Mineral deposits consisting of CaCO3 and Mg(OH)(2) develop on steel surfaces immersed in seawater, whenever a cathodic protection is applied. The kinetics of their formation depends on various factors, such as temperature, applied potential, electrolyte composition and stirring. In this study, the cross effects between the deposition of CaCO3 and that of Mg-containing compounds was investigated by varying the [Ca2+] concentration of the electrolyte. Chronoamperometry and electrochemical impedance spectroscopy (EIS) were used to monitor the formation of the deposits whereas scanning electron microscopy was performed to characterise the deposits. At a potential of -1.0 V/SCE, an increase of temperature accelerated the deposition of CaCO3, whereas at -1.2 V/SCE it modified the composition, favouring Mg(OH)(2). The presence of SO42- anions proved to hinder the deposition of CaCO3. For instance, at a concentration of 50% [Ca2+](ref) (standard seawater reference), a compact deposit covered totally a steel surface immersed in a sulphate free solution, whereas less than half of the surface was covered when SO42- ions were present. (C) 2004 Elsevier Ltd. All rights reserved.