Microstructural Characterization of Fe-Al InhibitionLayer in Hot-Dip Galvanized Sheet Steel

• Main Authors: Chih-Kai Chen, 陳志凱
• Other Authors: 林招松
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/84482384885167455592

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === Hot-dip galvanized and galvanealed steel sheets have excellent corrosion resistance, and can be found in body panels and related components in the automobile industry. Large amounts of hot-dip galvanized steel plates are also used in structure, home appliances, and tool applications. To optimize the hot-hot dip galvanize process, it is essential to develop a quick and effective microstructural evaluation method. In this study, an effective technique to determine the effect of aluminum content in the zinc bath, bath temperature, strip entry temperature, and dip-time on the interface microstructure has been established. Results showed that Fe-Zn intermetallic compound can be observed in chemically color etched cross-sectional OM samples: ζ and δ were observed from the zinc coating towards the steel substrate. However, Γ and Fe-Al phases were not observed under OM due to its limited resolving power. In contrast, via cross-sectional TEM, selected area diffraction, EDAX, and GA-XRD analyses, higher aluminum content can be found at the zinc-steel interface in the form of granular and lamellar Fe2Al5 intermetallics. Also, open circuit potential can be a time-saving technique to analyze the presence of Fe-Al phase. Furthermore, the non-uniform corrosion behavior can be observed in the open circuit potential curve. Selective layer on the coating can be removed while performing open circuit potential in 4wt% HCl to observe each layer morphology and crystallinity in SEM and GA-XRD. In summary, when bath temperature, strip entry temperature, and hot-dip time increase, the extent of transformation from Fe-Al to Fe-Zn phases increases, in low aluminum content zinc bath (0.12 wt%). In mid-to-high aluminum content zinc bath (0.16 wt%, 0.20 wt%), in the condition that Fe-Al inhibition layer have not transformed to Fe-Zn intermetallics, the higher the bath or strip entry temperature, the higher thickness in the Fe-Al inhibition layer.