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

2018-08-25 09:33:15

合金 Nb Zr corrosion oxide



本文以Zr-1Sn-0.3Nb-0.3Fe-0.1Cr(NZ2)和Zr-1Sn-1Nb-0.3Fe(NZ8)两种新锆合金为对象,对采用不同中间退火工艺组合制备的板材进行了堆外高压釜内4000C/10.3MPa蒸汽中和3600C/18.6Mpa含锂(7.0×10-5)水中的均匀腐蚀性能和吸氢性能研究;进行了高压釜内5000C/10.3MPa蒸汽中非均匀腐蚀—疖状腐蚀性能研究。文章结合合金微观组织和第二相研究以及腐蚀后氧化膜微观组织结构的研究,分析了两种新锆合金的耐蚀机理。
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。
文章依合金在腐蚀转折前后吸氢分数分别保持恒定值为出发点,建立了氢增重动力学模型,进而进行了吸氢性能研究。结果发现在蒸汽中NZ2氢增重速率最低,在含锂水中NZ8合金的氢增重速率最低。吸氢分数和腐蚀增重共同决定了氢增重,两个参数变化的不一致性导致腐蚀增重和氢增重变化规律的差异。
通过对合金基体微观组织结构的研究,发现Nb在与Fe、Cr共存时,Nb于基体中的固溶度发生异常改变,即随退火温度升高在基体中的固溶度减少,两种合金的合金元素Fe、Cr、Nb主要存在于Laves沉淀相中;结合腐蚀性能研究结果,明确提出Zr(Fe,Cr)2Laves沉淀相有利于提高合金在蒸汽中的抗腐蚀性能,含铌的Zr(Nb,Fe,Cr)2或Zr(Nb,Fe) 2Laves沉淀相则有不利的影响;在含锂水介质中则刚好相反。
通过对腐蚀后氧化膜微观组织结构的研究及腐蚀机理的分析,证明腐蚀转折是氧化膜孔洞、裂纹发展的必然结果,由压应力稳定的四方或立方ZrO2对氧化膜起保护作用,其向单斜ZrO2的转化是孔洞、裂纹产生的根源,氧化膜晶粒织构特征、微晶特征及其从柱状晶到再结晶球化和长大均与氧化膜内ZrO2相变诱发的应力松弛密切相关;发现镶嵌于氧化膜中的Laves沉淀相的继续氧化将对周围氧化物产生附加张应力,这一张应力将促进其周边氧化膜中四方或立方ZrO2向单斜ZrO2的转化,在Laves沉淀相含铬时,由于铬对吸氢的促进作用,将在Laves沉淀相附近的氧化膜中形成氢化物,附加张应力和氢化物进一步促进在氧化膜中形成孔隙或裂纹,因此合金中的第二相应细小弥散均匀分布,依其继续氧化过程中不对氧化膜产生进一步破坏为宜,同时应减少合金中铬的添加量;合金氧化膜中的阻挡层对转折之后的腐蚀起决定作用,提出在合金基体中含有适量铌的前提下,锡与铌的共同作用抑制了锂对阻挡层的破坏作用,铌含量较高的NZ8合金在蒸汽中和含锂水中与改进Zr-4合金相比产生差异的原因比较复杂,需考虑氧化膜第二相氧化后微区中铌的富集、氧化层锡的富集等多种因素,系统的机理尚需探索;作者认为合金元素铌抑制疖状腐蚀出现的原因与氧化膜电导率提高以及加入铌后氧化膜机械性能提高从而减少氧化膜开裂有关。


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.