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.

Outline Industrial context and objectives Experimental apparatus and protocol Results Conclusion

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

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

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

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)

Experimental apparatus and protocol 1 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

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 1 000 frames/s.

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.

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

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

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)

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

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

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.

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

Conclusion Forced wetting of steel substrates by a liquid zinc alloy (0.18 wt% Al + 0.01 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.

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