Mg2+ + 2(OH)- → Mg(OH)2 and Ca2+ + HCO3- + OH- → H2O + CaCO3. These are typically the main compounds in calcareous deposits. It obviously has several advantages compared to the conventional coatings, since the environment-friendly calcareous deposit coating is formed by the elements(Mg2+, Ca2+) naturally present in seawater.
Recently, electrodeposition technologies that enables steel structure surface to be protected like a coating method have been studied in developed countries. However, there are some difficulties to maintain both a corrosion resistance for a long period of time and a strong adhesion between deposits and base metal.
In this study, environmental friendly calcareous deposit films were formed by an electrodeposition technique on steel substrates in natural seawater. The influence of current density, coating time, seawater temperature and use of steel mesh on composition ratio, structure and morphology of the electrodeposited films were investigated by Scanning Electron Microscopy(SEM), Energy Dispersive Spectroscopy(EDS) and X-Ray Diffractor(XRD).
Concludingly, the results of these experiments obtained were as follows 1/2 O2 + H2O + 2e- → 2OH- and 2H2O + 2e- → H2 + 2OH-. These reactions increase the pH at the metal / seawater interface. The high pH causes precipitation of Mg(OH)2 and CaCO3 in accordance with the formular Three quarters of the earth is sea. Resources for food and energy with minerals exist abundantly in the sea, which explains why human-being is interested strongly in this world of treasure.
Cathodic protection is one of the successful ways to prevent corrosion of steel structures in marine environments. The unique feature of cathodic protection in seawater is the formation of calcareous deposits on cathodic metal surface. Deposits are a consequence of pH increase of the electrolyte adjacent to metal surfaces by cathodic current. The composites of calcareous deposits are mainly calcium carbonate(CaCO3) and magnesium hydroxide(Mg(OH)2).
The principles of calcareous deposit formation in seawater had been known for a long time. Cathodic reduction reactions associated with cathodic protection in seawater generate OH- at the metal surface in accordance with the formular 1. The composites of calcareous deposits was mainly calcium carbonate (CaCO3) and magnesium hydroxide(Mg(OH)2). The formation of calcareous deposits was the consequence of pH increase of the electrolyte adjacent to metal surface influenced by cathodic current.
2. The Mg compositions, in general, were getting decreased regardless of current density but Ca composition was increased with the running time of electrodeposition.
3. Cacareous deposited films formed in Mg-free solutions were showed that corrosion potential was most noble potential value compared to other films formed in natural seawater, Ca-free solutions and Ca+Mg solutions.
4. Mg(OH)2 compounds with brucite structure shaped as flat type were formed at the initial stage of electrodeposition, however, CaCO3 compounds with aragonite structure shaped as flower type was formed in large scale. Besides, Mg(OH)2 compounds were much more formed at 5 A/m2 of current density compared to the 3 A/m2 and 4 A/m2.
5. The apparent general tendency was that the weight of calcareous deposits precipitated was modestly higher at 48 °C than at 28 °C, and it was some 30 % regardless of current density. This means that calcareous deposits are formed more readily on metal surfaces in warm water than in cold, and the other environmental conditions are relatively less influential.
6. The Ca/Mg ratio in the calcareous deposits was higher at 48 °C than at 28 °C. This reveals that the interfacial pH at the metal surface becomes lower with temperature due to the higher solubility of Mg(OH)2 in high temperature, whereas that of CaCO3 decreases.
7. The Ca/Mg ratio was a decreasing tendency with current density increased, which is probably related with the supersaturated calcium ion at the pH of ambient surface seawater, and precipitation of Mg(OH)2 at high pH greater than 9.5.
8. The increased nucleation and growth of uncleus for calcareous deposits at high temperature might be caused by the more active reaction of ion with increasing temperature.