The long-term effectiveness and failure mechanism of the chemically bonded phosphate ceramic coatings with different thickness

In this study, the chemically bonded phosphate ceramic coatings (CBPCs) with different thickness (100 µm, 200 µm, and 300 µm) are prepared on the mild steel (Q235). Potentiodynamic polarization testing and electrochemical impedance spectroscopy were carried out to study the synthetic effect of coating thickness and etching time (12, 24, 36, and 48 hours) on the corrosion behavior of CBPCs. The surface and cross section microstructure of CBPCs before and after different immersion time in 3.5 wt.% NaCl solution is investigated to better understand the corrosion behavior. Results revealed that the enhanced coating thickness can effectively postpone the corrosive factors infiltration into the substrate and prohibit the corrosion of the mild steel. From the potentiodynamic polarization testing results, the protection efficiency of the coating greatly increases with the increase in etching time. In addition, an increase in 10 000 and 1000 orders of magnitude of impedance value for the CBPCs with 12 hours and 48 hours etching time has been observed, respectively, indicating the CBPCs can effectively protect the mild steel from corrosion damage even with long-term service.