Study On Corrosion Resistance Of Nickel And Nickel Alloy Tube With Different Welding Process Conditions

Owing to nickel and its alloys having high strength,plasticity,porgability and mechanical properties in hot state,so welding pipe of nickel and its alloy are widely used in making sodium hydroxide,chlor-alkali industry,food processing industy,water trement and various equipments of high temperature halogen and salt corrosion environment.The study of corrosion resistance of pure nickel stright pipe joints in different corrosive environments is of great theoretical and practical significance for expanding their application.The joint is the weakest area of the whole nickel pipe,and the corrosion resistance of the joint plays an important role in the practical application of the nickel tube in the harsh corrosive environment.In this paper,the contrastive studies of the corrosion resistance of joints obtained by plasma arc welding(PAW)and plasma arc welding+tungsten inert gas arc weldng(PAW+TIG)in two corrosion environments were conducted.One was carried out in high temperature molten salt at 900 ?.The hot corrosion behavior and heat-corrosion resistance of two kinds of joints and base metal in two kinds of molten salts of 75%Na2SO4-25%NaCl and 75%Na2SO4-25%K2SO4 were compared and analyzed,and the mechanism of hot corrosion was further discussed.The other was performed by electrochemical measurement technology at room temperature.The corrosion behavior and characteristics of two kinds of joints and base metal with different soaking times in Cl-solution of different pH values were disscessed,and the chemical composition and electronic structure information of passive film formed in different corrosion solutions were characterized by X-ray photoelectron spectroscope(XPS).The main conclusions are as follows:(1)The microstructure observation of samples shows that base metal consist of homogenous and small austenite with an average grain size of 47.32?m and the PAW joint is columnar crystal structure with an average grain size of 463.57?m.The weld metal is reheated and recrystallizatized due to the arrangement of the TIG arc reheating.So the coarse columnar srystal structure with an average grain size of 547.32?m in the PAW+TIG joint.(2)For the corrosion specimens in mixed 75%Na2SO4-25%K2SO4 molten salt,experimental results show that the grain sizes and thermal corrosion resistance of the welds obtained by the two welding methods have little difference,which are lower than that of base metal.The hot corrosion kinetics of the specimens follow a linear power laws and the corrosion layers on the surface of base metal and joints are detected to be mainly composed of NiO and Ni3S2.Cross-sectional morphologies and corresponding elemental maps indicate that corrosion layers scale/substrate interface are composed of duplex oxides and sulfides.According to these results,a cooperating mechanism of oxidation and sulfides for hot corrosion of pure nickel is confirmed.That is,the distribution of internal oxides and sulfides,located inside the inner surface oxide film,extends to the substrate of the sulfidation-oxidation cycle accompanying with electrochemical corrosion mechanism and sulfur plays an important role in the process of hot corrosion.For the corrosion specimens in mixed 75%Na2SO4-25%NaCl molten salt,experimental results show that there is no incubation period with no selectivity and order.The corrosion initiation mainly occurs along the edges with appearing more holes,cracks and spalling and corrosion products are dominated by NiO and Ni3S2.The corrosion kinetics of specimens follow a parabolic power laws with corrosion resistance of three specimens being in order:base metal>PAW+TIG joint>PAW joint.Hot corrosion process includes the cycle enhancement of internal sulfidation-oxidation and chlorination-oxidation,in which chlorine and sulfur play an important role in the process of hot corrosion.(3)Soaked in the 1mol NaCl solution(pH=7.2),the surface of three specimens occurs activation-passivation-repassivation transformation and the passive film consists of NiO and Ni(OH)2.The eleltrochemical test results shows that electrochemical corrosion behaviors of all specimens keep the consistent change trend with the increase of grain size and the prolongation of immersion corrosion time and corrosion resistance of three specimens is in order:base metal>PAW joint>PAW+TIG joint.The local pitting of the passive film on the samples is due to the autocatalytic cycle enhancement of diffusion-ad sorption-transmission-dissolution of the erosive Cl-ions.Soaked in the lmol NaCl+H2SO4 solution(pH=0.13),the surface of three specimens occurs activation-passivation transformation and the passive film consists of NiO and Ni2O3.The eleltrochemical test results shows that electrochemical corrosion behaviors of all specimens keep the consistent change trend with the increase of grain size and the prolongation of immersion corrosion time and corrosion resistance of three specimens is in order:base metal>PAW joint>PAW+TIG joint.The localized corrosion along the grain boundary of the passive film on the samples is due to the further promotion of the addition H+ for autocatalytic cycle effect of diffusion-ad sorption-transmission-dissolution of the erosive Cl-ions.For pure nickel joint,the grain boundary size is negative effect in the corrosion media containing H+,that is,the larger the grain size,the better the corrosion resistance.Soaked in the lmol NaCl+NaOH solution(pH=13.8),the surface of three specimens occurs activation-passivation-repassivation transformation and the passive film consists of and Ni(OH)2.The eleltrochemical test results shows that electrochemical corrosion behaviors of all specimens keep the consistent change trend with the increase of grain size and the prolongation of immersion corrosion time and corrosion resistance of three specimens is in order:base metal>PAW joint>PAW+TIG joint.In the alkaline corrosion medium containing OH-,the addition of OH-greatly reduces the autocatalytic cycle effect of Cl-,which makes the formation rate of the passivation film on the surface of the surface is greater than the dissolution rate.And the densification and stability of the passive film are high with slight surface corrosion morphology.The grain boundary size is positive effect in the corrosion media containing OH-,that is,the larger the grain size,the worse the corrosion resistance.In the Cl-ions solution,the autocatalytic effect of Cl-ions of diffusion-ad sorption-transmission-dissolution process on the corrosion reaction of the matrix is different with different pH values,and the corrosion characteristics are different.