The formation and growth of the iron-zinc compound phases, especially Γ
1-Fe
5Zn
21 and Γ-Fe
3Zn
10, at 773K in galvannealed coatings on the titanium-stabilized interstitial free (IF) steel and the interstitial-free rephosphorized (IFP) steel have been investigated by means of the cross-sectional SEM observation. The formation order of Γ
1 phase and Γ phase was different between the substrates; on the IFP steel substrate, the formation of Γ phase was retarded, which resulted in the prior formation of Γ
1 phase. The formation and growth of Γ
1 phase, on which little influence of phosphorus in steel was observed, is considered to be controlled by the diffusion of iron atoms across the Γ
1/δ
1 interface, which is controlled by the diffusion-coefficient change and the iron-concentration gradient of δ
1 phase. The reactionrate analysis reveals that Γ phase on the IF steel substrate grows by moving the Γ/substrate interface toward the substrate by the diffusion of zinc atoms from the coating into the substrate and that early formation and rapid growth in the earlier stages of alloying reaction are caused by moving Γ/Γ
1 interface toward the Γ
1 phase by the diffusion of iron atoms from the substrate to the coating. Both formation and growth of Γ phase on the IFP steel substrate were retarded, comparing to those on the IF steel substrate, which suggest that both solid-soluted and grain-boundary segregated phosphorus in the IFP substrate interferes the zinc diffusion from the coating into the substrate.
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