摘要
本研究的目的在探討2205雙相不銹鋼在硫酸/鹽酸混酸水溶液中之選擇性溶解現象，以及其對2205雙相不銹鋼之腐蝕疲勞裂紋萌芽行為的影響。由於雙相不銹鋼兩相的化學組成及結晶構造的不同，使得兩相在環境中的抗腐蝕能力及機械性質亦不盡相同。經動電位極化曲線量測及表面形貌分析結果可發現，2205雙相不銹鋼在硫酸/鹽酸混酸水溶液中會發生選擇性溶解現象；並且該現象會影響2205雙相不銹鋼在不同荷重頻率之腐蝕疲勞行為。

2205雙相不銹鋼在硫酸/鹽酸之混酸水溶液中經動電位極化曲線測試後發現，活性-鈍態轉換區是由兩個明顯且分離的活化峰所組成。經定電位蝕刻及表面形貌觀察後得知電位較低的活化峰主要為肥粒鐵相發生選擇性溶解，而較高的活化峰則為沃斯田鐵相發生選擇性溶解。此外，當硫酸及鹽酸的成分於0.25 M~2 M之間變動時，上述兩活化峰的峰值電位及最大電流密度值亦會隨之變動。但是當鹽酸濃度高於1.2 M時，兩活化峰之間的鑑別度則隨之下降。另外，兩相相界間因化學成分差異而引起的電位差異，會導致伽凡尼效應而造成晶界附近肥粒鐵相的加速腐蝕。

在發生選擇性溶解以及反覆施力的條件下，2205雙相不銹鋼表面發生優先腐蝕之組成相所形成的應力集中位置會因選擇性溶解作用而被消除，並導致腐蝕疲勞裂縫萌芽於腐蝕速率較慢的組成相。實驗結果發現，於開路電位及肥粒鐵相發生選擇性溶解之電位下，腐蝕疲勞裂縫主要萌芽於溶解速率較慢之沃斯田鐵相；於沃斯田鐵相發生選擇性溶解的電位下，腐蝕疲勞裂縫則萌芽於肥粒鐵相。

在選擇性溶解的條件下，荷重頻率對於雙相不銹鋼之腐蝕疲勞行為呈現複雜的效應。在較高的荷重頻率下，選擇性溶解可消除應力集中位置(或裂縫萌芽位置)，致使腐蝕疲勞裂縫不會萌芽於凹下之相，穿晶破裂是主要疲勞裂縫的成長模式。在較低的荷重頻率下，於肥粒鐵相發生選擇性溶解時，疲勞裂縫也可在肥粒鐵/沃斯田鐵相界間萌芽並傳遞。至於在低荷重頻率及沃斯田鐵相發生選擇性溶解之電位下，視選擇性腐蝕之程度不同，疲勞裂縫可以在肥粒鐵或沃斯田鐵相萌芽，應力集中效應是關鍵因素；在沃斯田鐵相發生選擇性溶解的條件下，裂縫大致以穿晶模式傳遞。在肥粒鐵或沃斯田鐵相發生選擇性溶解的電位下，初始穿晶破斷面會因選擇性溶解而呈現沿晶腐蝕的特徵。

Abstract
The purpose of this study was to investigate the selective dissolution and corrosion fatigue behaviors of 2205 duplex stainless steel (DSS) in H2SO4/HCl mixed acid solution. Due to the differences in chemical composition and crystalline structure between a and r phases, the corrosion resistance and mechanical properties were dissimilar. Potentiodynamic polarization measurement and surface morphology analysis show that selective dissolution of one of the constituent phases of 2205 DSS could occur at certain characteristic potentials in H2SO4/HCl solution. Furthermore, selective dissolution would affect the corrosion fatigue behavior of 2205 DSS at various loading frequencies.

Potentiodynamic polarization measurement indicated that there were two distinctly separated anodic peaks appeared in the active-to-passive transition region of 2205 DSS in H2SO4/HCl solution. After potentiostatic etching and surface morphology analysis, the anodic peak appeared at lower potential was associated with the preferential dissolution of a phase while that for r phase at a higher anodic peak potential. In the solutions with the concentration ranging from 0.25 to 2 M for H2SO4 and HCl, the anodic peak potential and the corresponding current density for selective dissolution of each phase were greatly influenced by the change of electrolyte concentration. However, the evolution of separate anodic peaks became less distinguishable when the concentrations of HCl exceeded 1.2 M. At open circuit potential (OCP), galvanic corrosion resulting from the potential difference of the constituent phases caused the enhanced corrosion of ferrite at the ferrite/austenite phase boundary.

Under selective dissolution and cyclic loading condition, stress concentration sites formed in the phase preferentially corroded, which induced by the intrusion/extrusion fatigue mechanism, would be eliminated by selective dissolution. Therefore, corrosion fatigue crack was prone to initiate in the phase with lower dissolution rate. At OCP and at the potential where a phase selectively dissolved, corrosion fatigue crack initiated in r phase. On the contrary, corrosion fatigue crack initiated in a phase at an applied potential at which r phase dissolved preferentially.

The effect of loading frequency on corrosion fatigue behavior of 2205 DSS was complicated under selective dissolution condition. At higher loading frequency, selective dissolution would eliminate stress concentration sites (or fatigue crack initiation site). Therefore, corrosion fatigue crack would not initiate in the concave phase. The cracks mainly propagated transgranularly. At lower loading frequency, corrosion fatigue crack could also initiate and propagate along a/r boundary at potential where a phase selectively dissolved. At the potential where r phase selectively dissolved and at a lower loading frequency, corrosion fatigue crack initiated and propagated transgranularly in either a or r phase depending on the extent of selective dissolution of r phase. The intensity of stress concentration developed was the main factor affecting the corrosion fatigue crack initiated in r phase or not. The initially transgranular mode of fracture surface became faceted as the crack continued to grow. This was attributed to the selective dissolution of either a or r phase under controlled potential condition.

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