本研究旨在探討市售之201及444 不銹鋼(S.S.)之孔蝕敏感性，並以304 S.S.做為對照組，分別在25、250及2500 ppm Cl-及0.1 M Na2SO4 + 2500 ppm Cl-兩種水溶液中進行孔蝕敏感性的比較。在0.1 M Na2SO4 + 2500 ppm Cl-水溶液中，將原材及固溶熱處理的試片以動電位極化試驗搭配試驗後的表面形貌觀察，來決定臨界孔蝕溫度(CPT)，藉此比較晶粒大小對孔蝕敏感性的影響；在25、250及2500 ppm Cl-水溶液中，量測三種原材之動電位循環極化曲線，比較不同材料的Enp，並觀察蝕孔的表面及橫截面形貌，最後探討不同材料之間造成耐孔蝕性差異的原因。
以動電位CPT決定法量測三種材料之CPT，結果顯示，在0.1 M Na2SO4 + 2500 ppm Cl-水溶液中，201、304、444 S.S.原材之CPT分別為36、35、36 oC；經熱處理後CPT分別下降為30、30、28 oC，在0.1 M Na2SO4 + 2500 ppm Cl-水溶液中，三種材料之耐孔蝕性質並無明顯差距，顯示硫酸鈉具有一定的抑制孔蝕效果，從有無添加硫酸鈉之極化曲線比較也能獲得證實。各材料經1050 oC持溫30分鐘的固溶熱處理後的平均晶粒大小上升，因而導致孔蝕敏感性上升，這是由於粗晶材料中的Cr擴散速率比細晶的慢，較少Cr擴散至表面形成鈍化膜，所形成的鈍化膜化學性質較不穩定。從孔蝕起始位置的觀察得知，蝕孔在晶粒內及晶界皆會生成，常常起始於非金屬介在物，像是碳化物或氧化物，將動電位極化試驗後的試片以SEM觀察，在低於CPT之溫度沒有觀察到蝕孔產生，而在高於CPT的情況下，觀察到的蝕孔大小多在10 μm以下。
在25、250及2500 ppm Cl-水溶液中，隨著溶液中添加的氯離子濃度增加，三種材料的孔蝕起始電位Enp與孔蝕保護電位Epp皆明顯下降，顯示孔蝕敏感性上升。2500 ppm Cl-水溶液中，三種材料皆發生孔蝕，蝕孔大小約50 μm；250 ppm Cl-水溶液中，只有201 S.S.表面有大型蝕孔；25 ppm水溶液中三種材料皆只有10 μm以下的小型蝕孔，顯示氯離子濃度較低時，金屬離子周圍不容易吸引到足夠的氯離子與之反應。250 ppm Cl-水溶液中，444 S.S.之Enp略高於304 S.S.，但十分相近；2500 ppm Cl-水溶液中，304 S.S.之Enp略高於444 S.S.。在25 oC下，2500 ppm Cl-水溶液中，以SEM觀察試驗後的蝕孔橫截面形貌，444 S.S.的蝕孔發展深度最淺，且無向水平方向發展的趨勢，孔蝕敏感性最低；而304 S.S.與201 S.S.在此環境下蝕孔向下發展最深，並有向水平方向發展的趨勢，易產生較大的腐蝕量，。從試驗後表面及橫截面觀察結果顯示，在氯離子水溶液環境中，材料的耐蝕性差異，耐孔蝕性質444 S.S.＞304 S.S.＞201 S.S.。從化學成分差異進行比較，444 S.S.相較於304 S.S.，因含有1.92 wt %的Mo元素，Mo會在表面附近形成MoO42-，改變鈍化膜的電性，進而排斥侵蝕性陰離子，使鈍化膜更穩定；201 S.S.相對於304 S.S.，含有較高量的Mn，Mn是一種活性較高的元素，容易氧化，且其氧化物結構鬆散，無法有效保護內層，易被環境中的溶液入侵。

In this study, we investigated the pitting corrosion susceptibility of AISI 201 and 444 S.S., took 304 S.S. as the control group. The solution used in this research were 25,250, and 2500 ppm Cl- and 0.1 M Na2SO4 + 2500 ppm Cl- aqueous solution. The determination of the critical pitting temperature (CPT), observation of the pit morphology and the comparison of pitting initiation potential (Enp) are the main means to estimate the pitting corrosion susceptibility.
In 0.1 M Na2SO4 + 2500 ppm Cl- aqueous solution. The CPT of as-received materials of 201, 304 and 444 S.S. are 36, 35 and 36 oC. The CPT of heat treatment samples are 30, 30 and 28 oC. After the heat treatment, the grain size grew. Because the grain growth influence, the passive film on surface contain less Chromium. That leads to the rise of the pitting corrosion susceptibility. With surface observation, we found that is no pit on sample surface below CPT. With temperature higher than CPT, some pits could be found. The magnitude of it were about 10 μm.
In 25, 250, and 2500 ppm chloride solution, the Enp and Epp dropped with chloride content increased. In 2500 ppm chloride content, large pits were found on all three materials. The diameter were about 50 μm. The Enp of 304 S.S. is higher than 444 S.S. In 250 ppm chloride solution, large pits were only found on 201 S.S. The Enp of 304 and 444 S.S were very close. In 25 ppm chloride solution, there are only small and swallow pits on all three materials. The magnitude of them were mostly smaller than 10 μm. From observation of cross section of pit morphology. We found the pits on 201 S.S. and 304 S.S. grew deeper and likely to grow horizontally. While the pits on 444 S.S. are shallow and unlikely to grow horizontally. Over all, in chloride solution, the pitting corrosion resistance 444 S.S.> 304 S.S.>201 S.S.

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