Black rust staining is a phenomenon that occurs on zinc coated steel sheets during shipment or storage in damp conditions. The effect of coating thickness and wet storage conditions on the occurrence of black rust staining of Cr-free resin coated steel samples has been investigated 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 photoelectron techniques (XPS) for element analysis with depth profiles. Black rust formation is strongly related to coating thickness and is enhanced by the humidity of atmosphere.