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 a s a pretreatment before organic coating of aluminium, zinc and alumini um/zinc alloys. In this work we have studied the composition and the c hemical state of pretreated Al-43.4Zn-1.6Si-coated strip steel. Auger and x-ray photoelectron spectroscopy were used to characterize the sur face 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 an d make the surface richer in zinc. In the chromating step that follows , the chromic acid causes dissolution of Al and Zn from the surface an d precipitation of a complex chromium hydroxide layer containing small amounts of fluoride, aluminium, tungsten and molybdenum ions. Chromiu m exists in the valance states Cr3+ and Cr6+. In the final pretreatmen t step, which consists of a chronic acid after-rinse, additional, main ly hexavalent, chromium is deposited. The chromate coating is thicker in regions indented by the temper rolling than in surrounding areas. T he temper rolling, which is performed with textured rolls after metall ization of the strip, breaks the surface aluminium oxide film and incr eases the reactivity of indented regions. The chromate coating is also thicker in the Zn-rich interdendritic regions than on the Al-rich den drite arms. This is probably due to the microstructure of the metal co ating in interdendritic areas.