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Mouillabilité de surfaces hétérogènes (fer/oxyde) par un alliage de zinc liquide

Abstract : Wetting phenomenon takes place in several processes among which continuous galvanizing of steel sheet for car bodies. In this process, the steel sheet is immersed in a zinc bath in order to cover it with a protective zinc layer. But before that, the cold rolled steel sheet is annealed in a reducing atmosphere in order to eliminate the cold working and the superficial iron oxides improving thereby the galvanizing reactions. New steels, called advanced high strength steels, have been developed in order to improve vehicle safety without increasing their weight and therefore fuel consumption. These steels contain high amounts of oxidizable alloying elements that segregate during the annealing to the surface where they form oxides particles that are not wetted by liquid zinc. Therefore it seems important to understand the wetting by liquid zinc of heterogeneous surfaces composed by metallic iron and oxides. For this purpose, we studied the spreading of liquid zinc droplet at very short time using a high speed camera by means of a dispensed drop apparatus. In our experiments we have used different iron – silicon alloys and a commercial steel, annealed at high temperature before the wetting experiments, leading to surface coverage by oxides lying from 0 to 100%. Wetting is described using two parameters: the spreading diameter and the contact angle. We have focused on the comprehension and modelling of the evolution of these two parameters during the first time of contact and at the end of spreading. The influence of both the surface coverage by oxides and the initial kinetic energy of the droplet has been investigated. The results obtained for the wetting of the model heterogeneous surfaces are extrapolated to substrates annealed in the same conditions as in industrial continuous annealing.
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Jules Diawara. Mouillabilité de surfaces hétérogènes (fer/oxyde) par un alliage de zinc liquide. Matériaux. Ecole Centrale Paris, 2011. Français. ⟨NNT : 2011ECAP0042⟩. ⟨tel-00658768⟩

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