Calcareous deposits were formed on steel under cathodic protection conditions in artificial seawater at various potentials from 0.9 to - 1.4 V/SCE. The deposition kinetics was analysed by chronoamperometry measurements and the calcareous layers were characterized by electrochemical and electrohydrodynamical impedance spectroscopies, scanning electron microscopy observations and X-ray diffraction analyses. At 20 degreesC, the deposits were composed of aragonite CaCO3 when formed at potentials E comprised between -0.9 and - 1.1 V/SCE, of brucite Mg(OH)(2) and aragonite when formed at - 1.2 V/SCE, and only of brucite when formed at potentials E less than or equal to 1.3 V/SCE. However, the in situ impedance techniques demonstrated the presence of a Mg(II)-containing porous layer along with the aragonite deposit at E greater than or equal to 1.1 V/SCE. In seawater enriched with Mg2+, the deposition of aragonite was almost totally inhibited, and the behavior of the film containing Mg(II) could be described. (C) 2003 Elsevier Science Ltd. All rights reserved.