本研究主旨在探討不同環境參數對AISI 347 不銹鋼腐蝕疲勞性質之 影響，探討的環境參數包括溶液之酸鹼度、溫度、氯離子及抑制劑的影 響。實驗中比較空氣及五種水溶液中之高週疲勞及疲勞裂縫成長的差異 性，並以電化學試驗觀測在不同環境下其電化學行為之變化情形。此外， 亦利用光學式顯微鏡(OM)及掃描式電子顯微鏡(SEM)觀察疲勞破斷面， 以了解裂縫的生成及成長模式。 實驗結果顯示，AISI 347 不銹鋼在空氣及五種水溶液中之高週疲勞 行為以H2SO4 水溶液及3.5% NaCl 中有最明顯之影響，在此兩種環境中 的高週疲勞壽命皆有明顯地降低，其中又以H2SO4水溶液中的降幅最大， 而室溫純水、80oC 純水及3.5% NaCl + inhibitor 中之高週疲勞壽命與空氣 中之差異則不顯著。而在長裂縫成長實驗(stage II)方面，空氣中及五種水 溶液中之裂縫成長速率差異不大。由此顯示其高週疲勞壽命主要消耗在 裂縫起始階段，而不在裂縫延伸階段。 H2SO4水溶液及3.5% NaCl對347不銹鋼高週疲勞壽命主要的影響皆 在於裂縫起始及短裂縫成長(stage I)階段。3.5% NaCl 主要的影響在於氯 離子使試棒表面形成蝕孔造成應力集中效應而縮短了裂縫起始所需的時 間，而H2SO4水溶液的影響在於降低pH 值加劇了循環應力與腐蝕環境的 加成作用，並分解具保護作用的鈍化膜，進而促使短裂縫之生長，並加 速短裂縫通過過渡區進入長裂縫階段，縮短裂縫起始所需時間，因此整 體疲勞壽命有最明顯之下降。此外，在3.5% NaCl 中添加抑制劑可以抑 制蝕孔生成於光滑試棒表面，防止裂縫由蝕孔處生成，因而明顯提升了 高週疲勞壽命，而抑制劑對於金屬表面的吸附作用亦可隔絕腐蝕環境的 影響，減少腐蝕作用的程度。 The aim of this study is to investigate the influence of environmental factors, including pH value, temperature, chloride, and pitting inhibitor, on the corrosion fatigue properties of AISI 347 stainless steel. In particular, the high-cycle fatigue (HCF) and fatigue crack growth (FCG) behavior in air and five aqueous environments were made a comparison. The effect of environmentally assisted cracking mechanisms on the degradation of fatigue resistance was characterized. The electrochemical properties in five aqueous environments were also made a comparison. Fractography and microstructural analyses with optical microscopy (OM) and scanning electron microscopy (SEM) were conducted to determine the corrosion fatigue crack initiation and propagation modes. Results showed that the fatigue strength of AISI 347 in H2SO4 and 3.5% NaCl solutions was lower than that in air, water, 80oC water, and 3.5% NaCl with inhibitor; especially fatigue strength in H2SO4 was the lowest. However, the FCG rates in all environments were almost the same. These results indicated that the initial fatigue cracking stage controlled the HCF life of AISI 347. The H2SO4 and 3.5% NaCl solutions had more detrimental effects on the HCF of AISI 347 in crack initiation and stage I cracking stages as compared with other aqueous environments. The fatigue-strength reduction in 3.5% NaCl solution resulted from the formation of corrosion pits as the stress concentrations for premature fatigue crack initiation. The lower pH value in H2SO4 would dissolve the protective passive surface film and enhance the synergism between corrosive environment and cyclic stresses leading to the shoter fatigue life by reducing the periods of stage I cracking and transition from stage I to stage II cracking. Adding pitting inhibitor in 3.5% NaCl solution can prevent formation of corrosion pits on specimen surface and extend HCF life. This is due to the fact that the inhibitor added in 3.5% NaCl solution can prevent pitting formation by reacting with the metal ions to form a protective film on specimen surface.
