预渗氢对Zr-4合金在360 ℃/LiOH水溶液中耐腐蚀性能的影响

Effect of Pre-charging Hydrogen on Corrosion Resistance of Zircaloy-4 in LiOH Aqueous Solution at 360 ℃

摘要:

核反应堆运行时，锆合金包壳的腐蚀和吸氢相伴发生，被锆合金吸收后的氢不但会影响锆合金的力学性能，同时对锆合金的耐腐蚀性能也会产生一定的影响，其影响程度与合金成分和腐蚀条件密切相关。合金中的氢对Zr-4（Zr-1.3Sn-0.2Fe-0.1Cr, 质量分数，%）合金在LiOH水溶液中的耐腐蚀性能影响机理尚不完全清楚，所以本文开展了预渗氢影响Zr-4合金在LiOH水溶液中耐腐蚀性能规律及其机理的研究工作。采用气相渗氢法或电解渗氢法对Zr-4合金样品进行渗氢，制备了低氢（20~120 μg/g）和高氢（120~250 μg/g）样品。将渗氢和未渗氢的样品同时放入高压釜中进行360 ℃/18.6 MPa/0.01 M LiOH水溶液中的腐蚀试验。采用高分辨扫描电子显微镜（HRSEM）观察氧化膜断口、内表面和外表面形貌；用卷曲法测量氧化膜内应力；采用二次离子质谱（SIMS）测定Zr-4样品氧化膜中Li+浓度沿深度方向的分布。研究结果表明：3种氢含量样品在LiOH水溶液中腐蚀时均发生明显的腐蚀转折，但样品中的氢含量越高，转折后的腐蚀速率越小，即耐腐蚀性能越好。与未渗氢样品相比，渗氢样品的氧化膜更加致密，氧化膜/金属（O/M）界面的起伏程度更小，氧化膜显微组织的演化更慢。渗氢样品氧化膜中的应力水平较低，且随氧化膜厚度增加应力下降的趋势更平缓，同时氧化膜中的Li+浓度更低，且沿氧化膜深度方向上Li+浓度下降得更快。这说明合金中的氢可抑制Li+在氧化膜中的扩散、降低氧化膜中的应力水平，延缓氧化膜中空位扩散凝聚形成孔隙、孔隙连通发展成为微裂纹的过程，从而提高了Zr-4合金的耐腐蚀性能。

Abstract:

During the operation of nuclear plants, the reaction of Zr and water produces ZrO2 and hydrogen atoms, some of which enter the zirconium alloys. Hydrogen in zirconium alloys affects not only mechanical properties, but also corrosion resistance. The degree of influence is closely related to alloy composition and corrosion conditions. The corrosion mechanism Zr-4 (Zr-1.3Sn-0.2Fe-0.1Cr, mass fraction, %) alloy in LiOH aqueous solution affecting by hydrogen is still controversial. Therefore, the effect of pre-charging hydrogen on the corrosion resistance of Zr-4 in 0.01 M LiOH aqueous solution was investigated.Two batches of Zircaloy-4 specimens were pre-hydrided using gaseous or electrolytic hydrogen charging methods to obtain the predetermined hydrogen level: less than 120 μg/g H and more than 120 μg/g H. Then all the as-received specimens and hydrogen-charged specimens were corroded in lithiated water with 0.01 M LiOH at 360 ℃ and 18.6 MPa in a static autoclave. The mechanism about the effect of hydrogen on the corrosion behavior of zirconium alloys was discussed based on weight gain, microstructure of oxide films, compressive stress and Li+ depth profile in the oxide film. Results showed that Zircaloy-4 specimens with 20~250 μg/g hydrogen exhibited better corrosion resistance compared with the as-received ones. The corrosion resistance of Zircaloy-4 became better with increasing the hydrogen content. The integrity of the oxide film on the hydrogen-charged specimens was better than that on the as-received specimens, which indicates a slower microstructural evolution of the oxide film on the hydrogen-charged specimens. The hydrogen-charged specimens had a less undulate oxide/metal interface. Compared with the as-received specimens, the compressive stress in the oxide film on the hydrogen-charged specimens was lower and showed a less gradual gradient with the increase of oxide thickness. The concentration of Li+ in the oxide film on the hydrogen-charged specimens was lower, and it dropped down quickly along the depth of oxide film. In this way, the microstructural evolution of the oxide film was retarded and the corrosion resistance of Zircaloy-4 was improved by pre-charging hydrogen.