Issue 22

V. Di Cocco, Frattura ed Integrità Strutturale, 22 (2012) 31-38 ; DOI: 10.3221/IGF-ESIS.22.05 33 shown in Tab. 1. Zn-Sn and Zn-Ti baths were Fe saturated to reproduce industrial conditions. C Si Mn P S Al 0.090 0.167 0.540 0.10 0.004 0.051 Table 1 : Galvanized steel chemical composition (wt%). Before the galvanization steels samples were polished by degreasing and rinsing with alcohol. Subsequently specimens were dipped in an aqueous solution 50% HCl at 25°C for 10 minutes, washed in fresh water, fluxed in an aqueous solution containing 280 g/l ZnCl 2 and 220 g/l NH 4 Cl at laboratory temperature for 2 minutes and then dried for 10 minutes at 50°C. Then specimens were dipped for 15, 60, 180, 360 and 900 seconds. Bending tests were carried out on the galvanized specimens. They are characterized by 25mm of calibrated length in bending angle deformation control up to half-deformation about 34°, which corresponds to a deformed half-angle without load of about 30° in not galvanized specimen. All the calibrated lengths are characterized by constant values of bending moment at each half-deformation values. Finally, in order to identify the damaging mechanisms of each investigated coating bath, longitudinal sections of the bended specimens were metallographically obtained and observed by means of an optical microscope (LOM). Damage level was evaluated in terms of “radial cracks density” (cracks number/length) considering 6 images for each specimen ( damage level was obtained as the mean value of 24 measurements, with a very high repeatability). Crack paths were also evaluated analysing their interactions with Zn-based intermetallic phases. As a consequence, damage evaluation was considered strongly connected with cracks nucleation: authors are conscious of the limit of this definition that does not take into account the crack growth in the different phases. R ESULTS AND DISCUSSION n Fig. 2 equilibrium phases diagrams of Zn-Sn and Zn-Ti systems are shown. The first diagram indicates a decrease of melting temperature addictioning Sn nearby the eutectic compositions. The second diagram shows that the addition of Ti leads to an increase of melting temperature. It implies the use of a small addition of Ti to avoid troubles in solidification because of the lower temperature of specimen in dipping operations. Furthermore, Zn-Ti diagram is not totally known for high values of Ti yet. ( a) (b) Figure 2 : Equilibrium phases diagrams [11]: a) Zn-Sn alloys, b) Zn-Ti alloys. Coatings carried out from bath with Sn and bath with Ti are externally very different. The reason can be found in their different composition and in microstructures. Coatings obtained by Zn-Sn bath are characterized by bright smooth I