Hot corrosion behavior of NbSi2/SiO2-Nb2O5 multilayer coating on Nb alloy

Highlights: • A multilayer coating was formed on Nb alloy by HAPC combined with MAO. • The coating consists of a SiO₂-Nb₂O₅ ceramic outer layer and a NbSi₂ inner layer. • The coated Nb alloys shows superior hot corrosion resistance due to SiO₂-Nb₂O₅ layer. • The superior hot corrosion resistance is due to formation of glassy corroded scale. A NbSi₂/SiO₂-Nb₂O₅ multilayer coating, consisting of a NbSi₂ inner layer and a SiO₂-Nb₂O₅ outer layer, was prepared on Nb alloy through halide activated pack cementation (HAPC) and microarc oxidation (MAO). The hot corrosion behaviors of the single NbSi₂ coated and the NbSi₂/SiO₂-Nb₂O₅ coated Nb alloys were comparatively investigated with a mixture of Na₂SO₄/NaCl (3:1 wt.%) salt at 900 °C. The growth kinetics of the corroded scales follow the parabolic laws, and the parabolic rate constants of the NbSi₂ coated and the NbSi₂/SiO₂-Nb₂O₅ coated specimens were 2.16 × 10⁻³ mm s⁻² and 1.04 × 10⁻³ mm s⁻², respectively. During the corroding process, the SiO₂-Nb₂O₅ outer layer that serves as a barrier promotes the formation of continuous SiO₂ scale, and isolates the NbSi₂ layer from the corrosion salt. The lower corrosion rate of the NbSi₂/SiO₂-Nb₂O₅ coated specimens is attributed to the formation of the protective SiO₂ corroded scale.