Study on the microstructures and corrosion behavior of plasma sprayed dicalcium silicate coatings

In this study, dicalcium silicate coatings on stainless steel substrates were prepared by atmospheric plasma spraying. Immersion tests in 3.5 wt% NaCl solution and artificial seawater were carried out to evaluate the protection performance of coatings. XRD, SEM and EDS were used to analyze the phase compositions and microstructures of the coatings. During the immersion, various calcium carbonate crystals appeared on the surface of coatings. The pores and micro cracks in dicalcium silicate coatings were filled with the hydration products, producing denser coatings. A silica-rich layer under calcium carbonate crystals was observed in coatings immersed in artificial seawater but not in coatings immersed in NaCl solution. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) plots indicated that the immersion process in NaCl solution and artificial seawater both increased the corrosion resistance of Ca2SiO4 coatings. Further, the artificial seawater immersion is more in favor of the corrosion resistance of Ca2SiO4 coatings compared with the saltwater immersion. Salt spray tests were also used to evaluate anti-corrosive characteristics of the coatings. There was no visible rusty spot during the whole test, while calcium carbonate crystals appeared on the surface of Ca2SiO4 coatings.