In the present work, we examined the structure and protectiveness of rust layers formed on friction stir welded (FSWed) high phosphorus carbon steels, compared with those on SMA490AW and COR-TEN weathering steels. Rust layers were grown on base materials and FSWed materials under a wet/dry cyclic corrosion test. XRD revealed that the rust layers on all the materials consisted of Fe3O4 (magnetite), α-FeOOH (goethite), β-FeOOH (akaganeite), and γ-FeOOH (lepidocrocite), and main constituents of the rust layers were goethite and magnetite. On the base materials, the fraction of protective goethite increased with increasing P concentration in steel when the fraction was compared among SMA490AW, COR-TEN, and 0.1C-0.1P specimen, whereas for 0.1C-0.3P, 0.3C-0.1P, and 0.3C-0.3P specimens, the fraction decreased compared with that on 0.1C-0.1P specimen. The corrosion current of rusted base materials decreased with increasing P concentration when compared among SMA490AW, COR-TEN, and 0.1C-0.1P specimen. In contrast, the corrosion current of 0.1C-0.3P, 0.3C-0.1P, and 0.3C-0.3P specimens was comparable to that of 0.1C-0.1P specimen, indicating the improved protectiveness of rust layers formed on high phosphorus carbon steel. In addition, the protectiveness of rust layers on the FSWed materials was comparable to, or higher than that on the base materials, in particular, the protectiveness was apparently higher, independent of the fraction of goethite when the materials were subjected to the FSW below A1. The improved protectiveness for the FSWed specimens demonstrates that FSW does not deteriorate the corrosion resistance of rusted high phosphorus carbon steels.