Characterization of calcareous deposits in artificial sea water by impedances techniques: 2-deposit of Mg(OH)2 without CaCO3

Application of Taguchi method for the characterization of calcareous deposits formed by pulse cathodic protection

“…For steel structures in seawater, the usual applied protection potential varies from À0.8 to À1.2 V/SCE (where SCE stands for saturated calomel electrode), resulting in the reduction of dissolved oxygen onto the metallic surface, and is accompanied for the more cathodic potentials by hydrogen evolution. Both reactions produce hydroxyl ions, which permit the magnesium hydroxide precipitation, provided that the interfacial pH reaches the critical value of 9.3 [1]. Moreover, these reactions lead to changes in the inorganic carbonic equilibrium at the metallic interface, favor carbonate ion over hydrogen carbonate and allow calcium carbonate precipitation.…”

Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

“…Calculations for the pH required for the deposition have yielded values of pH 7.3-8.7 for the CaCO3 polymorphs (calcite and aragonite) and pH 9.3-11.25 for brucite (Mg(OH)2 [6,7,[9][10][11] and the composition achieved dependent on the potential applied, temperature, and flow of the electrolyte (ASTM D1141 [12] in our case) [9,[13][14][15]. In previous studies it has been established that the deposit has a two layer structure made up of a fine layer of Mg-rich brucite followed by a Ca-rich polymorph [7,14,[16][17][18][19][20][21].…”

“…Leeds and Cottis [22] found that a CaCO3 only deposit occurred at a potential of −0.7 V (SCE) and Barchiche et al [19] suggested an aragonite deposit would be formed at a potential −0.9 ≤ E ≤ −1.1 V (SCE), a combination of aragonite and brucite at −1.2 V (SCE) with brucite the only phase forming at a potential more negative than −1.3 V (SCE) in the 10-30 °C range with rotating electrodes. The transition from a Mg(OH)2 to CaCO3 composition is still not fully understood and several explanations of the mechanism have been proposed [10,11,[16][17][18]. The most widely adopted is the inhibiting effect of Mg 2+ ions on the nucleation and growth of calcite and the nucleation of aragonite and once the formation of the brucite layer depletes the zone of Mg 2+ ions the calcite and aragonite can nucleate and grow [16,18].…”

“…The most widely adopted is the inhibiting effect of Mg 2+ ions on the nucleation and growth of calcite and the nucleation of aragonite and once the formation of the brucite layer depletes the zone of Mg 2+ ions the calcite and aragonite can nucleate and grow [16,18]. Other explanations involve a change in the electrostatic properties of the brucite layer [10,11,17].…”

“…Generally, as far as calcareous deposits are concerned, the design philosophy for CP is to have a high initial current density to promote their rapid formation [23] and, thereby, achieve a more economical CP system [10]. The application of CP has limitations: due to structural complexity it is possible to have potentials more negative around the anodes and more positive at remote or shielded locations.…”