Composition Modulated Multilayer Zn–Fe Alloy Coatings on Mild Steel for Better Corrosion Resistance

Abstract

Composition modulated alloy (CMA) of Zn–Fe coatings were developed on mild steel galvanostatically from chloride bath containing sulphanilic acid (SA) and ascorbic acid (AA) through single bath technique (SBT). The properties of CMA coatings were found to depend on the thickness of individual layers and switching cathode current densities (SCCDs). The CMA (Zn–Fe) coating, having 120 layers, deposited at 20 and 50 mA cm−2, were found to show the least corrosion rate (1.545 × 10−2 mmy−1) compared to monolithic alloy (32.5 × 10−2 mmy−1) of the same thickness. The improved corrosion resistance of multilayered coatings was due to the fact that the defects and failures occurring in a single layer in the deposition process is covered by the successively deposited coating layers, and hence the corrosive agent path is extended or blocked. Further, the high corrosion resistance of CMA Zn–Fe coatings was attributed to the “dielectric barrier” of the coatings, evidenced by dielectric spectroscopy and Mott–Schottky's plot. The corrosion rate was found to increase at high degree of layering, and is attributed to less relaxation time for redistribution of metal ions in diffusion layer, during plating. In other words, at higher layer thickness, the CMA coating tends to become a monolithic. CMA coatings were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM).