Oxidation Mediumon Formation and Properties of High Frequency Oxide Film of Zirconium Alloy
Zirconium alloys are considered as cladding materials for current and future nuclear reactors because of their excellent cor-rosion resistance,high temperature mechanical properties,neutron stability and good chemical compatibility with nuclear fuel.Howev-er,when zirconium alloy is used as cladding material,the inner wall cladding is usually eroded by fission products,and the outer wall is subjected to high temperature and high pressure water corrosion,fretting wear and foreign body erosion,which requires the forma-tion of a wear-resistant and anti-corrosion protective layer on the surface.Therefore,in this paper,zirconium alloy tubes were oxidized under the oxidizing conditions of 615 ℃ for 40 min in air,pure oxygen and atomized oxygen.The effects of three oxidizing media on the formation process,microstructure and properties of the oxide film were compared and studied.The microstructure of the oxide film was characterized by X-ray diffraction analysis(XRD),back scattering electron analysis(BSE)and transmission electron microscope analysis(TEM).The formation mechanism of the oxide film under atomized oxygen condition was analyzed.The hardness,wear resis-tance and corrosion resistance of the oxide film were evaluated by in-situ micro/nano mechanical test system and high temperature and high pressure water corrosion test,respectively.The results showed that:In the air,the pure oxygen and oxygen atomization under three kinds of medium,zirconium alloy tube surface could form a continuous uniform layer of oxide film,which under the condition of atomic oxygen could obtain the most thick layer of oxide film,and this was because the atomic oxygen containing oxygen possesses higher oxidation activity,zirconium and oxidizing reaction matrix faster,and oxygen concentration of oxygen atomization was the high-est,Therefore,a thicker oxide film could be formed in the same time.The XRD results showed that the oxide films formed under the three oxidation atmospheres are all composed of a large amount of monoclinic zirconia m-ZrO2 and a small part of tetragonal zirconia t-ZrO2.This was because zirconia exists mainly as a monoclinic phase at atmospheric pressure.However,due to the influence of the ma-trix during the formation process of zirconia,a large compressive stress was generated,which made part of the tetragonal phase stable.Therefore,part of t-ZrO2 also appeared in the diffraction peak of XRD.At the same time,the oxide film formed in atomic oxygen atmo-sphere was examined by TEM.The results showed that the outer oxide film was mainly monoclinic equiaxed grain with microcracks and other defects,which were caused by the volume expansion caused by the transformation of tetragonal phase to monoclinic phase,and some cracks and pores were produced.With the oxidation process,the thickness of the oxide film increases gradually,and the corre-sponding compressive stress in the film also increased gradually.The compressive stress resulted in the formation of a columnar grain with a preferred orientation.In this way,each columnar grain grew perpendicular to the oxide film/metal interface,and compressive stress could be uniformly applied to each columnar grain,thus keeping the oxide film compact without cracks and holes.Some tetrago-nal equiaxed grains could be stabilized at the oxide film/metal interface due to the maximum compressive stress.Moreover,the nano-hardness and wear resistance of the oxide film formed under the three kinds of media were also tested.The test results showed that the oxide film formed under the three kinds of atmosphere had a good improvement compared with the original sample.The average nano-hardness of the sample oxidized under the atomized oxygen,pure oxygen and air medium was 14.35,12.06 and 11.03 GPa,respective-ly.The hardness of the zirconium alloy tube before oxidation was about 2 GPa.It was showed that high frequency after the formation of the oxide film layer oxide nano-hardness had greatly improved,in which oxygen atomization conditions nano-hardness value maxi-mum,comparing the same increase by about 700％,this might be because the central oxygen atomization under the conditions of for-mation of the oxide film and the density of interface was best.The wear volumes after oxidation in atomized oxygen,pure oxygen and air were 6.530×109,10.055×109 and 12.031×109 nm3,respectively,while the wear volumes of the original samples were15.231×109 nm3,indicating that the wear resistance of the oxide film formed in the three kinds of media had been significantly improved.The wear volume under atomized oxygen was reduced by about 60％compared with the original sample,and the wear resistance was the best.Fi-nally,a 72 h autoclave corrosion test was carried out on zirconium alloy tubes under three different atmospheres at 400 ℃ and 10.3 MPa steam.The results showed that the corrosion weight gain of the original sample was 16.23 mg·dm-2,while the corrosion weight gain of the oxidized sample in air,pure oxygen and atomized oxygen media was 3.26,2.90 and 1.76 mg·dm-2,respectively.Compared with the original sample,the three kinds of media had obvious decrease,indicating that the oxidation film formed after high frequency oxidation could greatly improve the corrosion resistance of the zirconium alloy tube,and the improvement of the zirconium alloy tube under the atomized oxygen was the most obvious.
NETL
NSTL
万方数据
