Study Of Spangles On Hot-dip Galvanized Zn-Al-Sb Alloy Coatings

The corrosion of steel causes much loss. Hot-dip galvanizing is the most effective way to protect steel from corrosion. In order to obtain a required coating in this process, minor aluminium is usually added into the metal bath. The hot-dip-coated surface often exhibits a structure consisting of very large grains, termed 'spangles', when lead, antimony, bismuth, tin is/are added into the bath alloyed with aluminium. Spangles are special phenomenon of hot-dip galvanizing and have been received widely attention from institutes and researchers. Up to now, however, from the point of view of the formation mechanism and its details of were speculative, and the knowledge of spangle was only fragmentary in domestic studies.In this thesis, the spangles on batch hot-dipped coating were investigated. Spangle morphologies, formation mechanism, surface segregation of alloying elements, precipitation of second phase particles, corrosion resistance and corrosion mechanism were investigated using SEM, BSE, EDS, AFM, XRD, XPS, NSS and EIS. The results show that:Spangle is the result of dendritic growth of zinc layer, and each spangle represents a single-crystal zinc grain. On the basis of macroscopic surface appearance, spangles may be classified intod three types:shiny spangle, feathery spangle and dull spangle. The spangle crystallographic classification lies into three types according to the correlation between the basal plane (0001) and the inclined angleβwith respect to the steel sheet plane:1)β=0°, the spangle presents 60°symmetry dendrite structure with snow-flake or six-fold star structure.2) 0°<β<90°, the spangle presents inclined hexagonal structure or shiny/dull divided morphology.3)β=90°, the spangle presents dull and rough orthogonal-dendrite morphology. Both dendritic primary and secondary arms of spangle grow along the preferred direction (1010) of zinc crystal, the ones which are parallel to the thermal direction will posess sustained growth. The preferred direction and thermal gradent determine the formation of shiny spangle, feathery spangle and dull spangle in a single spangle area. Columnar-to-equiaxed transition (CET) Zn-0.2Al alloy was derived, whenas, no CET was found in Zn-0.2Al-0.1Sb alloy under the same directional solidification condition. This indicated that the addition of Sb (or Pb) restrains CET and reduces the amount of nucleus in unit volume and that Sb (or Pb) poisons these available nucleation sites, resulting in large spangle grains on hot-dip galvanized coatings.The coating with spangle can be produced by the addition of Sb (or Pb). As we know however, Pb is poisonous, and the research of application of Sb is more valuable. But overmuch of Sb addition would affect the quality of the coating by forming brittle AlSb phase. In this paper, solidification structures of Zn-Al-Sb master alloys under different cooling condition were investigated. The results show that solidification microstructure of the alloys are refined significantly as the cooling rate increases, and the eutectoid decomposition of metastableζtoβ:ζ→β+ηis inhibited under water cooling. Under equilibrium solidification condition, Zn-Al-Sb ternary phase diagram at ambient temperature can be divided into 4 phase zone:β-Sb3Zn4+Eβ+Zn; AlSb+Eβ+Zn; AlSb+Zn; Zn+AlSb+EZn+Al·The coating surface usually exhibited three kinds of spangles:shiny, feathery and dull spangle, of which extensively antimony surface segregation was detected. Batch hot dip galvanized coatings hold longer dipping time, thicker coating and slower coating solidification rate, thus resulting in a rougher coating surface and a greater surface segregation, and the transition of metastable intermetallic precipitate to stable state occurs more completed. In this paper, surface segregation rule of alloying elements on typical coating with spangle obtained in batch hot dipped Zn-0.05Al-0.2Sb bath was investigated. The results show that the segregation degree increased from shiny, feathery to dull spangle. Aluminium segregated toward the outer spangle surface in the form of Al2O3 layer with the thickness of 20nm, while antimony segregated toward the surface in the form ofβ-Sb3Zn4 particles with the thickness of 29nm. Final distribution of the precipitate is correlated with spangle morphology, which depends on interaction of crystal orientation and growth direction of the nucleated zinc grain in the zinc layer. Theβ-Sb3Zn4 particles randomly distribute on the shiny spangle surface;β-Sb3Zn4 particles and snow-like dendritic segregation of antimony exists in the dendritic secondary arm spacings of feathery spangle surface;β-Sb3Zn4 particles and dendritic segregation of antimony spread extensively on the whole dull spangle surface.Corrosion resistance in 5%NaCl solution differs from spangles. From the results of Tafel polarization and EIS under different immersion time it was concluded that both the polarization resistance Rp and low-frequency impedance ZLF decreased from shiny, feathery to dull spangle, and corrosion resistance of spangle in 5%NaCl solution decreased from shiny, feathery to dull spangle in turn. The difference of corrosion resistance of spangle follows into three factors:the crystallographic orientation of crystal plane, surface roughness, the amount of alloying elements and precipitates segregated on the spangle surface. There are three stages during the corrosion process of spangle immersed in 5%NaCl solution, they are:the initial immersion stage involves the dissolution of superficial metallic oxide/hydroxide layer and the mass transfer of reaction particles; the middle immersion stage of which the relaxation of corrosion products layer takes place and, the later immersion stage involves the dissolution of zinc associated with the diffusion process, which controls the zinc dissolution.