reduction reaction on the metal surface.Finally,combined with the potentialpH diagram of Fe-H2O system and the analysis of cathodic reaction kinetics,the mechanism of local galvanic corrosion initiation and propagation of NdFeB was further understood.