熱交換器用鋼料硫氯混合鹽熱腐蝕後於氯化鈉水溶液之腐蝕

The 3.5wt% NaCl Aqueous Corrosion of Heat-exchange Steel After Hot Corrosion with Pre-coating NaCl/Na2SO4 Mixture

本研究選用SB450、STP A24及STP A26三種鋼料，經空氣單純氧化及沉積三式2mg/cm2 NaCl/Na2SO4腐蝕鹽於750℃、850℃進行高溫腐蝕後，藉由動態極化曲線、腐蝕形態之觀察及生成物鑑別分析等方式，討探鋼料於高溫腐蝕後在常溫5%NaCl電解液環境之腐蝕行為。實驗結果顯示：合金原材於之icorr隨鉻含量之增加而減少，腐蝕電位則隨鉻含量之增加而上升，呈貴重(noble)之趨勢。其中STP A26之icorr最小且明顯鈍化(passivation)，整體而言具有最佳的抗蝕性。當沈積鹽含有NaCl時，由於氯氧化反應，氧化皮膜呈多孔狀而易產生間隙腐蝕，以致icorr較原材高2~3倍、循環極化曲線再現性變差，腐蝕電位偏低，接近合金原材。於空氣單純氧化之氧化皮膜較為緻密，故不易發生間隙腐蝕，以致icorr與原材相近，循環極化曲線再現性僅次於原材，惟抗蝕性較前者佳。此外，合金受含NaCl之熱腐蝕時，鉻較鐵易受氯之作用，含鉻愈高之合金其氧化層愈鬆散多孔，以致icorr相較於原材明顯增加。在NaCl及Na2SO4混合鹽之沈積環境中，隨熱腐蝕溫度及時間之增加，高溫腐蝕氣氛將轉變為空氣單純氧化或完全Na2SO4之沈積，反應趨向氧化反應而有較緻密之氧化皮膜，以致icorr降低，腐蝕電位上升。此外，當試片有Fe1-xO殘留時，由於Fe1-xO為高溫殘留相，且具高缺陷結構，以致icorr明顯高於無Fe1-xO之試片。

The electrochemical corrosion behavior of three Fe-Cr alloys (containing SB450, STP A24, and STP A26) after hot corrosion with 2mg/cm2 NaCl/Na2SO4 deposits at 750℃ and 850℃ were studied in 3.5% NaCl aqueous electrolyte. The corrosion mechanism was evaluated in terms of scale morphology by OM and SEM, XRD, and dynamic polarization curve measurements. The results showed that the higher Cr content in alloy the lower icorr and the higher Ecorr poetntial which tends to be more noble. The lowest icorr and a passivation characteristics, STP A26 displayed the best corrosion resistance of the three alloys. Because the oxy-chlorination took place when hot corrosion with the NaCl deposits, scales become more porous and easy to corrode, leading to an increase of icorr by a factor of 2 as compared to the raw materials. Thus, the reproduction of cyclic polarization curve gets worse, and the Ecorr decreased. On the other hand, the scales are more dense in simple oxidation, so that crevice corrosion is more difficult to occur. As a result, icorr will be close to that of the raw material, and the reproduction of cyclic polarization curve is second to raw materials, however the corrosion resistance is highly improved. As increasing temperature and exposure duration, the dominating hot-corrosion environment change to simple oxidation in air or Na2SO4-deposit. The overall reaction is favorable to form dense oxide scales, which reduces the icorr but increases the Ecorr. However, the high-defective Fe1-XO scale retained on the alloy cause a significant increase in the icorr as compare to the specimen without Fe1-XO.