The corrosion behaviour of a mild steel in agricultural irrigation water was investigated in the reference solution constituted of 0.2 g l−1 NaCl solution and the inhibiting effect of sulphate ions was examined by addition of 0.27 g l−1 Na2SO4 to the reference solution. The voltammograms obtained with rotating disc electrode showed a cathodic plateau. This limiting current was due mainly to the reduction of dissolved oxygen. In parallel to this reaction, the hydrogen evolution reaction is also taking place for a higher cathodic polarisation potential. At the corrosion potential, however, the diffusion is not the process that determines the corrosion rate. The electrochemical impedance spectroscopy (EIS) at the corrosion potential showed three parallel RC circuits linked hierarchically. The corrosion rate in the presence of sulphate ions was 20 times as fast as in the reference solution. That is, the enhancement of galvanic coupling between local cells prevails on the surface blocking effect of sulphate ions revealed by electrohydrodyanamical (EHD) impedance measurements. In fact by EHD impedance we have shown that sulphate ions actually induce the behaviour identified as partially blocking surface. The active surface areas were roughly estimated to correspond to the crystallographic grain size of mild steel used in this study.