Study on anti-corrosion performance of composite modified magnesium ammonium phosphate cement-based coatings

Corrosion of metals not only fails the overall stability of a building but causes significant economic losses. Spraying anti-corrosion coatings is the simplest and most effective method of corrosion protection. Magnesium phosphate cement-based coatings are attractive due to their fast-setting speed, high strength and durability. This paper aims to improve the corrosion protection properties of magnesium ammonium phosphate cement-based coatings. Zinc oxide and aluminum tripolyphosphate were used as additives at 3% and 4% by mass of magnesium oxide. Meanwhile, metakaolin and aluminum tripolyphosphate at 15% by mass of magnesium oxide was added as admixtures for compound modification. In addition, the electrode potential and current density, impedance spectra, passivation films and phase composition of the materials were investigated using LPR, EIS, SEM and XRD/TG-DTG measurements. Comparing the anodic slope Ba of the polarization curve shows that the Zinc oxide mixed metakaolin specimens(Zn-CSA) have the highest Ba value, indicating that the coating prevents rusting by inhibiting the anodic electrochemical reaction. The EIS impedance spectroscopy results show that sulfate aluminate cement as a compound modifier of the admixture can effectively improve the corrosion protection of the coating. The addition of ATP is effective in inhibiting the transfer process of the coating in the charge. Furthermore, the addition of Zn is to improve the corrosion resistance of the coating by enhancing the shielding properties of the coating. SEM results show that the Zn-CSA surface is very dense, with a lamellar distribution of crystals consisting of tiny crystals closely packed at the bottom and a lamellar distribution at the top. This effectively prevents the erosion of the substrate by corrosive media. The XRD/TG-DTG results show that the composite modified coating consists mainly of incomplete MgO, the main hydration product MAP, Mg2SiO4 and MgCaSiO4. Moreover, Zn-CSA showed the lowest porosity before and after immersion in the coating porosity test. In the salt spray test, the composite-modified coating showed good corrosion resistance, with no bulging, flaking or rusting of the coating surface after 1440 h of salt spray time. This study has important implications for improving corrosion resistance and reducing corrosion costs.