The objective of electro-galvanizing was to replace hot dip galvanizing (HDG). In the current investigation, Zn, Zn-Al composite and Zn-Ni alloy electrodeposition was done in acidic sulphate bath at the pH of 3.5 onto mild steel both in direct current (DC) as well as pulsed current mode to explore whether the two techniques would lead to a difference in corrosion behavior of the plated films. A systematic cyclic voltammetry (CV) study was done to get the range of current density (CD) for deposition i.e. -50, -150, -180, -250 mA/cm2. Pulse deposition was done after the DC deposition at the average current density of -180 mA/cm2 at different duty cycles, frequencies and peak current density (PCD). After deposition, coating thickness, adherence and structure was analysed thoroughly. The thickness of all the coatings was found to be in the range of 10 µm to 42 µm. After the analysis of coating, the corrosion behaviour was obtained by potentiodynamic polarization followed by Tafel extrapolation and electrochemical impedance spectroscopy (EIS) with 10 mV AC potential in between 105 to 0.01 Hz in 3.5 wt% NaCl. For pure Zn deposits, best corrosion result was found to be 0.35 mm/a and 0.102 mm/a for the films deposited at -180 mA/cm2 CD and a combination of duty cycle 25 % i.e. PCD of -720 mA/cm2 and frequency of 75 Hz respectively. Best adhesive property was also obtained at -180 mA/cm2 current density whereas for pulse deposition, it shows good properties at -720 mA/cm2 PCD at frequency of 25 Hz with 25 % duty cycle.
For Zn-Al deposition, Al nano-powder (< 80 nm) was added to synthesize the composite coatings. Best adherence of coating was also obtained at -180 mA/cm2 current density for DC deposition and for pulse deposition, it was found for -720 mA/cm2 PCD at the frequency of 200 Hz with 25 % duty cycle. The best corrosion outcome was obtained at a current density of -180 mA/cm2 with 0.37 mm/a corrosion rate. Whereas less corrosion rate (0.13 mm/a) was obtained at high peak current density i.e. -720 mA/cm2 at 200 Hz frequency with 25 % duty cycle. done by different technique. A coarse grain structure was acquired and the coating&rsquos corrosion resistance was somewhat improved as compared to Zn coatings. After obtaining the effect on Zn and Zn-Al composite coating, Zn-Ni coating was attempted to check whether the corrosion resistance can further be improved. From the PDP, EIS and adherence analysis, the best-performing film was found to be deposited at -180 mA/cm2. For PCD, the optimized parameter was obtained at low frequency (25Hz) for high (-720 mA/cm2) and medium (-360 mA/cm2) PCDs whereas at low PCD (-240 mA/cm2), it was obtained for medium frequency (75 Hz).
Furthermore, a Hull cell deposition was used to investigate and evaluate the impact of current pulsing and industrialization of the process based on the optimal parameter achieved. The good corrosion properties were obtained for Zn, Zn-Al and Zn-Ni coating at -180 mA/cm2 current density compared to hot dip galvanized sheet. The salt spray test is also a typical process for industrial validation which was used for Zn-Al composite coating. It has been found that after 96 hours of salt spraying at a rate of 0.8 cm3/s, the deposits remained unaltered.