An artificial patina on weathering steel produced with a 10% sulphuric acid solution and a natural patina formed over a period of 2 years in an urban environment were compared with a 44-year-old natural patina on an outdoor weathering steel sculpture. The samples and sculpture are exhibited and studied in two of the most polluted Croatian cities, Zagreb and Sisak. Various techniques used to examine the patina included scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and in-situ electrochemical impedance spectroscopy (EIS) using a paste electrolyte cell. A widely accepted criterion for the protective effect of the patina on weathering steel, the ratio of goethite to lepidocrocite (α/γ), was found to be 0.10 < 0.38 < 0.71 for 2-year-old natural, 2-year-old artificial and 44-year-old natural patina on the investigated sculpture. Equivalently, the corrosion rates determined by EIS were approximately 37, 22 and 12.5 μm year-1. The in-situ EIS technique allowed confirming the efficiency of artificial patination by H2SO4 in immediately attaining about twice as protective patina than that formed naturally after 2 years. It also confirmed about three times as efficient protection offered by a 44-year-old patina except on the moss-covered surface.