Electrochemical and surface morphology characterizations of zinc electroplated coatings

Abstract

Acid baths containing sulfate still lack industrial purposes, although they are classified as low toxicity and represent smaller risks to the environment and operators when compared to the traditional alkaline baths. So, the influence of zinc concentration (Zn²⁺), current density (i), temperature (T), and pH levels were examined during the Zn electroplating process in sulfate baths over process efficiency, coating morphology, and corrosion resistance. The current efficiency decreased as decreasing i (from 30.5 mA/cm² to 7.5 mA/cm²). Increasing both Zn²⁺ and T promoted a reduction in the average grain size with nodules structures spread over the coating surface as shown by scanning electron microscope (SEM) analysis. Increasing the bath acidity resulted in a higher roughness profile and a more fragile coating, while the substrate was not completely covered at low i as shown by energy-dispersive x-ray spectroscopy (EDS) analysis. As expected, two capacitive arcs were observed in the electrochemical impedance spectroscopy (EIS) test. Therefore, among the completely covered samples, the conditions Zn²⁺= 50 g/L, 30.5 mA/cm², 50 °C, and pH = 2.5 presented greater deposit homogeneity, grain refinement and increased impedance modulus (2850.26 Ω.cm² at 30 mHz).