Formation of out-burst structure in hot-dip Zn-5mass%Al alloy coating

The formation of the "out-burst" products which is consisted of Al-Fe intermetallic compounds (IMCs) grown locally and abnormally, has been studied to compare with the structure of thin alloy layer normally formed on the sheet steel in a hot-dip Zn-5mass%Al alloy coating at the bath temperature of 450 degrees C. The formation of the out-burst products is not due to the localized growth of thin alloy layer consisted of mainly Zn containing Al3Fe(Al13Fe4), but due to the growth of Al-Fe IMCs with mainly Zn containing Al5Fe2 formed at ferrite grain boundaries near the steel surface. These Al-Fe IMCs are considered to be changed from Fe-Zn IMCs which have been formed by the grain boundary diffusion of Zn. And during changing to Al-Fe IMCs, Zn discharged from Fe-Zn IMCs further diffuses to the adjacent ferrite grain boundaries and also into the adjacent ferrite grains. As a result, Fe-Zn IMCs are formed around Al-Fe IMCs which have been formed at the grain boundaries, and changes again to Al-Fe IMCs. This growth behavior of Al-Fe IMCs formed at the grain boundaries is induced From the results obtained in the previous works that the changing rate from Fe-Zn IMCs to Al-Fe IMCs in a molten Zn-5mass%Al alloy bath at 450 degrees C is very high.