Issue 48

C. Bellini et alii, Frattura ed Integrità Strutturale, 48 (2019) 740-747; DOI: 10.3221/IGF-ESIS.48.67 744 Considering all the unetched observations, the mean thickness of the coating can be measured for each one of the dipping times, and the values are shown in Figure 3. 0 100 200 300 400 500 600 0 200 400 600 800 1000 Phase thickness [  m] Dipping time [s] Figure 3 : Coating thickness kinetics. As shown in Figure 3, up to the dipping time of 360 s the kinetic formation of coating seems to be like the kinetic governed by interdiffusion phenomena, but for the higher investigated dipping time a sharp increase of thickness is observed. Furthermore, at 15 s of dipping time, the minimum measured thickness is about 80 μm. This value is very high if compared to the traditional steel galvanizing and it is due to the high presence of silicon in the GS500 which leads to high reactivity of galvanizing alloy in the Zn bath. Often, for 60 s of dipping time the thickness of the coating is about 30 μm in a commercial low carbon steel, that is more than two times lower. It means that the hot-dip galvanizing of DCIs is much more performing than the traditional steels both in terms of costs and in terms of time to galvanizing. I NTERMETALLIC PHASES FORMATION KINETICS onsidering the etched section (Figure 4), all the traditional intermetallic phases which characterized the traditional Zn coatings are observed. In particular, the main phase seems to be the ζ for all investigated time conditions, and the η phase seems to be negligible for higher dipping time. Moreover, the δ phase seems to be present in all investigated dipping time. In Figure 5, the kinetic formation of different intermetallic phases is shown, and the kinetic of ζ phases seems to be similar to the behaviour of the whole coatings shown in Figure 3. It means that the ζ phase is the main phase of the coatings generated in the DCI. Furthermore, the presence of η phases seems to be negligible for 15, 60 and 900 s, whereas the δ phase thickness seems to be almost constant starting from 180s up to 900s. Finally, considering the presence of graphite in the coating, the presence of void is also observed in the coatings like the graphite in many different shapes. As shown in Figure 6, not all the dark zones are characterized by the graphitic carbon presence; for instance, as observed in the chemical elements maps, the dark area in the upper right zone of the Figure 6 is characterized by a presence of a low quantity of carbon. It could be due to the presence of desegregation of carbon of some graphite nodules in the coating. T HERMAL AND GRAPHITE DAMAGE he formation of coating in investigated GS500 leads to a formation of a very damaged brittle δ phase as observed in the Figure 7, where the red arrows show the radial crack due to the thermal effect. These cracks are dominant, in terms of number of cracks, but are present only in the δ phase. No thermal cracks are observed in the ζ phase. Furthermore, it is observed how the carbon of graphite tends to dissolute in the coating (nodules on the right of Figure 7). The upper side of this nodule seems to extend toward the radial direction. The new shape of the graphite tends to be like C T