Development of Fe-Al intermetallic phases in hot-dip Zn–6Al–3Mg alloy coating on steel sheets

B1. Advanced Steels
Hiroki Yokoi , Naoki Takata1, Suzuki Asuka1, Makoto Kobashi1
1 Department of Materials Process Engineering, Nagoya Univeristy
Introduction/Purpose: Hot-dip Zn–Al–Mg ternary alloy coatings exhibit superior corrosion resistance and their hot-dip galvanizing technologies are being applied to building steel parts. In the Zn–Al–Mg alloy coating on steel parts, the Al-rich Fe–Al intermetallic phase layer consisting of Fe4Al13 and Fe2Al5 phases are often formed. These intermetallic phases show brittle properties whereby controlled formation of the Fe–Al intermetallic phases is required to maintain the sufficient adhesiveness of the Zn alloy coating layer. In present study, we examined the formation sequence of Fe–Al intermetallic phases and their related growth process during the hot-dip galvanizing process.
Methods: Interstitial-free (IF) steel sheets were hot-dipped in a Zn–6Al–3Mg (wt.%) alloy melt at different temperatures (400, 460 and 500°C) and dipping times (from 2 to 3600 s) under N2/H2 atmosphere. The prepared samples were characterized by SEM, XRD, EDS and EBSD.
Results: Microstructural characterization revealed the initial formation of a continuous Fe4Al13 phase layer, followed by the local growth of Fe2Al5 phase toward both steel and Zn alloy melt sides during the hot-dipping process. After long-term hot-dipping, anomalous growth of the Fe2Al5 phase was accompanied by significant dissolution of Fe into the Zn alloy melt, resulting in the loss of thickness of the IF steel sheets. The observed formation sequence of Fe–Al intermetallic phases at the interface between solid Fe and liquid Zn alloy can be rationalized using a Zn–Al–Mg–Fe quaternary phase diagram calculated [1,2] in the present study.
Conclusions: Anomalous growth of Fe–Al intermetallic phase observed in the hot-dip Zn-6Al-3Mg alloy coating could be associated with a phase transition from Fe4Al13 to Fe2Al5 phases.
Selected references

[1] P. Liang, T. Tarfa, J. A. Robinson, S. Wagner, P. Ochin, M. G. Harmelin, H. J. Seifert, H. L. Lukas, F. Aldinger, Thermo. Acta 314 (1998) 87–110.
[2] J. Nakano, D. V. Malakhov, S. Yamaguchi, G. R. Purdy, CALPHAD 31 (2007) 125–140.