Corrosion of a mild steel in agricultural irrigation waters in relation topartially blocked surface

The corrosion behaviour of a mild steel in agricultural irrigation water wa s investigated in the reference solution constituted of 0.2 g l(-1) NaCl so lution and the inhibiting effect of sulphate ions was examined by addition of 0.27 g l(-1) Na2SO4 to the reference solution. The voltammograms obtaine d with rotating disc electrode showed a cathodic plateau. This limiting cur rent was due mainly to the reduction of dissolved oxygen. In parallel to th is reaction, the hydrogen evolution reaction is also taking place for a hig her cathodic polarisation potential. At the corrosion potential, however, t he diffusion is riot the profess that determines the corrosion rate. The el ectrochemical impedance spectroscopy (EIS) at the corrosion potential showe d three parallel RC circuits linked hierarchically. The corrosion rate in t he presence of sulphate ions was 20 times as fast as in the reference solut ion. That is, the enhancement of galvanic coupling between local cells prev ails on the surface blocking effect of sulphate ions revealed by electrohyd rodyanamical (EHD) impedance measurements. In fact by EHD impedance we have shown that sulphate ions actually induce the behaviour identified as parti ally blocking surface. The active surface areas were roughly estimated to c orrespond to the crystallographic grain size of mild steel used in this stu dy. Published by Elsevier Science Ltd.