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, TOFSIMS) 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.