鐵鉻（鎳）不銹鋼於硫氯混合鹽之熱腐蝕

The Hot Corrosion of Fe-Cr-(Ni) Stainless Steels with pre-coating Sulfate/Chloride Mixtures

本研究以430、304及310不銹鋼於表面分別沉積2mg/cm2為基礎之NaCl，及附加Al2(SO4)3或AlCl3使總重量為3 mg/cm2的NaCl/AlCl3和NaCl/Al(SO4)3混合鹽，進行750℃和850℃之熱腐蝕試驗，藉此了解在不同腐蝕氣氛與合金元素之交互影響及作用。結果顯示，二種合金於各種混合沉積鹽之熱腐蝕，NaCl仍為主要的腐蝕源，而硫之作用則在於促使沿晶腐蝕之發生。在單純NaCl之熱腐蝕、304及310呈現典型內侵蝕，其侵蝕深度隨溫度增加而提高，430則為均勻腐蝕之形態；310因含鉻量最高，故其金屬損失最低，304則因內侵蝕而呈現最大之總侵蝕深度。於NaCl/AlCl3之熱腐蝕，合金的腐蝕形態及侵蝕深度、相似於單純NaCl，惟因NaCl/AlCl3之共晶溫度甚低，以致引發更多的Cl-參與反應，而使內侵蝕孔洞最為粗大。在沉積NaCl/Al2(SO4)3狀態時，因硫與氯同時與合金反應，且因晶界處兼具高能量與擴散快之特性，進而導致304於750℃出現大量沿晶腐蝕，並造成極大之內侵蝕深度；但在850℃時，304之熱腐蝕行為則由內侵蝕轉變為金屬損失之型式。

The hot corrosion behavior of the three stainless steels containing 430, 304, and 310 was studied at 750℃ and 850℃ in air with 2mg/cm2 NaCl or 3mg/cm2 NaCl/AlCl3 or NaCl/Al2(SO4)3 having the weight ratio of 2:1. Effects of deposited salts and alloying elements on the hot corrosion of the steels were investigated. The results showed that among those different salt deposits, NaCl was the main source of corrosion. In addition, sulfur plays an important role in enhancing intergranular corrosion. The morphologies of 304 and 310 coated with 100% NaCl showed typical internal attack, while 430 exhibited uniform attack. The total depth of attack increased with increasing temperature for both 304 and 310. The 310 which with the highest Cr content showed less metal loss, while 304 with a large amount of subscale attack exhibited the deepest total depth of attack. The corrosion morphology and depth of attack of alloys with NaCl-AlCl3 deposits are similar to those of NaCl deposit. However, due to the low eutectic temperature of NaCl-AlCl3 and more Cl- involved in the molten salt, alloys exhibited the severe internal oxides or voids. For alloys coated with NaCl/Al2(SO4)3, S2- and Cl- may react with alloys simultaneously. In addition, the diffusion of S2- and Cl- at the grain boundary are much faster than those in the grain, resulting in the intergranular corrosion of 304. At 750℃, comparing to the other two salts, 304 and 310 revealed lesser metal loss in NaCl/Al2(SO4)3 deposits. The hot corrosion behavior changed from the internal attack to the metal loss at 850℃.