含稀土金屬耐候鋼經加速大氣腐蝕模擬試驗後銹層性質分析

Characterizations of Rusts Formed on Weathering Steels Containing Rare Earth Metals after Accelerated Atmospheric Corrosion

利用大氣腐蝕加速模擬試驗方法，對含有稀土金屬元素的耐候鋼施以乾溼循環暴露及烘烤處理，其銹層化學組成以及微觀組織以XRD、SEM、EDS等加以分析，而其電化學穩定性則以動電位極化曲線之測定來分析，以探討合金元素對銹層生成過程的影響與作用。試驗結果顯示，稀土金屬含量0.0016 wt%耐候鋼裸材有較負的腐蝕電位。經過乾溼循環暴露試驗後的耐候鋼銹層組成為γ-FeOOH、Fe3O4和α-FeOOH；銹層橫截面形貌顯示其具有內外兩層結構存在，且元素分佈有差異，然而銹層尚未穩定，保護性不佳。乾溼循環暴露試驗後再施以150℃×15天烘烤處理，銹層組成改變，由非晶質銹及Fe3O4組成；銹層缺陷及裂痕顯著減少，且銹層與基材介面的附著性獲得提升。烘烤處理後再進行反覆乾溼循環試驗，耐候鋼銹層轉為結晶相，銹層緻密性及附著性更佳。經不同模擬暴露試驗之耐候鋼在0.05 wt% NaCl水溶液中所測得的極化曲線可以反映各種不同銹層的穩定性。

The chemical compositions and structures of rusts formed on the surfaces of weathering steels (WSs) containing rare earth (RE), after laboratory-accelerated tests with and without baking treatments, were characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Potentiodynamic polarization analyses were employed to evaluate the stability of the rusts formed under different conditions. The effect of rare earth element on the atmospheric corrosion resistance of the steels was explored. Experimental results showed that the bare steel containing 0.0016 wt% RE had the lowest corrosion potential in a 0.05 wt% NaCl solution. After an alternating wet/dry exposure in salt fog generating from a 0.05 wt% NaCl solution, the as-formed rust on the steel consisted of α-FeOOH、γ-FeOOH and Fe3O4, while the inner and outer scales had different chemical compositions and structures. After a further baking treatment at 150 oC for 15 days, the rust underwent a phase transformation from crystalline to amorphous phase and Fe3O4. In addition, the defect density decreased, and the adhesion strength between the substrate and the rust increased. A further cyclic wet/dry exposure after the baking treatment caused another phase transformation of the rust from amorphous to crystalline phase again. A more compact and better adherence between the substrate and the rust was observed. The potentiodynamic polarization curves of various steels obtained in 0.05 wt% NaCl solutions revealed the stability of the corresponding rusts formed under different exposure tests.