PSI - Issue 3

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

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2. Material and experimental procedure Specimens used in the galvanizing operation have been obtained from 3 mm thick commercial carbon steel plates. Table 1 shows the steel chemical composition.

Table 1: Chemical composition of the steel used as substrate (wt%). C Si Mn P

S

Al

Fe

0.090 Bal. Before galvanizing, steels samples have been degreased and rinsed with alcohol. Subsequently they have been pickled in an aqueous solution 20% H 2 SO 4 at 50°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 they have been dried for 10 minutes at 100°C. After this procedure, they have been immediately dipped into the galvanizing bath, at 440 ± 2 °C for different durations (respectively 15, 60, 180, 360, 900 seconds). Finally, they have been water cooled. Two different baths have been considered for galvanizing operations: the first bath is characterized by 3%Sn while second one is characterized by a 0.5%Ti. All baths have been homogenised for 2 days at 460°C. Bending test have been performed by means of a non-standard device and they have been repeated at least three times for each considered dipping duration. Tests are performed using an electromechanical 100kN testing machine and considering a crosshead displacement equal to 35 mm that corresponds to a bending half-angle equal to 30°. In order to identify the damaging mechanisms for each investigated dipping duration and loading conditions, longitudinal sections of the bended specimens have been metallographically prepared and observed by means of an optical microscope (LOM). The damage level has been evaluated in term of “cracks density” that corresponds to the cracks number contained in a millimetre of deformed arc (Kim). 3. Experimental results and discussion Figure 2 shows the schema of specimens used for phase analyses and bending tests. Sides at the end of specimens have been used to perform the metallographic observation of intermetallic phases in the kinetic analysis. 0.167 0.540 0.010 0.004 0.051

Clamping hole (diameter=5mm)

For metallographic analysis

Calibrated length (50 mm)

Thickness (3 mm)

Fig. 2. Schema of specimens.

3.1. Analyses of intermetallic phases Phases contained in the Zn-Sn coatings are the same observed in the coating obtained from pure Zn baths characterized by traditional  ,  and  . In figure 3 the kinetics of coating formation shows a more or less linear dependency of thickness on the dipping time. This is due to the high reactivity of baths probably caused by the presence of high contents of Sn (3% wt) and the presence of high content of Si in the composition of steel. This is confirmed by presence of a well developed  phases (rich in iron) since 15 s of dipping time, as well as the presence of  phases which is the main phases of coatings in terms of thickness. Thickness of phase  weakly increases with the increasing of the dipping time due to different mechanisms of formation due mainly to wettability of the bath. Presence of 3%Sn in the zinc bath increases the fluidity of bath and at the same time allows to obtain a quasi-constant thickness of outher phase.