預鍍鎳對低碳鋼熱浸鋁化層其顯微結構與組成相之影響

Effect of Nickel Pre-plating on the Microstructure and Phase Constitution of a Hotdipped Aluminide Coating on Mild Steels

本研究採用預鍍鎳之低碳鋼熱浸於670℃之純鋁鋁湯。藉由掃描式電子顯微鏡（scanning electron microscope, SEM）、X-ray繞射（X-ray diffraction, XRD）、能量分散光譜儀（energy dispersive X-ray spectroscopy, EDS）與電子背向繞射（electron backscatter diffraction, EBSD）技術，分析預鍍鎳對低碳鋼熱浸鋁化層其顯微結構與組成相之影響。研究結果顯示，低碳鋼上20μm之預鍍鎳層在經過熱浸5秒後將溶解為12μm，並且在鎳層上方形成約5μm之連續Ni2Al3 + NiAl3 + （Fe,Ni）2Al9介金屬層與散布於純鋁塗層中的NiAl3相。當熱浸時間增加至60秒時，部分鎳鍍層被完全消耗完，使得Ni-Al相開始轉變為FeAl3與（Fe,Ni）2Al9相。隨著熱浸時間持續增加，鋁化層將由外側少量的FeAl3與內側主要的Fe2Al5相組成。對照沒有預鍍鎳之低碳鋼熱浸鋁化層可以發現，在經過長時間熱浸後，預鍍鎳之介金屬層其組成相及厚度與無預鍍鎳之介金屬層相同。

A nickel pre-plated mild steel was coated by hot-dipping in a molten bath containing pure Al at 670 ℃. The effect of the pre-plated nickel layer on the microstructure and phase constitution of the aluminide layer was investigated using a combination of scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). The results showed that the thickness of the nickel pre-plating layer was reduced from 20 μm to 12 μm due to dissolution after a hot-dipping of 5 seconds. Also, a continuous layer, composed of Ni2Al3 + NiAl3 + (Fe,Ni)2Al9, formed above the nickel plating layer, while a small amount of NiAl3 phase scattered in the aluminium topcoat. In the aluminide layer after a hot-dipping of 60 seconds, a portion of the nickel plating layer was consumed, leading to phase transforms from Ni-Al to FeAl3 and (Fe,Ni)2Al9. As the dipping time increased, the aluminide layer was composed of FeAl3 in a minor amount and Fe2Al5 in a major amount. In comparison with the mild steel hot-dipped with aluminide but without nickel pre-plating, the phase constitution and the thickness of the intermetallic layer with nickel pre-plating at a long dipping time were the same as those in the layer without nickel pre-plating.