1. Forced wetting of steels by liquid Zn-Al alloy
J.-S. Diawara*, M.-L. Giorgi*,
J.-B. Guillot*, A. Koltsov**, D. Loison**
6th International Congress HTC
6-9 May 2009
* École Centrale Paris – Laboratoire de Génie des Procédés et Matériaux
** ArcelorMittal Research S.A.

2. Outline Industrial context and objectives
Experimental apparatus and protocol
Results
Conclusion

3. Continuous galvanizing process
Annealing conditions T
800°C N2
95 vol.% H2
5 vol.%
PH2O
38 Pa

4. Industrial problem Annealing: Aims: Recrystallization of the steel.
Protective atmosphere
(N2-H2) to avoid oxidation of iron.
However:
Selective oxidation of alloying elements (Mn, Si, Al, Cr, P…)
FEG-SEM image of IFTi steel surface after annealing

5. Objectives Forced wetting
Improvement of the wetting
Forced wetting
metallic iron partly covered by oxide particles
liquid zinc alloy
Variation of the kinetic energy of a zinc droplet impacting the steel surface

6. Materials Chemical composition of IFTi steels
Polished up to 1 µm
Chemical composition of the zinc alloy
Zinc droplet mass: 80 ± 0.5 mg C Mn Si P Al Cr Ti B Ni N 2
117 9 13 27 17 74
0.1 8
5.2
Average composition of the IFTi steel studied (x 10-3 wt.%) Al Fe
0.18 ± 0.05
0.010 ± 0.001
Average composition (4 trials) of Zn-Al-Fe alloy in weight% measured by Atomic Absorption Spectroscopy (SpectraAA, Varian)

7. Experimental apparatus and protocol1
Gas atmosphere: N2-H2, frost point -60°C (1 Pa H2O)
Annealing
Melting and spreading of the droplet
Excess pressure from 15 to 50 mbar to release the liquid metal droplet

8. Spreading sequence of the Zn-Al droplet on the steel surface
Capillary
Steel surface
Excess pressure P= 15 mbar, V0 = 0.8 m/s, KE = 2.8 x10-5 J, t = 15 s
The flight and the impact of the droplet on the surface was followed by a high-speed camera (CMOS, pco. 1200hs) at a rate of frames/s.

9. Measurements
Mean contact angle is determined by averaging left contact angle and right contact angle
*Drop Snake method programmed as a plug-in for ImageJ
* A. F. Stalder, G. Kulik, D. Sage, L. Barbieri, Hoffmann P., (2006) Colloids Surf, A Physicochem. Eng. Asp. 286:92.

10. Measurement of the impact velocities
Sequence of droplet falling onto the substrate between t = 0 to 6 ms before the contact

11. Kinetic energy and We number
Excess pressure (mbar) 15 27 37 50
V0 (m/s)
0.8 ± 0.1
1.2 ± 0.6
1.4 ± 0.2
1.5 ± 0.3
Kinetic energy (x 10-5J)
2.8 ± 0.3
6 ± 3
8 ± 2
9 ± 3 We 35 48 52
Impact velocities and kinetic energies during the droplet fall calculated from the images depending on the excess pressure
We > 1 , Spreading is mainly controlled by kinetic energy

12. Characterization of the surface after annealing
FEG-SEM image of IFTi steel surface after annealing
Roughness of The IFTi steel surface after annealing (Interferometric Microscopy)
EDS analysis of the oxide particles
Ra (nm)
Rt (nm)
9 ± 2
31 ± 7
Mn, Si, Al…
Average roughness (5 points)

13. Dimensionless diameter
KE = 2.8 x10-5 J
Increase of the spreading diameter when increasing the kinetic energy

14. Contact angle
Fe/Zn
Popel et al. 1975
Tarasova et al. 1976
Increase KE
Decrease of contact angle when increasing the kinetic energy

15. Reactive wetting SEM image of the interface Zn/Steel
SEM image of the triple line
Interfacial layer formation pinned the triple line.
Prevent the receding of the droplet.
Concentration profile of Fe, Zn and Al.

16. Summary of the wetting experiments
Kinetic energy (x10-5 J)
2.8 ± 0.3
6 ± 3
8 ± 2
9 ± 3
Static contact angle (deg)
35 ± 5
20 ± 4
15 ± 4
14 ± 3
D/D0
0.59 ± 0.04
0.70 ± 0.03
0.86 ± 0.14
0.94 ± 0.10
D/D0_max
0.62 ± 0.04
0.74 ± 0.05
0.91 ± 0.13
1 ± 0.14
Average contact angle (left and right) for 3 series of trials measured when the droplet reached an equilibrium state after 1000 ms of contact

17. Conclusion
Forced wetting of steel substrates by a liquid zinc alloy (0.18 wt% Al wt% Fe).
Sequences of falling and spreading of the droplet onto the surface by varying the impact velocity.
Evolution of the contact angle and the dimensionless diameter with spreading time.
Increasing the impact velocity of the droplet causes an increase of the final and maximum spreading diameter and a decrease of the final contact angle.

18. Thank you for your attention