Corrosion Resistance Of Air Plasma Sprayed Ca₂SiO₄ Coating

Marine engineering materials,especially metal structural materials suffer from different levels of corrosion damage.Thus,measures must be taken to protect the metal materials served in the harsh marine environment.Organic paint might age and then spall when it works for a long time under the exposure of sunshine or in seawater,so its durability needs to be improved.Coatings need good wear resistance and electrical insulation in some special conditions.Alloy coatings exhibit good corrosion resistance in marine environment,except for that the good electrical conductivity.The oxides such as Al₂O₃ and Al₂O₃/TiO₂ showed good corrosion resistance,but there are some pores and cracks in coatings,which would weaken the corrosion resistance of coatings.Ca₂SiO₄ might show a better corrosion resistance as the main composition of anti-corrosion cement.Dicalcium silicate is a kind of gel material,which would react with water to generate hydrated calcium silicate gel.Hydrated calcium silicate gel would fill the pores and micro cracks in the structure and improve the density of coating.It might be benifite to the corrosion resistance and electrical insulation.In this study,Ca₂SiO₄coating was sprayed on stainless steel substrate and the corrosion resistance of the as-sprayed coating was studied in salt water.At the same time,Al₂O₃ coatings were produced by air plasma sprayed technology as comparison.The influence of bond coat on corrosion resistance of ceramic coatings were also studied.In the current work,dicalcium silicate powders were synthesized by solid state reaction using calcium carbonate and silicon dioxide.Immersion test in 3.5 wt%NaCl solution and simulated ocean water were carried out to evaluate the protection performance of coatings.XRD,SEM and EDS tests were used to analyze the phase compositions and microstructures of the coatings after immersion.Potentiodynamic polarization curves and electrochemical impedance spectroscopy?EIS?plots were also analyzed.Salt–spraying test was carried out to study the protection property of coatings.From the results of above investigation,it is concluded that as follows:1)Ca₂SiO₄ coating reacted with water to release Ca?OH?₂.High Ca²⁺concentration promoted to form rhombic calcite.As the slow dissolution of CO₂ gas,Ca²⁺ concentration decreased,spherical vaterite formed on the coating surface.Only aragonite formed on Ca₂SiO₄ coating when immersed in simulated ocean water, which was due to the existed Mg²⁺.2)The pores and micro cracks in Ca₂SiO₄ coatings were filled with its hydration products,namely C-S-H gel,to make the coating denser.The corrosion current density of Al₂O₃ coatings went up sharply after immersed,and the microstructure became looser and porous with obvious rusty spots during the immersion test.3)Ca₂SiO₄ coatings released Ca?OH?₂ in NaCl solution to promote the formation of calcium carbonate crystal and passivation film which improved the corrosion resistance.4)There were also passivation behavior in Ca₂SiO₄ coatings and Al₂O₃ coatings after immersed in simulated ocean water.The pH of the simulated ocean water was higher than 3.5 wt%NaCl solution,which promoted the formation of passivation film of the substrate.5)The coatings without NiCr bond coat showed a better corrosion resistance than those with bond coat.6)Ca₂SiO₄ coating shows a better corrosion resistance than Al₂O₃ coating especially in 3.5 wt%NaCl solution.