報告書番号

T95038

タイトル（和文）

高温高圧水中における酸化鉄皮膜の安定性評価への交流インピーダンス法の適用

タイトル（英文）

A C IMPEDANCE METHOD OF EVALUATING STABILITYOF SURFACE OXIDE FILM FORMED ON HIGH-PURITY IRON IN HIGH-TEMPERATURE WATER

概要 （図表や脚注は「報告書全文」に掲載しております）

交流インピーダンス法を用い２５～２７０度のＮａＯＨ水溶液中における酸化鉄皮膜の安定性について調べ，皮膜の分極抵抗値と平衡論的な皮膜の溶解度との相関性について基礎的に検討し，以下の知見を得た。（１）．脱気水中で形成された酸化鉄皮膜の分極抵抗値は，温度の上昇とともに減少し，Ｆｅ３Ｏ４の溶解度が平衡論的に最小となるとき最大になり，分極抵抗値の大小は平衡論的に求めたＦｅ３Ｏ４の溶解度の計算結果と良い対応関係を示した。（２）・Ｆｅ３Ｏ４より安定な不動態皮膜Ｆｅ２Ｏ３が形成される環境下では，分極抵抗値はＦｅ３Ｏ４に比べ大きくなること，また，皮膜破壊条件下では最小となることから，分極抵抗値は酸化鉄皮膜の安定性と密接に関連することが明らかとなった。以上の結果より，交流インピーダンス法は，酸化鉄皮膜の安定性評価に対して有効な手段であることが示された。

概要 （英文）

SEVERAL TYPES OF CORROSION HAVE BEEN FOUND ON FEEDWATER TUBES IN LWR AND FOSSIL-FUEL POWER PLANTS DUE TO IRON ION RELEASE FROM THE FERRIC MATERIAL,AND IRON SCALE DEPOSITION ON THE TUBE SURFACE. SCALE DEPOSITION ACCELERATES THE FILM DISSOLUTION AND RUPTURE. CANSEQUENTLY,IT IS VERY IMPORTANT TO EXAMINE THE SURFACE CONDITIONS OF THE TUBE.VARIOUS CHEMICAL ANALYSES HAVE BEEN USED TO EVALUATE THE SURFACE-OXIDE-FILM CHARACTERISTICS ON THE TUBES,HOWEVER,EXACT ANALYSIS IS VERY DIFFICULT. THE POLARIZATION RESISTANCE AND THE ELECTRIC CAPACITANCE OF OXIDE FILM FORMED ON HIGH-PURITY IRON WERE MEASURED IN DEAERATED 1 10000 PPM AND AERATED 1000 PPM SODIUM HYDROXIDE SOLUTIONS AT 25 270 DEGREE. THE EFFECTS OF PH,TEMPERATURE,DO AND ELECTRIC POTENTIAL ON THE IRON-OXIDE FILM STABILITIES WERE EXAMINED. THE SOLUBILITIES OF THE IRON OXIDE WERE CALCULATED TO INVESTIGATE THE CORRELATION BETWEEN THE POLARIZATION RESISTANCE AND THE SOLUBILITIES. THE MAIN RESULTS ARE AS FOLLOWS: (1)A C IMPEDANCE TEST RESULTS SHOWED THAT THE POLARIZATION RESISTANCE OFFE3O4 FORMED IN DEAERATED SODIUM HYDROXIDE SOLUTION IS MAXIMUM WHEN THE CALCULATED SOLUBILITY OF THE OXIDE IS MINIMUM. THIS RESULT INDICATE THAT THERE IS A GOOD RELATIONSHIP BETWEEN THE PH DEPENDENCE OF POLARIZATION RESISTANCE AND THAT OF THE CALCULATED SOLUBILITY OF THE FE3O4. THE RESISTANCE DECREASES WITH INCREASING SOLUTION TEMPERATURE. (2)THE RESISTANCES OF FE3O4,PASSIVE FILM FORMED UNDER AERATED AND POLARIZED NOBLE POTENTIAL CONDITIONS WERE LARGER THAN THAT OF FE3O4, AND THE RESISTANCE WAS SMALLEST WHEN THE OXIDE FILM WAS DISSOLVED. THESE TEST RESULTS REVERAL THAT POLARIZATION RESISTANCE IS CORRELATED TO THE STABILITY AND MORPHOLOGY OF THE IRON OXIDE. THSES TEST RESULTS SUGGESR THAT THE TO EVALUATE THE STABILITY OF THE AC IMPEDANCE METHOD CAN BE USED TO EVALUATE THE STABILITY OF THE IRON-OXIDE FILM.

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