The mechanism of calcareous deposition on steel plates cathodically protected in seawater was investigated. When the cathodic current density was increased with constant total electric quantity passed, the amounts of both CaCO_3 and Mg(OH)_2 deposited reached maximum values. To account for this phenomenon, the zeta-potentials of colloidal particles of CaCO_3 and Mg(OH)_2 suspended in seawater were measured as a function of pH. With increasing pH, the amount of formed solids increased, but the zeta-potential decreased. These two mutually conflicting factors are closely related to the occurrence of the maxima in the above calcareous deposit amount vs. current density curve.
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