脱気高温水中における鋭敏化加工ステンレス鋼のSCC特性 -これまでの研究状況-
SCC Behavior of Sensitized and Cold-Worked Austenitic Stainless Steels in Deaerated High-Temperature Water -A Review of Current Studies

山田　卓陽（Takuyo Yamada） ，寺地　巧（Takumi Terachi）

要約
1970年代の沸騰水型軽水炉（BWR）で，再循環配管の溶接部近傍でオーステナイト系ステンレス鋼の応力腐食割れ事象が報告された．このSCCの材料因子は，溶接時の熱影響により粒界炭化物が生成し，それに伴う粒界Cr濃度の低下であるとされている。BWRの環境条件は酸素を含む腐食電位の高い条件であり，この環境下の鋭敏化ステンレス鋼のSCC挙動について多くの研究例がある．一方，加圧水型軽水炉（PWR）の１次冷却水は，水素添加により腐食電位が低く抑えられているため，ステンレス鋼のSCCはこの種の環境下では生じないものと長らく考えられてきた．しかし，強い加工を加えた場合には，非鋭敏化ステンレス鋼でもSCC進展を生じる場合があることが指摘されている．INSSでは，低腐食電位環境での冷間加工ステンレス鋼のSCC挙動に着目し研究を実施してきたが，類似の環境下で、鋭敏化と加工が複合した「鋭敏化加工ステンレス鋼」に関する検討も実施してきた．本報告では，まず軽水炉におけるSCC事例及び研究例を，主として(1) 腐食電位（と温度）の観点，(2) 鋭敏化と加工度の有無（複合条件を含む）の観点で分類し概説する．その中で特に脱気高温水中（低腐食電位）での鋭敏化加工ステンレス鋼のSCC特性，腐食電位依存性，温度の影響に関するこれまでの研究状況を紹介する．
それら結果，鋭敏化加工ステンレス鋼のSCC挙動は，腐食電位によって大きく異なり，脱気高温純水中およびPWR1次冷却材模擬水中のような低腐食電位環境下では，酸素を含む高腐食電位条件に比べて，SCCが顕著に抑制的であった．重要な材料因子である鋭敏化（炭化物、粒界Cr濃度）と加工の複合影響については，炭化物，粒界Cr濃度，加工の影響をそれぞれ分離した場合と，複合した場合を考える必要がある．またそのそれぞれの影響について，腐食電位による差異，および温度の影響を考慮に入れる必要がある．

キーワード    鋭敏化加工オーステナイトステンレス鋼，高温水中応力腐食割れ，腐食電位依存性，加圧水型原子炉, 沸騰水型原子炉

Abstract
In the 1970s, stress corrosion cracking (SCC) occurrence was reported in austenitic stainless steels (ASSs) near the weld heat affecting zone (HAZ) of recirculation piping in boiling water reactors (BWRs). The material factor for this SCC is believed to be the intergranular carbides formed by welding heat; these carbides affect the intergranular Cr concentration and cause it to decrease. BWR water is under normal water chemistry (NWC), in other words, there is a condition of high corrosion potential due to including oxygen, and there are many examples of studies on ASSs SCC behavior in this environment. On the other hand, the primary coolant of pressurized water reactors (PWRs) is normally hydrogenated to keep a low corrosion potential, and it has long been believed that SCC of ASSs does not occur in this type of environment. However, it has been pointed out that SCC growth may occur even in non-sensitized ASSs when subjected to strong (cold) working. INSS has focused its research on SCC behavior of cold-worked ASSs in environments with low corrosion potentials and has also conducted studies on “sensitized and cold-worked ASSs (including combined condition)”. In this paper, ASSs of SCC experience in light water reactors (LWRs) and its research examples are categorized and reviewed mainly in terms of (1) corrosion potential (and temperature) and (2) sensitization and cold-work (including combined condition). Especially, corrosion and temperature dependencies on SCC of sensitized and cold-worked ASSs are introduced in de-aerated and hydrogenated high temperature water (low corrosion potential).
 The SCC behaviors of sensitized and cold-worked ASSs greatly depend on the corrosion potential, and the SCC in low corrosion potential environments such as de-aerated high-temperature pure water and simulated PWR primary coolant water is significantly more suppressive than that at high corrosion potential. To clarify the effects of sensitization (carbide formation and intergranular Cr concentration) and cold-working on IGSCC, it is necessary to consider these effects separately and then combined. And then effects of corrosion potential and temperature should also be considered.

Keywords   sensitized and cold-worked austenitic stainless steel, stress corrosion cracking (SCC) in high-temperature water, corrosion potential dependence, pressurized water reactor (PWR), boiling water reactor (BWR)

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