Investigation of corrosion mechanism of ribbed mild steel bars coated with magnesium potassium phosphate cement paste

This study investigated the anti-corrosion performance of magnesium potassium phosphate cement (MKPC) paste applied to the surface of ribbed mild steel bars - which was exposed to simulated accelerated corrosive environment. Four electrochemical approaches were used including open-circuit potential (OCP), electrochemical impedance spectroscope (EIS), polarization resistance (PR) and potentiodynamic polarization (PDP) over a period of 5376 h (224 days). Moreover, visual inspection, optical microscope, and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) were used to assess the extent of corrosion on the samples. To understand the mechanism of corrosion protection of the coating system, X-ray photoelectron spectroscopy (XPS) was employed to characterize the chemical groups on the surface of mild steel, and the chemical changes in the coating layer were characterized using thermogravimetric/differential thermal analysis (TG/DTA) and X-ray diffraction (XRD). The MKPC paste coated bars were compared with not only uncoated bars, but also bars coated with ordinary Portland cement (OPC) that is known to passivate steel due to its high alkalinity. Results indicated that MKPC paste coating layer could effectively protect the ribbed mild steel bars, and its protectiveness significantly surpassed that of OPC. Both the de-passivation effects of chloride ions and carbonation of the OPC resulted in relatively severe corrosion of the OPC coated bars during the long exposure duration; while the anticorrosion merit of the MKPC paste coating layer could be attribute to a double-protection system- the dense microstructure of MKPC and the formation of an iron (III) phosphate passivation layer between the substrate steel and the MKPC paste coating layer.