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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Bending curves for all investigated dipping times show the presence of Sn in the bath allows to obtain coatings characterized by improved mechanical behavior. The increased resistance of Zn-Sn coatings is not due to values of thicknesses which are lower than thicknesses of Zn-Ti coatings, but it is probably due to characteristics of intermetallic phases. On the other hands, the ductility of intermetallic phased which characterizes the Zn-Sn coatings improves the mechanical behavior of coatings. In terms of residual plastic deformation, figure 6 shows that the residual plastic deformation of Zn-Sn coatings is lower than the residual deformation than Zn-Ti specimens for all investigated dipping time. 3.3. Phases damage LOM observations of bent coating sections show presence of radial cracks in the sides characterized by tensile stress. In the compression sides only seldom longitudinal cracks at steel-  interface can be seen for higher dipping time both in Zn-Sn and Zn-Ti coatings. In the tensile sides the main damage morphology is the presence of radial cracks (figure 7). In the Zn-Sn coatings, radial cracks are present in  phases and in  phase but not in the phase η. All the cracks in  phase generate at steel-  interfaces and then propagate in  phase until the  -  interfaces stopping the propagation  phase. All the cracks in  phase come from  phase and propagate in  phase until the  -η interfaces stopping the propagation η phase (figure 7a). In the Zn-Ti coatings, radial cracks are present in the  phase (figure 7b). Often  phase of Zn-Ti coatings is interested by the presence of longitudinal cracks which propagate into  phase, but not at steel-  or in  -multiphasic phases.

a) b) Fig. 7. Phases damage morphology: a) Zn-Sn coatings, b) Zn-Ti coatings. Considering the number of cracks for millimeter of deformed arc as damage parameter, in the figure 8 are shown the damages of bent specimens for  phases of both Zn-Sn and Zn-Ti coatings and for all dipping times. At lower dipping time the damage of  phases of Zn-Sn and Zn-Ti coatings are characterized by higher values. This is due to brittle behavior of  phase and to the thicknesses which represent a not negligible phases in the total of coating thicknesses. For Zn-Ti coatings, the damage of  phase decreases at higher values of dipping times because the thickness of multiphase layer is about 5 times greater than  phase thickness. For Zn-Sn coatings, the damage of  phase and of  phase, are lower at 900s of dipping times. This is the condition were the bending tests shows the higher bending moment. As a consequence, in terms of bending strength and general damage of intermetallic phases, the best conditions are Zn-Sn 3% bath and 900s of dipping time.