Role of buffering and complexing agents in zinc plating chloride baths on corrosion resistance of produced coatings

Corrosion behavior of zinc coatings produced from ammonium chloride + zinc chloride + potassium chloride bath (ACB) and zinc chloride + potassium chloride + boric acid bath (BAB) was studied using weight-loss and electrochemical techniques. It was established that the zinc coatings electrodeposited in ACB performs better than the electrodeposited coatings produced from BAB. The zinc deposition reactions in baths having ammonium chloride polarized to a greater extent than the bath blended with boric acid. Alternating current (AC) impedance studies demonstrated that the coatings produced in both types of solutions formed unstable film in corrosive media. This was corroborated by weight, loss as well as direct current (DC) polarization techniques. The presence of boric acid in the plating bath had little influence on charge-transfer resistance of zinc deposition. However, the electrochemical double-layer capacitance decreased with the increase of boric acid concentration in the bath. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) studies revealed that the coating produced in ACB has amorphous-type grains whereas the BAB coating showed a crystalline structure. It was concluded that improved corrosion resistance of the zinc coating deposited in the ACB was caused by the stronger complexing tendency of ammonium chloride with zinc cations, which resulted in a stow discharge of this ion during the deposition process