The Effect of Solution pH on Morphology and Anti-corrosion Properties of Hexafluorozirconic Acid Based Conversion Coating

A nanoceramic hexafluorozirconic acid based conversion coating free of phosphate salts was investigated. The effect of solution pH on coating morphology and corrosion resistance was evaluated. Corrosion resistance of nanoceramic conversion coating on cold rolled steel (CRS) substrates in different solution pH (3.5, 4.5 and 5.5) was studied utilizing DC polarization and Electrochemical Impedance Spectroscopy (EIS). The polarization tests revealed that sample was coated in pH=4.5 have the lowest corrosion current density and nobler corrosion potential in comparison with other coated samples. The results of EIS showed that this sample (pH=4.5) has the highest polarization resistance and the lowest capacitance. According to bode diagrams, this sample indicated greater phase angle (at high frequencies) and impedance modulus (at low frequencies). The morphology of the conversion coating surface formed at different pH was investigated using Field Emission-Scanning Electron Microscopy (FE-SEM). Images revealed that the morphological structure changed with increasing pH value.