Molybdate-phosphate conversion coatings to protect steel in a simulated concrete pore solution

For the corrosion protection of steel rebars in concrete, five different molybdate-phosphate conversion coatings were prepared, by changing the molybdate-phosphate ratio in the conversion baths. Their composition and morphology were studied with GD-OES, XPS, Raman spectroscopy and SEM-EDX and a schematic representation of their structure is suggested. Further, the corrosion resistance properties of the coatings were evaluated in a simulated concrete pore solution (CPS) containing chlorides, using anodic polarization measurements. Thanks to the elaborated surface analysis their corrosion behaviour could be correlated to their structure. The results show that by altering the molybdate-phosphate ratio the structure and, consequently, the corrosion protection properties change significantly. By optimizing this ratio a twofold protection mechanism could be attained, demonstrating both good barrier properties and consistent with active corrosion protection.