This work describes a study of the mechanism and corrosion rate of steel bars in concrete in seawater. It focused on influences of concrete crack width on macrocell effect and, on corrosion rate determined using a polarisation curve model. Experimental results are compared for corrosion behaviours of steel bars in two concretes. A mechanism is proposed to interpret the different influences, in the short and long term, of crack width on the macrocell effect. A test procedure is used to obtain polarisation curves of steel in concrete and these curves are found very different to those...