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镍基合金、特别是Hastelloy N(16Mo—7Cr—5Fe—Ni)为美国发展熔盐反应堆研制的一种耐蚀材料。大量文献是针对在高纯熔融氟化物燃料盐系中的试验结果,并证实一般主要发生的是选择性脱铬反应。本试验(静态)是在普通(未经特殊净化处理)熔融氟化物盐或硝酸盐中进行。通过金属物理等手段,详细研究了表面腐蚀层的结构、钼对晶界脱溶腐蚀的影响及应力腐蚀破裂的过程。表明除铬元素外,也存在钼元素的选择性脱溶腐蚀,并构成上述腐蚀现象的主要原因。本文着重说明了钼的双重作用:一方面随着合金中钼含量的提高,明显改变了钼、铬元素沿晶粒边界和通过晶粒内部脱溶腐蚀的相对速度,从而抑制了在高温氟盐中出现晶间腐蚀;另一方面钼、铬元素的脱溶反应造成表面腐蚀层基本上是一种疏松多孔的纯镍层。在熔融硝酸盐中,尽管Hastelloy N合金的一般腐蚀速度极低,但发现在张应力的作用下加速了局部晶界的脱溶腐蚀;而钼、铬元素沿晶界的选择性脱溶反过来又促进了裂纹的形成和扩展,引起晶间应力腐蚀破裂。
Nickel-based alloys, particularly Hastelloy N (16Mo-7Cr-5Fe-Ni), are corrosion resistant materials developed for the development of molten salt reactors in the United States. A large body of literature addresses the experimental results in the high-purity fused-fluoride fuel salt system and confirms that in general, selective dechromination reactions occur predominantly. This test (static) is carried out in ordinary (without special purification treatment) molten fluoride salts or nitrates. By means of metal physics and other means, the structure of the surface corrosion layer, the influence of molybdenum on the grain boundary dissolution and corrosion and the stress corrosion cracking process are studied in detail. It shows that besides chromium, selective desolventizing of molybdenum also exists and it is the main reason for the above corrosion. This article highlights the dual role of molybdenum: on the one hand, with the increase of molybdenum content in the alloy, the relative velocity of molybdenum and chromium along the grain boundaries and through the internal dissolution and decrystallization of the grains is obviously changed, In the intergranular corrosion; the other hand, molybdenum, chromium desorption reaction caused by surface corrosion layer is basically a loose porous pure nickel layer. In the molten nitrate, although the general corrosion rate of Hastelloy N alloy is very low, it is found that under the action of tensile stress, the desorption corrosion of local grain boundary is accelerated; while the selective desolvation of molybdenum and chromium along the grain boundary is reversed But also promote the formation and expansion of cracks, causing intergranular stress corrosion cracking.