Investigation of effect of solution pH, immersion time and solution temperature of zinc phosphate conversion coating on cold rolled steel: Morphological and electrochemical study

Phosphate conversion coating is one of the conversion coatings that have high applications indifferent industries due to the good corrosion resistance. In this study, anti-corrosion resistance of the zinc phosphate conversion coatings on cold rolled steel (CRS) with different immersion times (1, 5 and 25 min), different solution temperatures (37-57 ° C) and different pH values (1.7, 2.7, 3.7, 4.7 and 5.7) was studied, using direct current (DC) polarization and electrochemical impedance spectroscopy (EIS). Electrochemical results revealed that the best anti-corrosion resistance from zinc phosphate conversion coating was obtained in 47 °C, immersion time of 25 minutes and pH =2.7. In this point the polarization resistance (Rp) value is 225 times higher than the bare sample polarization resistance (Rp: 0.39 kΩ.cm2). Also the morphology of coated sample in optimum condition by using a field emission scanning electron microscope (FE-SEM) was evaluated. The granular structure and uniform distribution of crystals on the sample surface is a reason to confirm the obtained results from electrochemical tests for optimized sample.