Calcareous scales deposited in the organic coating defects during artificial seawater cathodic protection: Effect of zinc cations

In this work, the effect of Zn²⁺ cations on the calcareous scales during the cathodic protection of mild steel was studied. The Zn²⁺ cations were introduced into the epoxy and alkyd coatings in the form of zinc chloride. Also, the influence of Zn²⁺ cations was investigated in the zinc -rich primer coated mild steel. The study was conducted in the electrolyte and coating phases. In the first stage, pure calcareous scales deposited from artificial seawater (AS) were compared with zinc containing calcareous scales deposited from the zinc containing AS solution. For this purpose, scanning electron microscopy equipped with the energy dispersive spectroscopy and X-ray diffraction analyses were used to characterize the calcareous scales. Also, chronoamperometric, electrochemical impedance spectroscopy, and potentiostatic polarization were conducted to investigate the effect of Zn²⁺ cations on the calcareous scales. Results indicated that the Zn²⁺ cations have influenced the calcareous scales through altering calcareous deposited phases and phase proportions. Moreover, according to the electrochemical results, with increase in the Zn²⁺ cations concentration, an increase in calcareous scales inhibition was observed. In the coating phase, the effect of Zn²⁺ cations presented in the organic coatings and zinc-rich primer was studied. To evaluate this effect, the coated samples with an artificial defect were exposed to AS. Results showed that the presence of Zn²⁺ cations in the coatings led to a decrease in the brucite content of the scales and stabilization of the hydrated calcium carbonate phases over aragonite phases. Furthermore, presence of the Zn²⁺ cations in the form of zinc-rich primer leads to the brucite phase elimination in the calcareous scales.