Corrosion Performance Evaluation of Bi-layered Zn/Zn-ZnO-CaCO3 Composite Coatings on Mild Steel under Marine Conditions

Abstract

Mild steel, widely used in marine structural applications is highly susceptible to corrosion and biofouling. In this study, nano-CaCO₃ particles synthesized from periwinkle shells (PS) and eggshells (ES) were incorporated into a-Zn/Zn-ZnO matrix via electrolytic co-deposition to develop functionally graded coatings on mild steel. PS and ES CaCO₃ nano particles were incorporated into the bath in varying mass ratios from 0 to 4.56 g. The co-deposition process of Zn-ZnO-CaCO₃ composite coatings on zinc-coated mild steel was optimized using a central composite design of experiments (DoE) framework, targeting uniform coating thickness and nanoparticle dispersion. Electrochemical corrosion tests were performed on single-layer Zn-coated, uncoated and bi-layered Zn/Zn-ZnO-CaCO₃ composite coated mild steel in seawater marine environment. Polarization resistance of single-layer zinc coatings was assessed by Tafel analysis using potentiodynamic polarization. Electrochemical frequency modulation (EFM) was employed to determine instantaneous corrosion rates, polarization resistance and validate corrosion protection efficiency of the bi-layered Zn/Zn-ZnO-CaCO₃ composite coatings on mild steel. The bi-layered Zn/Zn-ZnO-CaCO₃ composite coating with ES-derived CaCO₃ (4.56 g) exhibited superior corrosion performance, with an average corrosion rate (CR) of 0.007 mm/year, corresponding to 93.15 % coating efficiency; and average polarization resistance (Rp) of 59,260.17 Ω.cm2, yielding 98.15% improvement in Rp over the uncoated mild steel. Scanning Electron Microscopy (SEM) and energy dispersion X-ray spectroscopy (EDS) examinations confirmed dense coating morphology and presence of zinc (Zn), oxygen (O), and calcium (Ca) in the coatings. The Zn underlayer provided sacrificial anodic protection, while the Zn-ZnO–CaCO₃ composite bi-layer served as physical barrier and bioactive interface. These results demonstrate that CaCO₃-modified Zn-ZnO coatings offer enhanced dual protection against electrochemical and microbial corrosion in marine environments.