Hot corrosion of surface-modified Sm₂Co₁₇ high-temperature magnet with Ni and Ni/Cr bilayer coatings in 75 wt% Na₂SO₄–NaCl mixture

Hot corrosion behavior of Sm₂(Co, Fe, Cu, Zr)₁₇-type high-temperature magnetic alloy without and with a protective coating of Ni and Ni/Cr bilayer at 500 °C in a corrosive mixture of 75 wt% Na₂SO₄–NaCl for 100 h was reported in this paper. The obtained results of time-dependent weight change established the parabolic growth of oxidized surface of bare magnets (8.87 mg·cm⁻²), which caused a rapid loss of magnetic properties measured both at room and high temperatures (500 °C). X-ray diffraction (XRD) analysis revealed the growth of oxides for bare magnet and Cr–oxides (Cr₂O₃), top layer of Ni/Cr bilayer coating, but in the case of single coating, Ni–sulfides formation indicated penetration of sulfur, which is further verified by electron probe microanalysis with energy dispersive spectroscopy (EPMA/EDS) study. Results showed that bilayer-coated samples (Ni/Cr) performed better than Ni-coated and bare samples.