Characterization of black rust staining of unpassivated 55% Al–Zn alloy coatings. Effect of temperature, pH and wet storage

Wet storage staining is a phenomenon that occurs on both zinc coated and 55% Al–Zn coated steel sheets during shipment or storage in damp conditions. Whereas zinc coated sheets form white corrosion products, 55% Al–Zn coated steel sheets form black corrosion products. The effect of temperature, pH and wet storage on the occurrence of black rust staining of unpassivated Aluzink samples has been investigated in the laboratory in terms of corrosion product formation and composition. A characterization of corrosion products formed has been performed mainly based on scanning electron microscopy with X-ray microanalyses (SEM/EDS) for morphological and quantitative analyses and X-ray diffraction techniques (XRD) for crystalline phase identification. Black rust formation is strongly related to alkaline pH regions and is enhanced by the temperature. All black panels show the presence of Bayerite (Al(OH)₃), mainly formed on the aluminum rich dendrite branches and a basic zinc aluminum carbonate (Zn₆Al₂(OH)₁₆CO₃·4H₂O) formed in the zinc rich interdendritic alloy regions in contact with air. Blackening of Aluzink surfaces is connected to differences in optical properties of embedded metallic zinc and/or aluminum particles of different shape and size in the corrosion layer.