J.-M. Mataigne (ArcelorMittal, Research Automotive Products, Maizières-les-Metz)

Surface chemistry and reactivity of skin-passed hot dip galvanized coating This paper aims at describing surface chemistry and reactivity of skin-passed hot dip galvanized coatings. GI coatings surfaces are covered by a very thin aluminum oxide 5 nm thick layer that precipitates just immediately after wiping. Anisotropic growing of zinc crystals during solidification induces a strong basal texture in GI coatings. Metallic aluminum is rejected in the last liquid during solidification, and so accumulates along zinc grain boundaries and close to the free surface. Skin-pass induced changes in GI coating surface chemistry, crystallography and reactivity have been assessed. Local coating analyses have been performed (XPS, TOF-SIMS) in order to describe local effects of roughness indentation during skin-pass on coating characteristics. A laboratory bi-crushing device has been used on not skin-passed GI coatings, in order to reproduce on large scales heavy deformations that exist locally when skin-pass roll roughness peaks indent the coating. Such a protocol allowed analyzing the way aluminum oxide is crushed down in the zinc coating during deformation, then re-build during ageing. XRD and EBSD experiments have been conducted illustrating zinc textures changes induced by deformation. Reactivity of such surfaces has been tested using probe molecules, like fatty acids