Distribution of aluminum in hot-dip galvanized coatings

Abstract

Hot-dip galvanized panels of low-carbon (LC) and interstitial-free (IF) steels were produced in a laboratory simulator with an average coating mass of 60 g/m 2 . Three pot aluminum levels were used, viz., 0.10 pct (by wt), 0.15 pct, and 0.18 pct. Metallography, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize coating and base steel microstructures. Wet chemical analysis and scanning transmission electron microscopy (STEM) were employed for compositional analyses. The aluminum content of the melt was found to be the predominant factor influencing the distribution of Al in the coating. At 0.18 pct melt aluminum, Al is partitioned between the aluminide inhibition layer at the coating-steel interface (ϳ80 pct) and the zinc overlay (ϳ20 pct). At 0.15 pct, it is partitioned among the aluminide layer (ϳ75 pct to 80 pct), zinc-iron (FeZn 13 , ) intermetallic layer (ϳ5 pct to 15 pct), and the coating overlay (ϳ10 pct). At 0.10 pct, the aluminum is divided almost equally between the overlay and the zinc-iron intermetallics. At the two lower aluminum levels is the distribution marginally influenced by the steel grade. The was found to not preferentially nucleate at the ferrite grain boundaries. When both the aluminide and occurred at the coating-steel interface, the particles appeared near discontinuities and thinner regions in the aluminide layer. The coating, relative to the melt, is enriched in aluminum because of its concentration in the aluminide and in the zinc-iron intermetallics. This enrichment increases with melt aluminum through an increase in the aluminum content of the aluminide layer and not of its thickness. In addition, a few tens-of-nanometers-thick layer enriched in aluminum, oxygen, and iron is observed on the outer surface of all coatings. The aluminum content in this layer also increases with an increase in the melt aluminum, but it contributes negligibly to the coating's content because of its extreme thinness.

Fe Solubility in the Zn-Rich Corner of the Zn-Al-Fe System for Use in Continuous Galvanizing and Galvannealing

“…[7], is now in good agreement with Table II. The intermetallic species were identified by EDX, the general compositions of which can be seen in Table V.…”

“…[6], a small third term was added, as shown in Eq. [7]: The effect of this addition is shown in Figure 4. The effect is to shift the maximum toward the Al side of the figure, but the Al-rich region remains virtually unchanged.…”

“…[1][2][3][4] Changes in Al concentration, for example, from 0.10 to 0.30 wt pct, have a significant impact on the structure and properties of the resultant coatings by influencing the formation of the so-called ''inhibition layer,'' a 50-to 250-nm-thick layer of the g-Fe 2 Al 5 Zn X intermetallic at the substrate/bath interface. [1,[5][6][7] This layer acts as a transient barrier to further reactions between the liquid Zn overlay and the steel. The effectiveness of Al in this role is complicated by the partitioning of Al between the coating and intermetallic compounds that form within the galvanizing bath.…”

“…The effectiveness of Al in this role is complicated by the partitioning of Al between the coating and intermetallic compounds that form within the galvanizing bath. [1][2][3][4][5][6] At the low bath dissolved Al concentrations commonly employed in the production of GA coatings (i.e., less than 0.14 wt pct at 460°C), the g-Fe 2 Al 5 Zn X layer is thin or discontinuous [1,[5][6][7] and inhibition breakdown (the dissolution/destruction of the g-Fe 2 Al 5 Zn X layer) occurs rapidly. This facilitates the transformation of the Zn overlay to the desired sequence of Fe-Zn intermetallics during the subsequent galvannealing operation.…”

“…[1,5,6] The bath dissolved Al concentration plays a central role in the kinetics of the nucleation and growth process of the inhibition layer, [9][10][11] as well as in the overall Al concentration in the coating. [7,12] In particular, it is believed that a high Al concentration in the coating has a detrimental effect on the longevity of welding electrodes in spot resistance welding, a commonly employed technology in manufacturing processes involving galvanized or GA steel sheet. [13] The intermetallic particles found in continuous galvanizing line (CGL) baths are determined by the Al concentration in the molten Zn: f-FeZn 13 at low concentrations; d-FeZn 10 Al Y at intermediate concentrations; and g-Fe 2 Al 5 Zn X at higher concentrations, [1][2][3][4][14][15][16][17][18] as illustrated in Figure 1.…”