Microstructure And Corrosion Resistance Of Yttrium Microalloyed B10 Cuporonickel Alloy

B10 copper-nickel alloy acts as one of the most important industrial alloys in marine engineering due to its excellent corrosion resistance to seawater.However,corrosion leakage accidents occur frequently in copper-nickel alloys in the process of service owing to the complex marine environment.The corrosion resistance of copper-nickel alloys in seawater mainly depends on the microstructure of the alloy and the properties of the corrosion products attached to the alloy surface.In recent years,rare earth microalloying is one of the effective methods to optimize the corrosion resistance of Cu-Ni alloys.In this paper,the active role of heavy rare earth Yttrium（Y）in nonferrous metals is utilized to add trace or a small amount of Y to B10 alloy,and the appropriate deformation and heat treatment process is combined to improve the alloy microstructure and optimize the grain boundary structure.In addition,the influence of Y microalloying on the corrosion resistance of B10 alloy is systematically analyzed by means of electrochemical technology,and the mechanism of corrosion resistance is discussed from two aspects:the optimization of alloy microstructure and the improvement of corrosion product film property.The results show that:（1）The as-cast microstructure of B10 alloy is typical dendrites.Rare earth Y can effectively refine the secondary dendrite spacing of the as-cast microstructure and with the increase of Y content,dendrites gradually appeared preferential growth.The grain size of homogenized annealing B10 alloy first increases with the increase of Y content and then decreases slightly.When the Y content is 0.021%,the grain size is the largest and the microstructure is the most uniform.The final annealing microstructure of the B10 alloy is a complete recrystallization microstructure containing a large number of annealing twins.When the Y content is 0.021%,the special grain boundary ratio is the highest,large-size grain clusters are formed obviously and the grain boundary structure is optimized significantly.It is considered that the rare earth inclusion and Ni₂Y phase formed in the alloy hinder dislocation movement,induce the formation of annealing twins,and improve the special grain boundary ratio.Moreover,the rare earth solid solution atoms make the increase of dislocation slip resistance and the probability of stacking fault,which is conducive to the occurrence of twin reactions and the formation of large-size grain clusters,so as to optimize the grain boundary structure.（2）Yttrium microalloying has a significant effect on the corrosion resistance of as-cast and final annealed B10 alloy.After immersion in 3.5%NaCl solution for 30 days,the corrosion resistance of B10 alloy increase first and then decrease with the increase of Y content.When the Y content is 0.021%,the corrosion resistance of the alloy is the best and the total resistance of the film is the largest.The corrosion product film formed on the surface of the alloy is a double-layer structure,the outer layer is loose and unprotected Cu₂（OH）₃Cl,the inner layer is Cu₂O with high density and strong binding force with the matrix.When the content of rare earth Y is 0.021%,the corrosion product film has the highest density,the best stability,the solid binding with the matrix,showing the best corrosion resistance.（3）The mechanism of the effect of Y microalloying on the corrosion resistance of B10alloy is mainly reflected in two aspects.One is that rare earth Y improves the microstructure uniformity of the alloy,optimizes the grain boundary structure of the alloy,and inhibits the intergranular corrosion,so as to improve the corrosion resistance of the alloy;the other is that rare earth Y improves the density and stability of the film and reduces the conductivity of ions and electrons in the film by promoting the diffusion of Ni in the film.In addition,rare earth Y effectively prevents the corrosion product film structure from being damaged,enhances the bonding force between the film layer and the matrix and protects the alloy matrix from being eroded by Cl^-by inhibiting the diffusion of Cl^-in the corrosion product film,and thus improves the corrosion resistance of the alloy.