During the hot dip galvanization process, a thin continuous Fe2Al5 layer at the zinc-steel interface prevents zinc-steel diffusion, which leaves the coating as essentially pure zinc. For this layer to form, the steel surface must be free of oxides.
Compared with conventional steels, advanced high strength steels rely on greater use of manganese (Mn), silicon (Si), and aluminum (Al), among other additions in alloys that are likely a minimum of 95% iron.
The typical annealing atmosphere used in a continuous galvanizing line has a dew point of around –40 °C.S-129 This dew point is sufficient to reduce any iron oxides on the surface, but the Mn, Si, and Al diffuse to the surface and oxidize.
This surface oxide layer, on the order of 40 to 50 nm thick, prevents the Fe2Al5 layer from forming. In addition, these oxides reduce wettability such that a continuous uniform layer of zinc cannot form during galvanizing. This leads to bare spots and poor coating adhesion.J-28
External SiO2 films are particularly detrimental to wettability. Preventing the formation of an external SiO2 layer promotes reactive wetting in the zinc galvanizing bath, and requires controlling the Si/Mn ratio to a maximum of 0.5.S-129
Increasing the dew point in the heating and soaking sections of the furnace from –40 °C to +10 °C is also effective in preventing the oxides from forming on the external surface. Instead, they will form internally just beneath the surface. This change results in a sufficiently clean surface for good phosphating and galvanizing. This strategy is critical in the production of higher strength galvanized advanced high strength steels.J-28
On the other hand, galvannealing requires there to be a diffusion reaction between zinc and iron. However, external oxidation of Si and Mn retards this diffusion. As such, the elevated dew point to promote internal oxidation is a critical strategy in the production of galvannealed coatings found on advanced high strength steels.J-28