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Cr-containing steel panels have been exposed to marine atmosphere for 2 years. The phase constituents and the microstructure of the rust layer have been characterized using scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), Raman spectroscopy, and transmission electron microscopy (TEM). Results show that there are many cracks and pores in the rust layer on the surface of mild steel exposed for 2 years, resulting in a higher corrosion rate. But a compact rust layer with a phase constituent of alpha Cr substitute ferrite oxyhydroxide (α-CrxFe1 –xOOH) has been formed on weathering steel panels with a Cr content more than 2 mass% exposed for 2 years. Furthermore, ion selective measurements showed that the rust layer of α-CrxFe1– xOOH can suppress the penetration of corrosive anions, and accordingly weathering steel panels have a lower corrosion rate in the marine atmosphere.