This study investigated the distribution of steel corrosion products in a cracked concrete beam, which was in service under coastal atmospheric conditions for more than 40 years. The thickness of corrosion-product-filled paste (CP) and the thickness of corrosion layer (CL) were measured. The migration behaviour of Fe in the concrete cracks and air voids are investigated. The mechanism of CP formation was revealed. The steel corrosion and corrosion-product-filled paste propagates faster in the tension region than in the compressive region of the beam. In the atmospheric environment, the corrosion products neither fill the cracks or air voids nor do they accumulate on the edge of the cracks or inside the air voids.