Corrosion Resistance of Magnesium Ammonium Phosphate Cement-Based Coatings Modified by Calcium Sulfoaluminate Cement on Carbon Steel in a Saline Medium

Abstract

Calcium sulfoaluminate (CSA) cement was used as an internal admixture to replace some of the magnesium oxide (MgO) in magnesium ammonium phosphate cement (MAPC) coatings. This experiment resulted in improved corrosion resistance and production cost savings for MAPC coatings on carbon steel surfaces. The corrosion resistance of coated carbon steel with different CSA doping concentrations was evaluated by electrochemical methods including Tafel polarization curves and electrochemical impedance spectroscopy (EIS). At the same time, 7-day flexural and compressive tests, Low-field nuclear magnetic resonance spectroscopy (NMR) test, X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric and differential thermogravimetric techniques (TG-DTG) were performed on the modified coatings. Neutral salt spray tests confirmed the improved corrosion resistance of the coated carbon steel. Based on the results of the study, the best ratio of CSA-modified MAPC coatings was finalized. The addition of CSA improved the flexural and compressive strength of MAPC. At later stages of immersion, there was an increase in the polarization resistance value, and a significant increase in the anodic slope of the polarization curve. Furthermore, the overall low-frequency impedance value, coatings resistance, and charge transfer resistance all experienced a substantial increase. The microstructural study revealed that Ca4Al6O12(SO4) gradually hydrated during immersion to produce amorphous hydrated calcium sulfoaluminate gel, significantly improving the anticorrosion performance of the coated carbon steel. After the coated carbon steel were exposed to a neutral salt spray environment for 1,440 h, there was no bulging or flaking of the coatings surface, and no rusting of the carbon steel surface. The modified coatings effectively served as a protective layer.