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.