Effect of Hydrogen Chloride Gas and Water Vapor on Hot Corrosion of Nickel at 923 K

Abstract

Hot corrosion of Ni in molten NaCl-KCl-Na₂SO₄ was investigated at 923 K in atmospheres of 1%HCl and/or 6%H₂O in O₂-N₂. Mass change measurement was accomplished by the thermogravimetric technique using a thermobalance with laser displacement meter. In the atmosphere without HCl, the mass gain of Ni with the molten salt was larger than that without it. In the atmosphere with HCl, the mass gain was quite large, and the mass gain with the molten salt was larger than that without it. With the molten salt, the higher the oxygen potential was, the larger the metal loss of Ni was. Addition of H₂O into the atmosphere increased the mass gain of Ni in the atmosphere without HCl, and incorporation of HCl decreased the mass gain. The scale formed on Ni in the molten salt in O₂-1%HCl-N₂ was NiO and was column-like in shape. The potential-pO²⁻ diagram of Ni in the molten salt in the atmosphere containing HCl was introduced. The diagram showed that the condition of this experiment provides stable dissolution of Ni into Ni²⁺, and subsequently, NiO formation from Ni²⁺ and O²⁻ decreases a_O²⁻, which causes change in equilibrium position on the potential-pO²⁻ diagram to Ni²⁺-stable region again. According to such a mechanism, it was concluded that hot corrosion of Ni in the molten salt in the atmosphere with HCl proceeds in high speed, accompanied by column-like formation of NiO in the salt layer.