Yao Meiyi

EFFECT OF ADDING Cu ON THE CORROSION RESISTANCE OF M5 ALLOY IN SUPERHEATED STEAM AT 500

The effect of Cu content on the corrosion resistance of Zr-1%Nb-xCu(x=0.05%- 0.5%,mass fraction) was investigated in superheated steam at 500℃and 10.3 MPa by autoclave tests.The microstructures of the alloys and oxide films on the corroded specimens were observed by TEM and SEM,respectively.The results showed that when the Cu content was below 0.2%, the corrosion resistance of the alloys was markedly improved with the increase of Cu content,while further addition of Cu did not lead to a further improvement in the corrosion resistance.When the Cu content was below 0.2%,the Cu mainly dissolved in theα-Zr matrix.And when the Cu content was more than 0.2%,part of Cu precipitated as Zr_2Cu second phase particles.When theα-Zr matrix was oxidized,the Cu dissolved in theα-Zr could delay the process that the vacancies in the oxide film diffused and coalesced to form pores,and the pores developed into micro-cracks. Therefore,the corrosion resistance of the alloys was enhanced.It can be concluded that the Cu concentration in theα-Zr matrix,rather than the second phase particles containing Cu,is the main reason that the addition of Cu improves the corrosion resistance of M5 alloy in superheated steam at 500℃and 10.3 MPa.

EFFECT OF Bi ADDITION ON THE CORROSION RESISTANCE OF Zr-1Nb ALLOY IN DEIONIZED WATER AT 360 ℃ AND 18.6 MPa

The effect of Bi contents on the corrosion resistance of Zr-1Nb-xBi (x=0.05—0.3, mass fraction, %) was investigated in deionized water at 360 and 18.6 MPa by autoclave tests. The results show that the corrosion resistance of Zr-1Nb alloy can be improved by adding Bi, and the more the Bi content is, the better the corrosion resistance is. TEM and EDS analyses on the microstructures of the alloys show that there are two types of second phase particles (SPPs), including ZrNbFe and β- Nb. The Bi contents have little effect on the type, size and amount of SPPs, 0.3%Bi can be completely dissolved in α-Zr matrix and has no influence on the solution content of Nb in α-Zr matrix. From the fracture and inner surface morphology of oxide films observed by SEM, it can be seen that the Bi dissolved in the α-Zr could noticeably slow down the microstructural evolution of oxide film, including the propagation of micro-cracks and the transformation from columnar grains to equiaxed grains in the oxide film.

STUDY ON THE CORROSION RESISTANCE OF Zr-0.7Sn- 0.35Nb-0.3Fe-xGe ALLOY IN LITHIATED WATER AT HIGH TEMPERATURE UNDER HIGH PRESSURE

The corrosion resistance of Zr-0.7Sn-0.35Nb-0.3Fe-xGe(x=0.05,0.1,0.2,mass fraction,%) alloys was investigated in lithiated water with 0.01 mol/L LiOH at 360℃/18.6 MPa by autoclave tests.The microstructures of the alloys and oxide films on the corroded specimens were observed by TEM and SEM.The results show that the corrosion resistance of the Zr-0.7Sn-0.35Nb- 0.3Fe alloys in lithiated water at high temperature under high pressure is markedly improved by Ge addition.The alloy with 0.1%Ge shows the best corrosion resistance.In Zr-0.7Sn-0.35Nb-0.3Fe-xGe alloys,there exists fine Zr(Fe,Cr,Nb)_2 and Zr(Fe,Cr,Nb.Ge)_2 second phase particles(SPPs) with a close-packed hexagonal crystal structure(hep) and coarse Zr_3Ge SPPs with a tetragonal crystal structure(TET).The oxide films formed on the Zr-0.7Sn-0.35Nb-0.3Fe-0.1Ge alloys corroded for 220 d are compact and thin.The micro-pores and micro-cracks are hardly detected and many ZrO_2 columnar grains exist in the oxide films formed on the Zr-0.7Sn-0.35Nb-0.3Fe-0.1Ge alloys. This indicates that the suitable amount of Ge could not only delay the process that the vacancies diffuse to form micro-pores and micro-pores develop to form micro-cracks,but also could retard the evolution from ZrO_2 columnar grains to ZrO_2 equiaxed grains.

