Influence of formic acid on the microstructure and corrosion resistance of Zn–Ni alloy coatings by electrodeposition

In this study the effect on the deposition efficiency and corrosion resistance of Zn–Ni electrodeposits after the addition of formic acid (0.03 to 0.53 mol/L) to the plating bath is investigated. The corrosion resistance was evaluated in an aerated solution of 2.0 mol/L NaOH and 0.5 mol/L NaCl using weight loss tests and electrochemical measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy measurements. The addition of formic acid to chemical bath enhanced the corrosion resistance of the Zn–Ni deposit in both solutions. Above 0.07 mol/L formic acid added to the solution, which corresponds to pH 3.0, the corrosion rate does not vary significantly. Improved corrosion resistance, by adding 0.07 mol/L formic acid, was found in deposits obtained from chemical bath with the same pH. The addition of formic acid to the chemical bath of the Zn–Ni deposit produces an enhancement in the efficiency of galvanostatic deposition when the pH of the bath is maintained at approximately pH 3.0. It was observed that the addition of formic acid changes the structure of the Zn–Ni deposit due to the presence of the Ni5Zn21 intermetallic phase.