Electroplating of Zn- Al composite coating at different current densities onto mild steel for improved corrosion resistance

Abstract

The objective of electro galvanizing was to replace hot dip galvanization method. The advantage of electro galvanizing is to reduce the thickness as compare to hot dip galvanization (HDG). The problem of heat affected zone (HAZ) may occur in HDG whereas in electro galvanizing, this problem away which arise in HDG. To increase the corrosion resistance of electrogalvanized deposition, alloying element was used. Alloying elements increase the strength, hardness and resistance to wear property. Here in this work Al nano powder as an alloying element was used to increase the corrosion resistance of the coating on the surface of MS. Zn-Al electrodeposition was done in acidic sulphate bath at the pH of 3.5 on the surface of MS. Zn-Al composite coating was done at selected current densities. The range of deposition current density was obtained by a systematic cyclic voltammetry (CV) study. Zn-Al composite coating was done at the specific current densities i.e. at -50, -150, -180, -250 mA/cm2. After deposition the corrosion test was done in 3.5 wt% NaCl. Corrosion rate of deposition was found using potentiodynamic polarization method. Best corrosion result was found at -180 mA/cm2 current density. Pulse deposition was done after the DC deposition at the average current density of -180 mA/cm2. The parameters of the pulse deposition were decided with respect to -180 mA/cm2 average current density. Pulse deposition occurred at different duty cycle, frequency and peak current density (PCD). After the deposition again corrosion test was done through potentiodynamic polarization technique. Thickness of coating was measured using surface profilometer and was found to be in the range of 10 μm to 42 μm. The characterization of the deposition was done by different technique. SEM images, elemental mapping and XRD data were taken and analysed. Corrosion rate was obtained by tafel extrapolation. Electrochemical impedance spectroscopy (EIS) test was done to find out the mechanism of corrosion. 3.5 wt % NaCl electrolyte was used for EIS test in the frequency range of 105 Hz to 10-2 Hz at 10 mV AC potential. The corrosion resistance of coating was slightly increased and also coarse grain structure was obtained when SEM images were compared.