304L不銹鋼於含鹽間隙內的穿晶應力腐蝕現象

Study of Trans-granular Stress Corrosion Cracking of 304L Stainless Steel under Saline Crevice Environment

本研究旨在探討SS 304L不銹鋼間隙腐蝕環境下，對後續發生應力腐蝕之影響，以作為壽限評估之基礎參數。本研究於SS 304L不銹鋼試片表面均勻噴上海鹽溶液並以扭力起子鎖上鐵氟龍耦接墊片，放置於溫度45℃及相對溼度45%的環境溫濕度控制箱中。
研究結果顯示，試片於耦接墊片覆蓋處較未覆蓋處有較大的腐蝕面積，且試片表面於耦接墊片之外加壓應力下，粗糙表面腐蝕達1000小時及光滑表面腐蝕達2100小時後，試片表面出現穿晶應力腐蝕裂縫。從剖面金相觀察裂縫發現，細小的二次裂縫在材料內以不規則的鋸齒狀成長，裂縫分岔而無特定的裂縫成長方向，本文旨在探討腐蝕機制。

This work was to study the corrosion behavior of 304L stainless steel in crevice environment with the deposition of sea salt. Poly-tetra fluoroethylene (PTFE) was used as a crevice former by a torque applied on the SS 304L surface. The assembled test devices were kept at the ambient temperature of 45 ℃with a relative humidity of 45%.
The results show that the corrosion is more severe for the area with crevice former than the open area without crevice former. In addition, trans-granular stress corrosion cracking was found in the severe corrosion area, where the specimen of rough surface was kept in test environment for 1000 hours and the specimen of polished surface for 2100 hours test. The cross-section metallographic images show secondary micro-cracks, of which reveal multiple directions and branches, develops inside materials with zigzag shapes. Also, the corrosion mechanism is discussed in this study.