EFFECT OF Cu CONTENT ON THE CORROSION RESISTANCE OF Zr-0.80Sn-0.34Nb-0.39Fe-0.10Cr-xCu ALLOY IN SUPERHEATED STEAM AT 500℃

The effect of Cu content on the corrosion resistance of Zr-0.8Sn-0.34Nb-0.39Fe- 0.1Cr-xCu alloys(x=0.05-0.5,mass fraction,%) was investigated in superheated steam at 500℃and 10.3 MPa by autoclave tests.The microstructures of the alloys are observed by TEM.The results show that(0.05—0.5)Cu addition has little effect on the corrosion resistance of the alloys.When x is below 0.2.the precipitates Zr(Fe,Cr,Nb)_2 with hcp structure and Zr_3Fe containing Cu with orthorhombic structure are detected.When x is above 0.2,besides Zr(Fe,Cr,Nb)_2 and Zr_3Fe containing Cu.the precipitate of Zr_2Cu with tetragonal structure is also detected.Zr(Fe,Cr,Nb)_2 precipitates are smaller than the precipitates containing Cu in size.The precipitates containing Cu are found in the alloy even with 0.05Cu,indicating that the Cu content inα-Zr matrix is very small.Therefore,the reason that the Cu content has little effect on the corrosion resistance of the alloys is maybe related to the lower Cu content inα-Zr matrix.

CORROSION BEHAVIOR OF Zr(Fe x , Cr 1-x ) 2 ALLOYS IN 400℃ SUPERHEATED STEAM

To study the corrosion behavior of second phase particles in zirconium alloys, Zr(Fex, Cr1−x)2 (x=1, 2/3, 1/3) metallic compounds which have the same composition as the second phase particles in Zr-4 alloy were prepared by vacuum non-consumable arc melting. XRD and energy filtered TEM were employed for analyzing the corrosion products, the element distribution and grain morphology after corrosion tests of Zr(Fex ,C r1−x)2 metallic compounds powder at 400 and 10.3 MPa superheated steam with different exposure times. The results show that Cr has a very strong effect on the corrosion resistance of Zr(Fex ,C r1−x)2 metallic compounds, increasing Cr content can improve the corrosion resistance. When Zr(Fex ,C r1−x)2 oxidation starts, zirconium oxide is formed while elements Fe and Cr are expelled from the zirconium oxide due to their low solid solubility in the oxide. α-Fe(Cr) and γ-Fe(Cr) are formed and then oxidized to the stable corrosion product (Fe, Cr)3O4. The different corrosion behaviors of metallic compounds will affect the microstructure evolution of zirconium oxide layer differently during the corrosion process, and hence affect the corrosion resistance of zirconium alloys.

Study on Influence of Tetragonal ZrO2 in Oxide Film on Corrosion Resistance of Zr-Sn-Nb Alloy

Abstract Zr-Sn-Nb alloy specimens treated in different ways were exposed to 0.01 mol/L LiOH aqueous solution at 350 °C, 16.8 MPa in autoclave. Spectroscopy Ellipsometry (S-E) was applied to investigate the effect of tetragonal ZrO2 (t-ZrO2) in oxide film on corrosion resistance of Zr-Sn-Nb alloy. It was found that the 580 °C/C. R/500 °C specimen corroded for 42 d and 190 d have more t-ZrO2 than other two specimens. But it is converse to the corrosion rate. It is suggested that the small amount of t-ZrO2 is a main factor to affect the corrosion resistance ability and the corrosion resistance is improved by the formation of t-ZrO2.

