The effect of solution pH on characteristics of Hexafluorozirconic acid-based conversion coating for hot dip galvanized steel

In this study, a commercial zirconic acid based pretreatment was performed on hot dip galvanized steel sheets via immersion process. The effect of pH on morphology, composition and electrochemical behavior of the conversion layer was investigated. Field emission scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDS) were carried out to study the morphology and composition of the surface after treatment. Also, DC polarization and EIS were used to evaluate the corrosion behavior of the treated surface in 3.5 wt.% NaCl. Electrochemical results revealed that the best anti-corrosion performance with the lowest current density and highest Rp value, belongs to the conversion coating obtained in acid concentration of 3% and pH= 5 for galvanized steel samples. EDS results disclosed that conversion coating consisted of: Fe, Zn, O, Zr and C confirming a dense ZrO2 layer is formed on galvanized steel samples. Furthermore, FE-SEM images showed that a uniform and compact layer of conversion coating is obtained on galvanized steel surface in pH=5 which corroborates EDS and electrochemical consequences.