熱交換器用鋼料硫氯混合鹽熱腐蝕之反應機制及形態

The Reaction Mechanism and Morphology of Heat-exchange Steel After Hot Corrosion with Pre-coating NaCl/Na2SO4 Mixtures

本研究選用SB450、STPA24及STPA26三種不同鉻含量之鋼料，經空氣單純氧化及沉積三式2mg/cm2 NaCl/Na2SO4腐蝕鹽於750℃、850℃之高溫腐蝕，藉由OM、SEM腐蝕形態之觀察及XRD生成物鑑別分析等方式探討其熱腐蝕行為。實驗結果顯示：合金於空氣單純氧化，主要為鐵元素之氧化，氧化物由外至內的順序為Fe2O3、Fe3O4、Fe1-xO，相較於熱腐蝕之試片，氧化皮膜最為緻密。在含有NaCl沈積之熱腐蝕環境，由於氯氧化及其後之氯化/再氧化的反應機制，導致氧化皮膜呈鬆散且多孔狀。於完全Na2SO4沈積鹽熱腐蝕，類似於空氣單純氧化，但其皮膜較空氣單純氧化呈多孔狀。愈高的反應溫度助長NaCl之揮發，於含氯化鈉之熱腐蝕，隨應反應時間之增加，反應機制逐漸由單純氧化所主導，因此氧化皮膜逐漸變厚且緻密。含鉻之合金受氯化之反應機制較為明顯，其氧化皮膜較鬆散。各式合金隨溫度上升、時間增加及Na2SO4之沉積，Fe1-xO之殘留量會增加；但試片鉻含量以及沈積鹽中NaCl之增加，Fe1-xO成長將受抑制。

The hot corrosion of three Fe-Cr alloy (containing SB450 and STPA24, STPA26) was studied at 750℃ and 850℃ with pre-coating NaCl/Na2SO4 mixture. The hot corrosion behavior was evaluated in terms of scale morphology by OM and SEM, XRD. The results showed that Fe is the main element which be oxidized in simple oxidation. The oxides from outside to inside are Fe2O3, Fe3O4 and F1-XO, and the scale in simple oxidation is denser than in hot corrosion. Because of the oxychlorination took place when deposited with NaCl, the scale became more porous. The hot corrosion with 100%Na2SO4 deposits is similar to the simple oxidation, but the scale is looser than that in simple oxidation. As temperature and time increased, the dominate corrosion mechanism tends to transfer to simple oxidation, and the scale become denser and thicker. Due to the Cr in alloy is easy to be effected by the oxychlorination, the scale become more loose for the Cr-containing alloys. As the temperature and exposure duration increased, the amount of Fe1-XO will increase. However, the Cr in alloy and the NaCl deposits will suppress the formation of Fe1-XO.