矽對熱浸鋁化鋼之高溫氣化行為的影響

Effect of Si on High-Temperature Oxidation Behavior of Hot-Dip Aluminized Low Carbon Steel

本研究探討低碳鋼（SB450）各經熱浸純鋁／鋁-7wt%矽／鋁-10wt%矽塗層後，於750和850℃靜置空氣之高溫氧化行為。結果顯示，矽之添加至熱浸鋁化塗層，會影響其顯微結構，使得合金化層轉換為細窄與平坦，與合金熱浸純鋁之合金化層厚度相比較，添加矽至鋁化塗層中使得其合金化層厚度從50μm縮小至15μm。三種鋁化塗層經組成相分析後，由表面至基材側依序為Al、FeAl3和Fe2Al5。經各別高溫氧化實驗後，三種鋁化塗層均於表面上形成一層連續性的氧化鋁皮膜；隨著氧化時間的增加，純鋁和鋁矽鍍層之合金化層厚度仍舊各自保持為90和70μm，並且伴隨著Kirkendall孔洞在合金化層與基材及與皮膜的界面處生成。藉由矽添加至鋁化塗層中，顯示隨著矽含量的增加能夠有效地抑制合金化層之FeAl2和Fe2Al5組成相轉換成為FeAl之速率。三種鋁化塗層試片於750℃之氧化動力學皆依循著拋物線率，同時碳鋼可藉由熱浸鋁化塗層的施加，將其在相同的氧化環境下的氧化增重量降低為1/10倍。HDAl-10Si經56小時高溫氧化後具有相對較高之氧化速率，歸因於脆性的Fe-Al-Si合金化層之破裂與內氧化現象的產生。

Low carbon steel was coated by hot-dipping into a molten bath containing pure Al, Al- 7wt%Si, Al-10wt%Si, respectively. Oxidation was studied at 750 and 850 ℃ in static air. After hot-dip treatment, it was shown that addition of Si in the molten Al bath was responsible for the formation of a thinner intermetallic layer and smoother interface between the aluminide layer and the steel substrate. The thickness of intermetallic layer was decreased from 50 to 15 μm. All the coating layers consisted of three phases, where Al, FeAl3, Fe2Al5 were detected from the external topcoat to the aluminide/steel substrate. After high temperature exposure, a continuous alumina scale was detected on the surface of the aluminide layer. The thickness of the aluminide layer was about 90 and 70 μm in each temperature test for Al and Al-Si specimens, respectively, while voids resulting from the Kirkendall effect formed at the interface between the aluminide layer and the steel substrate. The additions of Si into the coating layer were effective to restrained the transformation speed of FeAl2 and Fe2Al5 to FeAl. The oxidation kinetics followed a parabolic rate law at 750 ℃ for all kinds of coating specimens, while the weight gain of the hot-dip aluminizing coating on the surface of carbon steel is only 1/10 that of the carbon steel under the same oxidation condition. HDAl-10Si specimen has relativity higher oxidation rate for up to about 56 h. Due to the brittleness of the Al-Si aluminized coatings on the carbon steel severe cracking and internal oxidation occurred.