Nanoceria coating imperfections and their effect on the high-temperature oxidation resistance of a 304 stainless steel

The microemulsion method was employed in this work for synthesizing nanocrystalline cerium oxide particles. Average nanoparticle sizes of 3.45 nm were produced by these means. XPS determinations indicated that both Ce³⁺ and Ce⁴⁺ are present in the synthesized nanoceria particles, with an average amount of 22.8 % of Ce³⁺ ions. It was found that the nanocrystalline cerium oxide coatings lead to significant improvements (of 1–2 orders of magnitude) in the high-temperature oxidation resistance of 304 SS. In addition, it was found that by decreasing the nanoparticle mean size from 10 to 3.45 nm, the effect of the coating protection was drastically improved. The experimentally determined parabolic rate constants k _p at 1073 and 1273 K for 304 SS indicated a reduction of up to two orders of magnitude when nanoceria coatings with 3.45 nm in mean size were applied. Also, the scale thickness was reduced from 15 to 5 μm when oxidized at 1073 K for 442 h. Coatings with purchased nanoceria particles (10 nm) were not as effective as the coatings with synthesized nanoceria. Apparently, at increasing nanoceria sizes, the oxidation protection is significantly reduced. In addition, it was found that the method of dipping for coating 304 SS does not provide a uniform coverage with nanoceria particles. Fe-rich ‘islands’ develop during high-temperature oxidation indicating that some regions do not exhibit the protection that nanoceria can provide. In contrast, relatively thick-coating regions on the steel substrate exhibit minimal oxidation, and the resultant scale is fine grained and uniform.