Effect of divalent cations in low zinc ambient temperature phosphating bath

A new zinc phosphating bath, which produces coatings at relatively lower temperatures within a reasonable time by using of chemical accelerators has been devised. Improvement of the bath performance by the addition of divalent cations like calcium, manganese and magnesium has been studied. Bath formulation and operating conditions have been optimized by coating weight determinations. Corrosion resistance property of the resultant coatings has been evaluated in 1,000 ppm Cl⁻ by electrochemical methods such as potentiodynamic polarization and impedance measurements. Results of the electrochemical techniques have been complemented by salt spray, humidity and immersion tests. Porosity and roughness of the coatings have also been studied. Results show that the phosphating bath with manganese addition gives good coatings within 30 min. Studies have shown that the corrosion resistance of the resultant coatings are much superior than the conventional coatings.