Corrosion of metals and alloys in sulfate melts at 750°C

The corrosion of Ni, Co, Ni-10Cr, Co-21Cr, and IN738 was studied at 750°C in the presence of molten sulfate mixtures (Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄) and in an atmosphere consisting of O₂+0.12% SO₂-SO₃. The corrosion was observed to be similar for both Na₂SO₄-Li₂SO₄ and Na₂SO₄-CoSO₄ melts. The corrosion of Ni and Co tookplace by the formation of a mixed oxide plus sulfide scale, very similar to the corrosion in SO₂ or SO₃ alone. The initial stage for the corrosion of Ni-10Cr involved the formation of a thick NiO+Ni₃S₂ duplex scale, and Cr sulfide was formed during the later stages. A pitting type of morphology was observed for both Co-21 Cr and IN738. The pit was Cr sulfide at the beginning, and subsequently the sulfides oxidized to Cr₂O₃. A base-metal oxide layer was present above the pit, and this was observed to be formed very early in the corrosion process. A mechanism is proposed to explain this. In general, the formation of sulfides appears to be the primary mode of degradation in mixed sulfate melts.