PSI - Issue 3

Vittorio Di Cocco et al. / Procedia Structural Integrity 3 (2017) 224–230 Author name / StructuralIntegrity Procedia 00 (2017) 000–000

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 A barrier effect, which implies a shield of substrate to aggressive environment, due to complete embedding of substrate in the coating characterized by high adherence;  A cathodic protection effect, due to minor electronegativity of electrochemical potential of coating than of iron based substrate. Higher values of adherence are due to their formation mechanisms based on interdiffusion of zinc atoms in the bath and the iron atoms of substrate, which generates zones characterized by different chemical compositions, Marder 2000. Iron content decreases from substrate to surface. Thanks to the local chemical composition, the zinc based coatings are characterized by the presence of different metallic phases and different morphologies of microstructures due to importance of diffusion phenomena, Tzimas 2001. Nevertheless, the main parameters used in the zinc coating formation are the dipping time and the chemical composition of the bath. Overall, phases present in the traditional zinc coating are (Figure 1):   phase, characterized by higher Fe contents, and lower values of thickness. Sometimes thicknesses are not observable by LOM and they are due to the presence of a few rows of atoms;   phase, characterized by high hardness. It is a brittle phase where the Fe contents is about 80%. Its thickness range is about of tens microns.   phase is more ductile than  and  phases, and it is characterized by a columnar morphology which can degenerate to an unoriented morphology in case of higher dipping times.   phase is the most ductile coating phase. Its chemical composition is similar to the chemical composition of the bath because the formation of that phase is due to wettability of the bath and not to interdiffusion phenomena.

100  m

Fig. 1. LOM observation of coating section obtained by pure Zn Bath

All the intermetallic phases are characterized by different mechanical behavior, both in terms of maximum strength and in terms of ductility, especially the phases at high Fe contents are characterized by lower ductility, Duncan 1999. As a consequence, the external phases are characterized by high ductility and high attitude to the plastic deformation, Reumont 2000. In this work a comparison between coatings obtained from 3%Sn and 0.5% Ti baths has been investigated in order to evaluate the differences in terms of intermetallic phases and their kinetics at 15, 60, 180, 360 and 900s of dipping times. Furthermore, for all the investigated dipping times, bending tests have performed in order to evaluate the bending behavior of galvanized specimens and the damage of intermetallic phases, Duncan 1999, Kim 2000.