Forced wetting of steels by liquid Zn-Al Forced wetting of steels by liquid Zn-Al alloy 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.

OutlineOutline

Industrial context and objectives Experimental apparatus and protocol Results Conclusion

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Continuous galvanizing processContinuous galvanizing process

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Annealing conditions

T 800°C

N2 95 vol.%

H2 5 vol.%

PH2O 38 Pa

Industrial problemIndustrial problem

FEG-SEM image of IFTi steel surface after annealing

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…)

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ObjectivesObjectives

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

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Improvement of the wetting

MaterialsMaterials

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)

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Experimental apparatus and protocolExperimental apparatus and protocol

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Gas atmosphere: N2-H2, frost point -60°C (1 Pa H2O)AnnealingMelting and spreading of the dropletExcess pressure from 15 to 50 mbar to release the liquid metal droplet

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Spreading sequence of the Zn-Al droplet Spreading sequence of the Zn-Al droplet on the steel surfaceon the 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.

Capillary

Steel surface

MeasurementsMeasurements

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*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.

Mean contact angle is determined by averaging left contact angle and right contact angle

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

dropmtgVV

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Measurement of the impact velocitiesMeasurement of the impact velocities

Kinetic energy and Kinetic energy and We We numbernumber

)(2

1 20 JouleVmEC

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 15 35 48 52

Impact velocities and kinetic energies during the droplet fall calculated from the images depending on the excess pressure

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We > 1 , Spreading is mainly controlled by kinetic energy

LV

DVWe

0

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Characterization of the surface after annealingCharacterization of the surface after annealing

EDS analysis of the oxide particles

FEG-SEM image of IFTi steel surface after annealing

Roughness of The IFTi steel surface after annealing (Interferometric Microscopy)

Ra (nm) Rt (nm)

9 ± 2 31 ± 7

Average roughness (5 points)

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Mn, Si, Al…

Dimensionless diameterDimensionless diameter

Increase of the spreading diameter when increasing the kinetic energy

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Contact angleContact angle

Decrease of contact angle when increasing the kinetic energy

Fe/ZnPopel et al. 1975Tarasova et al. 1976

Increase KE

Reactive wettingReactive wetting

Interfacial layer formation pinned the triple line.Prevent the receding of the droplet.

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SEM image of the interface Zn/Steel

Concentration profile of Fe, Zn and Al.

SEM image of the triple line

Summary of the wetting experimentsSummary 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

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ConclusionConclusion

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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Thank you for your attentionThank you for your attention

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