304L不銹鋼SCC之裂縫周邊應變分佈研究

Study on Local Strain of 304L Stainless Steel around Stress Corrosion Cracking (SCC)

應力腐蝕龜裂（SCC）一直是防蝕的重要議題，此議題的討論主要遷涉到材料、週圍環境、及應力三大議題。在大氣環境下，由於腐蝕速率較慢，應力的影響相對明顯。特別是商用不銹鋼棒材及板材，在要求表面保有金屬光澤，又必需於最後製程進行矯直及平整的情況下，表面殘留應力，往往是存在的。本研究於SS 304L不銹鋼試片表面均勻噴灑海鹽溶液及乾燥後，以扭力施加於試片上，放置於溫度45℃及相對溼度45%的環境溫濕度控制箱中。長期腐蝕實驗後試片，以背向散射電子繞射（EBSD）進行研究。結果顯示，試片表面晶粒有較高的殘留應變，而其中腐蝕坑及裂縫週邊，特別是裂縫分岔處，晶粒的殘留應變較大，裂縫尖端的殘留應變則不如裂縫中段明顯。

Stress corrosion cracking (SCC) was an issue for preventing corrosions, where materials, environments, and stress are often discussed to evaluate SCC susceptibility. As the corrosion rate of SCC is very slow under atmosphere environment, the control of residual stress was important; especially, the commercial products of stainless steel possess residual stress, which thanks to the requirement of product straightness, flatness, as well as brightness. This study focuses on the residual stress of corrosion surface. The specimens were deposited with 0.1 g/m2 sea salt and kept at the temperature of 45 ℃ and the relative humidity of 45%. After the long term corrosion test, specimens were studied by electron backscatter diffraction (EBSD). The results show local plasticity of a grain as well local deformation increase with the present of corrosion pits; the grain around crack branch has a higher deformation than the grain around the crack front.