690合金在25℃的5M NaCl溶液中之電化學極化與應力腐蝕行為研究

Study on the Electrochemical Polarization and Stress Corrosion Cracking Behaviors of Alloy 690 in 5M Chloride Solutions at 25℃

本研究利用循環極化法建立了鎳基690合金在25℃，濃度5M之氯化鈉溶液中的實驗上之電位與pH值關係圖，並定義此圖中之免疫、均勻腐蝕、鈍態、和孔蝕等區域界線。結果發現：在pH值大於4時，鈍態區會被細分為完全鈍態區和不完全鈍態區。在極化實驗後，試片表面進行金相的觀察。發現孔蝕在整個實驗的pH值範圍內（0.3-8.52）均會發生，但均勻腐蝕則主要發生在較低pH值（<3）的溶液中。另一方面，利用慢速應變拉伸試驗所量測的試片機械特性如最大拉伸強度、破裂時的應變值和縮減面積比可發現在pH值<3時均會明顯地降低。而慢速應變拉伸實驗的結果亦與透過掃瞄式電子顯微鏡所觀察到的試棒斷裂表面與側面形貌相符。

An experimental, potential-pH diagram was constructed for the nickel-based Alloy 690 in 25oC concentrated (5M) sodium chloride solution, using an cyclic polarization method. The domains of immunity, general corrosion, passivation, and pitting in 5M NaCl solutions were defined. At pH > 4 the passive region subdivided into areas of perfect passivation, imperfect passivation, and pitting. After anodic polarization, the surface of each specimen was carefully examined metallographically. Pitting corrosion was observed over the entire pH range investigated (0.3-8.52) but general corrosion predominated at lower pH values (< 3). On the other hand, the mechanical properties, such as ultimate tensile strength (UTS), strain at fracture and the reduction area (RA) measured by the slow strain rate test (SSRT), decreased significantly at pH < 3. The SSRT results are consistent with fractography and side-view observations of the tested specimens by SEM.