Substrate, Surface Finish, and Flow Rate Influences on Calcareous Deposit Structure

Experiments have been performed where 1018 (UNS G10180), A710(UNS K20747), A537 (UNS K02400), and HY80 (UNS K31820) steel specimens, of surface finish corresponding to no. 120, 320, 600, and 1500 polishing, were polarized potentiostatically in seawater at –900 mV (SCE). The resulting calcareous deposits were analyzed with regard to structure, composition, and morphology, and they were found to be comprised of a relatively thin, inner Mg-rich layer that formed initially and a subsequent thicker, outer aragonite precipitate. The effect of this dual deposit structure, as well as variations in substrate, surface finish, and electrolyte flow rate, on current density decay was evaluated. The observed trends are discussed in terms of the film formation process and cathodic protection utility.