Nb基およびMo基超耐熱合金のLi腐食挙動

Corrosion behavior of Nb-based and Mo-based super heat-resisting alloys in liquid Li

斉藤 淳一  ; 森永 正彦*; 加納 茂機

Saito, Junichi; Morinaga, Masahiko*; Kano, Shigeki

高温アルカリ金属のフロンティア領域を開拓するために、高温液体Liの過酷な環境下で使用できる構造材料の研究開発を推進してきている。その条件下で使用される材料としてNbおよびMoの高融点金属を基とした超耐熱合金に着目した。液体リチウム中の腐食特性、クリープおよび引張強度等の高温での機械的特性は最も重要な特性の一つであり、高温(1473K)でのリチウム耐食性と機械的特性の多くの実験や解析により、NbおよびMo基合金の設計開発を進めてきた。本報告書は、これまで実施してきたNb基およびMo基超耐熱合金のLi腐食試験結果を系統的にまとめ、解析結果より腐食機構を提案するとともに、表面の腐食生成物の生成機構とNb基合金の表面クラックの生成機構を解明したものである。主な結果は下記のとおりである。(1)腐食生成物の生成機構は、溶出金属とLi中の窒素の間で最初に反応が起こり、腐食生成物(窒化物)が表面に析出し、その後、溶出金属どうしの反応が生じ腐食生成物(金属間化合物)が表面に析出する。これらの生成機構により、腐食生成物の成分を予測できることがわかった。(2)クラック生成メカニズムでは、化学ポテンシャル図を用いることによりクラック発生の原因となる3元系酸化物の形成を理解することができた。その結果、合金中の固溶酸素だけでなく試験中に表面から侵入してくるLiの濃度も3元系酸化物の形成に重要であることが明らかになった。

Research on structural materials which will be utilized even in the severe environment of high-temperature liquid alkali metals has been promoted in order to develop the frontiers of materials techniques. The super-heat resisting alloys which are based on refractory metals, Nb and Mo, are aimed as promising materials used in such an environment. The corrosion resistance in liquid Li and the mechanical properties such as creep and tensile strengths at high temperatures are important for these structural materials. On the basis of many expeliments and analyses of these properties at 1473K, the material design of Nb-based and Mo-based alloys has-been carried out successfully. In this report, all the previous experimental results of corrosion tests in liquid Li were summarized systematically for Nb-based and Mo-based alloys. The corrosion mechanism was proposed on the basis of a series of analyses, in particular, focussing on the deposition mechanism of corrosion products on the surface and also on the initiation and growth mechanism of cracks on the corroded surface of Nb-based alloys. The principal results are as follows. (1)For the deposition mechanism, a reaction took place first between dissolved metallic elements and nitrogen which existed as an impurity in liquid Li and then corrosion products (nitrides) precipitated on the metal surface. Subsequently, another reaction took place between dissolved metalic elements in liquid Li, and corrosion products (intermetallic compounds) precipitated on the metal surface. The composition of deposited corrosion products could be predicted on the basis of the deposition mechanism. (2)For the crack initiation mechanism, the chemical potential diagrams were utilized in order to understand the formation of Li-M-O ternary oxides which caused cracks to be formed on the corroded surface. Consequently, it was evident that not only the concentration of the dissolved oxygen in the alloy but also the concentration of Li which ...