The influence of alkali-degreasing on the chemical composition of hot-dip galvanized steel surfaces

The influence of dipping temperature and time on the surface chemistry of hot-dipped galvanized steel sheets during the alkaline degreasing process is investigated. The surface chemistry was monitored with scanning Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The results show high Al concentrations on the untreated surfaces, which are significantly reduced during alkaline degreasing. The same conclusions could be drawn for the carbon compounds that accumulate on the surface during storage. The measurements reveal a gradual reduction in surface Al as the alkali solution temperature and/or degreasing time are increased. When degreasing was conducted at 70 degrees C for 30 s the surface was practically free from Al, which was present only in small islands. Furthermore, the experiments showed that the thickness of the oxide film covering the surfaces before and after alkaline degreasing is approximately 20 angstrom. The main constituents of the film varied from ZnAl hydroxide/oxide to Zn hydroxide/oxide, before and after degreasing, respectively. Copyright (C) 2006 John Wiley & Sons, Ltd.