Issue 22

V. Di Cocco, Frattura ed Integrità Strutturale, 22 (2012) 31-3 8; DOI: 10.3221/IGF-ESIS.22.05 31 Sn and Ti influences on intermetallic phases damage in hot dip galvanizing V. Di Cocco DICeM - Università di Cassino e del Lazio Meridionale, Via G. Di Biasio 43, 03043, Cassino (FR), Italy v.dicocco@unicas.it A BSTRACT . Protection against metallic materials corrosion is one of the most important means to reduce both maintenance costs and environmental impact. In the last years new studies on chemical baths compositions and fluxes have been performed in order to improve processes, corrosion resistance and mechanical behavior of Zn based coatings. Chemical bath composition is often improved by the Sn addition which increases the fluidity of the melt. Ti addition makes the coatings to change color under appropriate heat treatment. In this work a comparative microstructural analysis, in Zn-Sn and Zn-Ti coatings, is performed to evaluate intermetallic phases formation kinetics and the influence of intermetallic microstructure on coating damage under constant bending deformation. K EYWORDS . Hot Dip Galvanizing; Intermetallic Phases; Damage. I NTRODUCTION rinciples of hot dip galvanizing have been unchanged since 200 years, when this coating process came into use. Actually, it is still one of most important processing techniques to protect metallic material against corrosion in many aggressive environments. New investigations on bath composition were performed to generate different types of coatings depending on specific purposes such as high plastic deformations or reinforced components. Zn and Zn-based coating formation is a diffusion driving phenomenon. Many intermetallic phases are generated by different Zn concentration varying from surface to substrate. Alloying components and their concentrations in the bath influence intermetallic phases formation. These are responsible for a brittle or a ductile coating - because of different phases behaviors and their thicknesses. Four intermetallic layers are usually observed in traditional Zn, Zn-Pb or Zn-Sn coatings, characterized by different Fe contents (decreasing from steel substrate towards surface). Zinc-based layer formation is obtained by interdiffusion of zinc and iron atoms. This process generates a different chemical composition coating, characterized by different intermetallic phases according with Zn-Fe diagram, as shown in Fig. 1a. From the iron-coating interface towards surface, Zn content increases. Therefore the zinc coating is a multilayer system mainly formed by four phases (Fig. 1b), characterized by different thickness and mechanical properties. Outer layer is a ductile  phase with maximum Fe content up to 0.03%. The following layer is named “  ” phase, which is isomorphous with a monoclinic unit cell. Its atomic structure contains a Fe atom and a Zn atom surrounded by 12 Zn atoms at the vertices of a slightly distorted icosahedron. The icosahedra link together to form chains and the linked chains pack together in a hexagonal array [1].  phase is a brittle one with Fe content up to 11.5 wt% and hexagonal crystal structure. The last phase is a very thin layer named “  ” and it is characterized by a Fe content up to 29 wt% (fcc). Coating formation is governed by physical (bath P