Further Insight into Interfacial Interactions in Iron/Liquid Zn-Al System

Abstract

In the galvannealing process, steel strips are immersed in molten zinc containing 0.100 to 0.135 wt pct Al at 450°C. The coating obtained is composed of a thin intermetallic compounds' layer called the inhibition layer (200 nm) covered with a thick zinc layer (10 lm). The nature of this inhibition layer has been investigated here for a galvanizing bath with a low Al content. The inhibition layer formed on industrial low-alloyed steels was characterized by transmission electron microscopy and atom probe tomography. The inhibition layer is composed of a thin Fe 2 Al 5 Zn x layer (20 nm), covered with a thicker d layer (200 nm). The Fe 2 Al 5 Zn x layer is discontinuous at the lowest bath Al content. Small precipitates (20 nm in diameter) with a stoichiometry corresponding to Fe 3 Al-containing Zn were also found for the first time in the d phase. The microstructure of the inhibition layer can be described with diffusion paths drawn in the Al-Fe-Zn ternary section at 450°C. This means that all interfaces of the inhibition layer are at thermodynamic equilibrium. The Fe 2 Al 5 Zn x layer is formed on the steel surface before the d layer. The nucleation and growth of the Fe 3 Al-Zn particles probably occur in the liquid metal at the same time as d.

High-Temperature Behaviour of Zn-Based Galvannealed Coatings on Steel

“…Price et al [13] reported that ~0.1 wt.% Al supported the formation of a FeAl 2 Zn x phase. However, Zapico-Álvarez et al [15] identified this non-uniform layer in a 0.11 wt.% Al coating to be built by a ~20 nm thick Fe 2 Al 5 -Zn x phase followed by a δ phase layer. As reported by several authors, 0.1 wt.% Al is the borderline composition which started to act in favour of the δ phase instead of ζ phase formation [13,15,21,25,26].…”

“…After galvanising in pure Zn, a well-known coating structure with η(Zn), ζ, δ and Γ phases is expected to form [6][7][8][9]. Gradual addition of Al in the range of 0.1-0.2 wt.% changes the formation of these phases significantly by forming an Fe-Al-based inhibition layer, usually reported as Fe 2 Al 5-x -Zn x [6,[10][11][12][13][14][15]. While, generally, a continuous inhibition layer is reported to form above 0.15 wt.%Al in the bath, several publications indicate that even lower Al content has an influence on the formation of the Fe-Zn alloy layer.…”

“…The studies of Price et al [13] report that such layers can be formed by FeAl 2 -Zn x phase particles. On the other hand, based on FIB/TEM analysis, Zapico-Álvarez et al [15] have stated that even such an incomplete inhibition layer is formed by the Fe 2 Al 5 -Zn x phase and is followed by a δ phase layer. For a 0.13 wt.% Al bath, Fe 2 Al 5 -Zn x and FeAl 3 phases have been reported to form the inhibition layer by Chen et al [10].…”

Operando kinetics of hydrogen evolution and elemental dissolution I: The dissolution of galvanized steel in hydrochloric acid

“…The use of a galvanized steel coating as a reference material also demonstrates the sensitivity of the AESEC technique to nanometric films. During the galvanization process with a molten 0.1 -1 at.% Al-Zn bath, an intermetallic film is formed between the zinc coating and the steel substrate, presumably in the form of Fe 2 Al 5 Zn x (0 < x < 1) [16][17][18][19][20]. This film, on the order of 20 -50 nm, inhibits the formation of Fe-Zn intermetallic phase or -outbursts‖ that are detrimental to the mechanical properties of the final coating.…”

Operando Kinetics of Hydrogen Evolution and Elemental Dissolution I: The Dissolution of Galvanized Steel in Hydrochloric Acid

“…The use of a galvanized steel coating as a reference material also demonstrates the sensitivity of the AESEC technique to nanometric films. During the galvanization process with a molten 0.1 -1 at.% Al-Zn bath, an intermetallic film is formed between the zinc coating and the steel substrate, presumably in the form of Fe 2 Al 5 Zn x (0 < x < 1) [16][17][18][19][20]. This film, on the order of 20 -50 nm, inhibits the formation of Fe-Zn intermetallic phase or -outbursts‖ that are detrimental to the mechanical properties of the final coating.…”