Ni-Cr合金のNa₂SO₄-NaCl混合塩による高温腐食

抄録

The effect of the salt composition on the Na₂SO₄-NaCl induced hot corrosion of Ni-5Cr, -10Cr and -20Cr binary alloys was examined by modified coating test in still air in a temperature range from 1173 to 1373 K. The corrosion loss and the depth of penetration were measured, and the role of NaCl in corrosion by Na₂SO₄ and the effect of chromium content on corrosion of nickel were discussed.
Corrosion losses of Ni-Cr alloys showed the smallest values at 100%Na₂SO₄ and increased gradually with increasing NaCl content, then indicated a maximum value at 100%NaCl, with the exception of Ni-5Cr alloys at higher temperatures. Corrosion loss of the Ni-5Cr alloy at 1373 K was very large at Na₂SO₄-rich salt compositions and decreased with increasing NaCl content. This behavior was very similar to that of pure nickel.
The total depth of penetration of Ni-Cr alloys increased with increasing NaCl content and had maximum values at the salt compositions of 50-75%NaCl. Addition of NaCl to Na₂SO₄ accelerated internal penetration. In an internal penetration layer, there existed chromium sulfides only in grain boundaries at 100%Na₂SO₄, and chromium sulfides particles also dispersed within grains at Na₂SO₄-NaCl mixtures.
Corrosion products were composed of NiO, Cr₂O₃ and NiCr₂O₄, and NiO increased in quantity with NaCl content. NaCl promoted spalling of scales.
At Na₂SO₄-rich salt compositions, an addition of chromium markedly suppressed accelerated oxidation of nickel, but it became less effective with increasing NaCl content. While chromium addition was still effective even at 75%NaCl mixture, no effect was obtained at 100%NaCl.