A Study On Microstructure And Corrosion Performance Of Pure Nickel N6 Welded Joints

Pure nickel N6 performs better than other metals in the high concentration of sodium hydroxide corrosion resistant experiment, so that it is widely used in evaporators’ producing process. However, poor heat conductivity, high melting point, exposed to high temperature for too long makes nickel welded joint to be the weak link of corrosion failure.This paper preliminarily explores pure nickel N6 corrosion resistant performance of welded joints under different heat input with filler wire and non-filler wire, and analyzes the corrosion resistance of welded joints and base metal comparatively. Tungsten argon-arc welding(TIG) method is adopted in this experiment. Weightlessness corrosion tests and electrochemical experiments were performed in a simulated industrial caustic soda evaporator discharge solution. OM, SEM, EDS and other test devices are used to obeseve and analyze the microstructures of pure nickel N6 welded joints and the surface morphology of the weight loss corroded specimen.In this paper, main work and achievements were as follows:(1)The microstructure analysis shows that base and weld metal substrate is austenite. Weld organization is bulky columnar dendrite. Heat affected zone is coarse isometric crystal grain and the base metal is fine. With the increase of welding current, welding joint of the grain size is bigger.(2)We use weight-loss method to calculate the corrosion depth of welding joint under different welding process. With the increase of welding heat input, welding joint of the corrosion depth is increased.(3)Specimens after immersion corrosion test on its surface observed pitting phenomenon exists.(4)Using electrochemical method, the electrochemical characters of pure nickel N6 and welding joints in the caustic soda evaporator are studied. Welded joint and the base is the phenomenon of passivation. With the increase of welding heat input from the corrosion current density increased gradually. In the same welding current, Corrosion current density is lower than the filler wire connector wire connectors do not fill.