Surface study of a chromate pretreatment before coil coating of Al-43.4Zn-1.6Si coated steel

Chromating is frequently used for temporary corrosion protection and as a pretreatment before organic coating of aluminium, zinc and aluminium/zinc alloys. In this work we have studied the composition and the chemical state of pretreated Al–43.4Zn–1.6Su‐coated strip steel. Auger and x‐ray photoelectron spectroscopy were used to characterize the surface of samples exposed to the chemicals from the different processing steps. In the first step, which is alkaline cleaning, hydroxy ions in the cleaner dissolve aluminium oxide from the surface of the strip and make the surface richer in zinc. In the chromating step that follows, the chromic acid causes dissolution of Al and Zn from the surface and precipitation of a complex chromium hydroxide layer containing small amounts of fluoride, aluminium, tungsten and molybdenum ions. Chromium exists in the valance states Cr3+ and Cr6+. In the final pretreatment step, which consists of a chromic acid after‐rinse, additional, mainly hexavalent, chromium is deposited. The chromate coating is thicker in regions indented by the temper rolling than in surrounding areas. The temper rolling, which is performed with textured rolls after metallization of the strip, breaks the surface aluminium oxide film and increases the reactivity of indented regions. The chromate coating is also thicker in the Zn‐rich interdendritic regions than on the Al‐rich dendrite arms. This is probably due to the microstructure of the metal coating in interdendritic areas.