Study On AlCuRE Alloy Grounding Material And Its Corrosion Behavior In Alkaline Soil

Grounding device is a kind of device that connects the conductors buried in soil with related facilities.It was adopted to ensure the safety of personal and equipment by introducing extra harmful current generated by electrical equipment or other related devices into the earth.Good electrical conductivity,low contact resistance with the soil,and long-term soil corrosion resistance of grounding materials was required during the whole service life,such as the commonly used galvanized steel and pure copper.Pure copper grounding grids exhibit excellent grounding performance and long service life due to their high conductivity and outstanding corrosion resistance.But the major concern of high costs and heavy metal pollution to soil has limited its wide application.Galvanized steel showed problems of poor anticorrosion ability and shorter service lifetime when used as grounding material despite of its low cost.Although pure aluminum showed high electrical conductivity and soil corrosion resistance,it is considered not suitable to use as grounding material due to its poor electrical conductivity resulted from oxide.Therefore,the key issue to realize the use of aluminum as grounding material is to improve the characteristics of surface corrosion products,which can ensure the long-term corrosion resistance as well as good grounding conductivity.A novel AlCuRE alloy was developed as grounding materials with aluminum matrix,small amount of copper(4-5wt%)and trace amount of rare earth elements(0.3wt%)by semi-continuous casting and hot extrusion processes.The existence of copper converted the aluminum oxide film into aluminum-copper alloy oxide,which solved the poor conductivity of pure aluminum.The addition of trace rare earth elements improves the solid solubility of copper in alloy,refines the second phase particles and reduces Al2Cu phase segregation.By combining shot peening and oxidation processes,the thickness and compactness of the oxide film increases and the diffusion of copper element to surface layer promotes,increasing the relative content of copper in surface oxide layer.Thus,AlCuRE alloy meets the requirements of grounding materials with high soil corrosion resistance and excellent grounding conductivity.The corrosion behavior of AlCuRE alloy and its welded joint were investigated with accelerated corrosion test,immersion test and electrochemical test in alkaline soil and in alkaline soil extract.Scan electron microscope(SEM)was applied to characterize the surface morphology before and after corrosion test.Optical microscope(OM)was employed to research the crystal structure.X-ray diffraction(XRD)and energy dispersive spectrometer(EDS)were used to analyze the components of corrosion products.Scanning kelvin probe(SKP)was applied to study the electrochemical properties.The effects of surface treatment on microstructure,morphology,composition,stress state,soil corrosion behavior and grounding conductivity of alloy were studied.The grounding and soil corrosion resistance of AlCuRE alloy with shot peening and oxidation were analyzed.The main conclusions are as follows:The AlCuRE alloy showed excellent corrosion resistance to alkaline soil and its extract at the initial stage of experiment,and changed-to steady pitting from metastable pitting at the later stage.When the chloride ion diffuses along the grain boundary,intergranular corrosion occurs in the matrix and finally exfoliation corrosion occurs.The main corrosion forms of AlCuRE alloy in alkaline soil are pitting,intergranular corrosion and local exfoliation corrosion with major products of aluminum hydrated oxide,aluminum copper oxide,aluminum sulfides and chlorides.The corrosion control step of AlCuRE alloy in alkaline soil is cathodic control,and the resistance of ions passing through the passive film is the controlling factor of cathodic reaction.In order to solve the problems of practical application with AlCuRE alloy as grounding material,MIG welding process was used to realize the good connection of materials.The weld zone(WZ)of AlCuRE joint showed obvious cast microstructure with equiaxed crystal in central zone and columnar crystal near fusion line.Some eutectic phases are precipitated at grain boundary in weld zone.The heat affect zone(HAZ)and the base metal(BM)suggested a typical microstructure of hot rolling microstructure.The surface potential of joint is different from matrix due to its composition and microstructure,which causes macro galvanic corrosion.The corrosion behavior of welded joint in alkaline soil extract is mainly pitting corrosion and exfoliation corrosion.The corrosion resistance of fusion line(FL)is the worst.Pitting mainly occurs in columnar crystal zone near the FL with aluminum oxides and sulfides as corrosion products.At the same time,the residual tensile stress in the FL also accelerates the corrosion of welded joint.The coarse particles formed by heterogeneous phase segregation in alloy will become the preferred region of pitting nucleation due to its effect on the integrity of oxide film.On one hand,the surface potential difference caused by material composition is the reason to accelerate corrosion by forming micro galvanic and macro galvanic corrosion.On the other hand,the oxide film is easily broken under tensile stress,and becoming a weak area in corrosion environment.The chloride ions can react with the matrix through being adsorbed on the surface of oxide film and diffused into the matrix from the weak area,resulting in the dissolution of matrix and the formation of metastable pitting.When chloride concentration increased to a certain value,metastable pitting will develop into steady pitting.Shot peening on the surface of AlCuRE alloy increases the defect density of the surface layer,refines the grain and the second phase particles of the surface layer,and change the surface layer of the alloy from original non-uniform stress to compressive stress.Due to the input of deformation energy,the initial oxidation reaction rate and the pitting tendency of the alloy were increased.The high density defects formed by shot peening provide a short-circuit diffusion channel for copper from matrix to surface layer,improving the relative copper content of the surface corrosion products.The dense and conductive aluminum copper oxide film was obtained by high temperature oxidation treatment.The fine-grained and high-density defects provide nucleation site and growth power for the formation of oxides,improve the thickness and compactness of oxide film on the alloy surface,reduce the corrosion current density of AlCuRE alloy in alkaline soil extract,improve the AC impedance modulus,inhibit pitting corrosion,and improve the corrosion resistance of AlCuRE alloy in alkaline soil.