Study On Formation Mechanism Of Hot - Dip Galvanized Coating With Ammonia - Free Plating

With the development of economy and the enhancement of environmental consciousness, hot dip galvanizing in ammonium-free fluxing process attracted more and more concern and attention at home and abroad, because of the environment friendly of ammonium-free fluxing process. Although the mechanism of action similar to the traditional zinc-ammonium fluxing process, but because the ammonium-free fluxing process without the ammonium chloride and addition other chemicals, will inevitably lead to the composition and thickness of the fluxing membrane on steel base surface distinctly different, as well as the speed of reaction with zinc bath and the speed of the reaction product break away from steel base and rise to the surface of zinc bath during hot dip galvanizing. All this will directly affect the growth speed、layer thickness and the distribution of each alloy layer.Based on this, in this paper, the tested samples were pretreated by ammonium-free fluxing, and the coatings were galvanized by the hot-dipping in pure zinc bath as well as Zn-0.05% Al bath under series of dipping parameters. The microstructure and the growth of the coatings were examined and analyzed by scanning electron microscopy(SEM) equipped with energy dispersive spectrometer (EDS) analyzer. Take the formation process of coating pretreated by ammonium-free fluxing as the subject to study the feature of the microstructure of the coating in dipping parameters under the conditions of ammonium free fluxing process. Summarized the formation rules of hot dipped galvanizing coatings under the conditions of ammonium-free fluxing process, and the mechanism of coating formation under the ammonium free fluxing process also was analyzed. The main conclusions were as follows:Under the conditions of ammonium-free fluxing process, hot-dip Zn-0.05% Al zinc bath in 450℃, the thickness increases with extension of the dipping time, the formation of the alloy phase order:zeta(ζ) → zeta(ζ)+delta(δ) → zeta1(ζ1)+zeta2(ζ2)+delta(8) → zeta1(ζ1)+ zeta2(ζ2)+delta(δ)+eta(η). The formation and mechanism of the alloy layer were similar to traditional hot dip galvanized, but no obvious Γ phase found in the coating. Under the ammonium- free fluxing conditions the optimal dipping time is 60 s-120 s to obtain uniform densify structure、suitable thickness and good appearance quality coating.Under the conditions of ammonium-free fluxing process, hot-dip Zn-0.05%Al zinc bath the best dipping temperature is about 450℃, the dipping temperature too low will led to the organize of coating uneven, if excess temperature the alloy layer will grows extra-thick. The inhibitory effects of Al on the nucleation and growth of ζ phase in the initial stage of galvanizing decreases with the increasing of dipping temperature. With increasing dipping temperature, the thickness of coatings increases.Under the conditions of ammonium-free fluxing process, the addition of 0.05%Al in zinc bath could significantly improve the appearance quality and reduce the thickness of the coating. And the addition of 0.05%Al in zinc bath could adequately reduce the alloy layer thickness, suppress the nucleation and growth of ζ phase in the initial stage of galvanizing, elements Al mainly exists in the growth front of alloy phase.Under the conditions of ammonium-free fluxing process, when dipping time with 0 to 300 s in pure zinc bath as well as Zn-0.05%Al bath, the alloy layer growth is mainly controlled by a mechanism of atomic diffusion rate. When dipping time in the Zn-0.05% Al bath less than 30 s, the alloy layer growth is mainly controlled by a mechanism of interface reaction.