Theory of the Mass Crystallization of Poorly Soluble Compounds on a Cathode's Surface in Hard Water

The formation of calcareous deposit determines the optimum regimes of hard water softening and cathodic protection. The mass transfer of $\text{O}{\text{H}}^{-}$, ${\text{CO}}_{\text{3}}^{\text{2}-}$, and ${\text{HCO}}_{\text{3}}^{-}$ ions in the diffusion layer caused by direct current is studied in this work with allowance for the crystallization of CaCO₃ and Mg(OH)₂ on a cathode's surface. It is shown that as the current density grows, the crystallization current of CaCO₃ is first attained, and the crystallization current of Mg(OH)₂ is then achieved. Simulations showed that the molar fraction of CaCO₃ in the cathode deposit shrinks, while that of Mg(OH)₂ increases as the current density grows; the molar fractions are equal at a current density of 1.4 A/m². The calculation results are proven by direct experiments to analyze cathode deposits formed in sea water at different densities of an external current.