STUDY OF ANISOTROPIC BEHAVIOR FOR ZIRCONIUM ALLOYS CORRODED IN LITHIATED WATER

Zirconium alloys of a hexagonal close-packed crystal structure have prominent anisotropic characteristic in comparison with metals of a cubic crystal structure and a strong texture is produced in sheet or tubular materials during the fabrication process.The anisotropic characteristic is bound to be reflected on the corrosion behavior of zirconium alloys.In order to investigate the effect of texture and compositions on the anisotropic growth of oxide layer formed on zirconium alloys and clarify the mechanism of improving corrosion resistance by adding Nb in zirconium alloys,Zr-4,N18 and ZIRLO zirconium alloys with different contents of Nb were adopted as the experimental materials. All the plate specimens of zirconium alloys 2 mm in thickness have a similar texture.Corrosion tests were carried out in a static autoclave at 360℃,18.6 MPa in lithiated water with 0.01 mol/L LiOH.The results show that the anisotropic growth of oxide layer on different surfaces of the specimens was only observed for Zr-4 specimen but not for N18 and ZIRLO specimens.The thickness of oxide layer devel- ops much faster on the rolling surface(Sn surface) than that on the surface perpendicular to the rolling direction(Sr surface) and the surface perpendicular to the transversal direction(St surface) for Zr-4 specimen after 90—100 d exposure,and the corrosion resistance on the S_r and S_t surfaces was much better than that on the S_n surface.However,for N18 and ZIRLO specimens the anisotropic growth of oxide layer was restrained by the addition of Nb,and the oxide thickness on these three different surfaces was the same after 280 d exposure.Therefore the corrosion resistance of N18 and ZIRLO sheet or tubular specimens was superior to Zr-4 corroded in lithiated water,because the oxide layers grew mainly on the S_n surface of the specimens.If making a comparison among Zr-4,N18 and ZIRLO specimens about the growth rate of oxide layers only on the S_r and S_t surfaces,it is shown that the growth rate of oxide layers increased with the increase of Nb content in these alloys.From a point of view for the improving corrosion resistance,the addition of Nb no more than 0.3%(mass fraction) is recommended.

EFFECT OF β-QUENCHING ON THE CORROSION RESISTANCE OF Zr-4 ALLOY IN LiOH AQUEOUS SOLUTION

To investigate the effect ofβ-quenching on the corrosion behavior,Zr-4 specimens wereβ-quenched and subsequently annealed at 480—600℃for 2—200 h.The corrosion tests were carried out in 0.01 mol/L LiOH aqueous solution at 360β/18.6 MPa.The microstructure of Zr-4 specimens and fracture surface morphology of oxide films on the corroded specimens were observed by TEM and HRSEM,respectively.The results show that Zr-4 specimens exhibit excellent corrosion resistance as Zr-Sn-Nb alloys in LiOH aqueous solution after increasing the solid solution content of Fe and Cr inα-Zr matrix by suitable quenching rate fromβ-phase to avoid the formation ofβ-Zr. When the cooling rate is too fast,the specimens show worse corrosion resistance due to the formation ofβ-Zr.After annealing at 480—600℃,the better corrosion resistance of Zr-4 specimens can be obtained with increasing annealing temperture and time due to the decomposition ofβ-Zr.

EFFECT OF THERMAL PROCESSING ON THE CORROSION RESISTANCE OF ZIRCONIUM ALLOY N18 IN LiOH AQUEOUS AT 360℃/18.6 MPa

The effect of thermal processing on corrosion behavior of N18 alloy has been inves- tigated by autoclave tests in 0.01 mol/L LiOH aqueous solution at 360 and l8.6 MPa, and the microstructures of these specimens were examined by TEM and SEM. The results show that the cor- rosion resistance of specimens is improved obviously by β phase quenching before cold rolling and annealing due to nano second phase particles (SPPs) precipitated dispersively, but when the inter- mediate annealing temperature increased to 740 before cold rolling and annealing, the corrosion resistance is lowered greatly due to SPPs coarsened to several hundred nanometers, where the cor- rosion rate accelerates markedly after the corrosion transition at 150 d exposure. The existence of β-Zr phase in the microstructures of specimens treated at 780 and 800 for 2 h in dual phase region is harmful to the corrosion resistance, but the corrosion resistance returns to a better level after the decomposition of β-Zr phase during the following processes of cold rolling and annealing at 580 to obtain fine SPPs.

IN SITU IMPEDANCE CHARACTERISTICS OF ZIRCONIUM ALLOY CORROSION IN HIGH TEMPERATURE AND PRESSURE WATER ENVIRONMENT

The electrochemical impedance spectroscopy (EIS) was used to analyze the corrosion evolution of pre- and post-transition of the annealed Zr-4 alloy in 0.01 mol/L LiOH aqueous solution at 360 /18.6 MPa. The results show that oxidation weight gain of Zr-4 alloy increases slowly before corrosion transition. When Zr is oxidized into ZrO2, the stress appears in the oxide film due to volume expansion. With gradually thickening of the oxide film, accumulation of micro-defects will relax the stress in outer layer of oxide film. As a result, the uniform and compact oxide layer turns into double layer with a porous outer layer and a dense inner one. Meanwhile the impedance spectra evolve from single arc into double capacitive arcs. When the oxide film forms macro-defects such as cracks, corrosion transition takes place, giving rise to an acceleration of oxidation weight gain and the rapid drop of impedance and corrosion potential. Corrosion potential and electrochemical impedance spectroscopy are able to obtain in situ some information of the transition point.