Impact of alkaline and acid treatment on the surface chemistry of a hot-dip galvanized Zn–Al–Mg coating

Abstract

The surface chemistry of an exemplary hot-dip galvanized Zn–Al–Mg coating comprising 1.6 wt.% aluminum and 1.1 wt.% magnesium was characterized by means of secondary and transmission electron microscopy as well as synchrotron radiation X-ray photoelectron spectroscopy. Lamellae of the coating prepared via focused ion beam technique were examined through energy dispersive X-ray analysis. Further the effect of alkaline and acid treatment respectively was investigated by employing time of flight secondary ion mass spectroscopy, energy dispersive X-ray analysis and glow discharge optical emission spectroscopy. Throughout this work it is shown that both alkaline (NaOH) and acid (H2SO4) solutions alter the chemical composition when applied to the coating surface. With the native Zn–Al–Mg system yielding a complex microstructure covered by a magnesium and aluminum rich oxide film, highly alkaline surroundings (pH > 13) caused a decrease of aluminum at the uppermost surface. Every deployed acid solution (pH < 2.4) lead to a preferred pickling of the magnesium compounds including the oxide layer plus the intermetallic MgZn2 phases of the coating. Exposure to both highly alkaline and acid environments corresponded to a higher proportion of zinc at the coating surface measured by all applied surface analytical tools indicating that the modification of the chemical composition took place on both nanoscopic and microscopic scale.