两种新锆合金堆外耐蚀性研究/Research on Out-of-pile Corrosion Resistance of Two Kinds of New Z
研究结果表明，与改进Zr-4和NZ8合金相比，采用较低温度中间退火工艺组合(5900C/5900C /5900C)的NZ2合金具有最佳的耐均匀腐蚀性能；对NZ8来说，采用6500C/5900C /5900C中间退火工艺组合的样品较其它工艺组合的样品耐均匀腐蚀性能更为优异，但在蒸汽中腐蚀时耐蚀能力较改进Zr-4和NZ2合金降低了近一倍。两种新锆合金在检验周期内(500小时)均无疖状腐蚀出现，与改进Zr-4合金在腐蚀8小时即出现疖状腐蚀相比，抗非均匀腐蚀性能大大提高了。
In this thesis, the out-of-pile uniform corrosion resistance and hydrogen pickup performance are investigated by means of autoclave test in 4000C/10.3Mpa steam and 3600C/18.6Mpa Lithiated water to the plate samples prepared by different annealing processing for alloys of Zr-1Sn-0.3Nb-0.3Fe-0.1Cr (NZ2) and Zr-1Sn-1Nb-0.3Fe (NZ8). The out-of-pile nodular corrosion resistance in 5000C/10.3Mpa steam is also tested to plate samples of two alloys. The mechanism of corrosion resistance is analyzed by combining the corrosion properties with observation results of microstructure, precipitates of alloys, and microstructure of oxide films.
The corrosion tests show that superior uniform corrosion resistance for alloy NZ2 can be obtained by adopting low temperature intermediate annealing combination (5900C/5900C /5900C), which is also the best one among alloys tested for, the better uniform corrosion resistance for alloy NZ8 is in condition of 6500C/5900C /5900C intermediate annealing combination processing, however its uniform corrosion resistance in steam is decreased by nearly 1-fold compared to alloys of Improved Zr-4 and NZ2.The nodular corrosion resistance of two new alloys is remarkably better than that of Improved Zr-4 ,no nodule occurs in test duration.
The model for hydrogen weight gain kinetics has been established on basis of hydrogen pickup fractions being constants respectively in pre-and post-transition stage during corrosion, and hydrogen weight gain performance has been investigated. It is found that alloy NZ2 has the lowest hydrogen weight gain in 4000C/10.3Mpa steam, whereas NZ8 has the lowest hydrogen weight gain in 3600C/18.6Mpa Lithiated water. The difference of hydrogen weight gain from corrosion weight gain is attributed to non-accordant change of hydrogen pickup fraction and corrosion weight gain during corrosion.
It is proposed that the solubility of Nb in a-Zr happens abnormal change when the element co-exists with Fe and Cr, i.e., the solubility decreases with increase of intermediate annealing temperature, and the alloying elements added to the two alloys mainly exists in the Laves precipitates. It is pointed out that precipitate Zr(Fe,Cr)2 is beneficial to improve the corrosion resistance , precipitate Zr(Nb,Fe,Cr)2 and Zr(Nb,Fe) 2 are harmful to the corrosion resistance in 4000C/10.3Mpa steam ,but those precipitates have reverse effect in 3600C/18.6Mpa Lithiated water.
From microstructure observation of oxide film and results of corrosion test, the mechanism of corrosion resistance is analyzed. It is confirmed that the transition of corrosion is the result of development of cracks and pores, the compressive-stabilized tetragonal or cubic ZrO2 protects oxide film, their transformation to monoclinic ZrO2 causes occurrence of defects such as cracks and pores, texture of grains in oxide, microcrystalline nature of oxide, morphology change of oxide, recrystallization of grains in oxide, and grain growth are also related to those phase transformation that resulted in relieving of compressive stress in the oxide film. It is found that the continual oxidation of Laves precipitates incorporated in zirconia will exert a tensile stress to vicinal oxide which accelerates the transformation of tetragonal or cubic ZrO2 to monoclinic ZrO2, and cause hydrides plate formation in case of Cr existed in Laves precipitates due to hydrogen pickup effect of Cr, these additional tensile stress and hydrides formed will also result in occurrence of cracks or pores, the disperse distribution of fine precipitates in the alloy and lowering Cr content added to the alloy may avoid this phenomenon. It is pointed out that the oxide barrier layer has a decisive role to corrosion in post-transition stage，the coupling effect of elements of Nb and Sn inhibits damage of this layer from Li+ penetration in Lithiated water corrosion providing that the matrix of alloy has a proper amount of Nb content, as for alloy NZ8 with relative higher Nb content, its corrosion resistance in 4000C/10.3Mpa steam is worst one among alloys tested, the mechanism should be further probed but Sn element rich layers in the oxide and possible Nb element rich micro-areas in the oxide should be considered in the mechanism model. It is proposed that the inhibited effect of Nb to nodular corrosion is associated with increase of electrical conductivity in the oxide and of improvement of mechanical properties of the oxide due to its addition to the alloys.