不同水媒環境下之AISI 347不銹鋼腐蝕疲勞行為

Fatigue Behavior of AISI 347 Stainless Steel in Various Aqueous Environments

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。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中有較明顯之影響，在此兩種環境中的高週疲勞壽命，皆有明顯降低現象，其中又以H2SO4水溶液中的降幅最大，而室溫純水、80℃純水及3.5% NaCl加抑制劑中之高週疲勞壽命與空氣中之差異則不顯著。而在疲勞裂縫成長實驗（Stage II）顯示：空氣中及五種水溶液中之裂縫成長速率差異不大。由此結果得知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。5% NaCl水溶液主要的影響，在於氯離子使試棒表面形成蝕恐造成應力集中效應，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水溶液中添加抑制劑，可以抑制蝕孔生成於平滑試棒表面，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. Results showed that the fatigue strength of AISI 347 in H2SO4 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 shorter 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.