This work focuses on the corrosion of cast iron in a drinking water network. The study is performed in water and in a sandy soil saturated with the same water. In the presence of corrosion products, the inner wall surface shows a lower corrosion rate than the external wall. In the two media, impedance diagrams are described by a high-frequency loop characteristic of a protective corrosion layer and a diffusion impedance loop at low frequencies associated with oxygen diffusion. Different models involving mass transfer through a porous layer are suggested for the inner and the external corrosion layers.