Si3N4セラミックスの88℃熱水中での腐食に及ぼす焼結助剤の影響

概要

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Corrosion and dissolution behaviors of hot-pressed Si3N4 ceramics at 88°C in lithiated and borated water that was prepared to simulate PWR primary coolant were studied using a small once-through type experimental loop. Experiments were carried out for two Si3N4 ceramics with different sintering aids composed of MgO and Y2O3/Al2O3, respectively, and also Al2O3 which is used as the component of the mechanical seals for PWR primary coolant pump. The weight loss of exposed Si3N4 specimen containing MgO was relatively large and reached to 2.1mg/cm2 after 1, 000h exposure. Mg was dissolved selectively into the test solution, but dissolution of Si3N4 grain itself was estimated to be negligibly small. The surface was deeply corroded with the depth of 70μm, and changed to porous one. On the other hand, the weight loss of 1, 000h exposed specimen containing both Y2O3 and Al2O3 was very small at around 0.12mg/cm2. This value was nearly equal to that of Al2O3 ceramics. Similar to the case of Si3N4 containing MgO, Y was dissolved selectively and the surface was changed to porous. The corrosion depth, however, was small, of the order of 2μm. It was thus estimated that the corrosion resistance of Si3N4 ceramics in hot water was largely dependent on the compositions of sintering aids.
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