The effect of zirconium on corrosion behavior of NiAl intermetallic compound in molten carbonate salt

The corrosion of NiAl alloys containing zirconium in molten carbonate salt at 650 degrees C was studied as a potential coating material on a stainless steel separator. A mixed layer of NiO, ZrO2, and Al2O3 was formed on the surface of the NiAl alloy during preoxidation at 1000 degrees C in air. The Ni2AlZr phase, precipitated along the grain boundaries of NiAl alloys containing zirconium, was the source of the formation of the outermost layer of Al2O3 only on the mixed oxide layer during the oxidation. The Al2O3 layer inhibited the dissolution of NiO on the NiAl-Zr alloy in molten carbonate salt. Therefore, the oxide layer formed on the NiAl-Zr alloy was more corrosion-resistant than the oxide layer on the NiAl alloy. As the amount of zirconium in NiAl-Zr alloy was increased, the corrosion rate greatly decreased due to an increasing amount of Al2O3 at the outermost surface layer oxide. The mixed oxide layer on the NiAl alloy was spalled off the alloy by air cooling after high temperature isothermal oxidation, but the spalling phenomenon of the oxide layer on the NiAl-Zr alloy with more than 0.84 at.% zirconium was greatly reduced by the improvement of the oxide adhesion due to the formation of oxide pegs of Ni2AlZr phase along grain boundaries, which were mechanically anchored to the surface oxide layer.