Influence of gas phase composition on the Hot Corrosion of steels and nickel‐based alloys beneath a (Ca‐Na‐K)‐sulfate mixture containing PbSO4 and ZnSO4

The corrosion of steels and of nickel‐based alloys was studied in exposure experiments at 600 °C beneath a molten CaSO₄‐K₂SO₄‐Na₂SO₄‐PbSO₄‐ZnSO₄ sulphate mixture in N₂‐5 vol.% O₂ with and without additions of 1000 vppm HCl, 1000 vppm SO₂ and 1000 vppm HCl in combination with 250 vppm SO₂. In the N₂‐5 vol.% O₂ atmosphere, the corrosion products are iron‐ and nickel‐rich but chromium‐free precipitates of oxides in the solidified melt. Additionally, pits filled with layered corrosion products are formed, growing into the metal substrate. These layers consists of less soluble chromium‐rich oxides, containing varying amounts of zinc, (ZnCr₂O₄) alternating with potassium‐rich sulfates, most probably K₂S₂O₇. The addition of 1000 vppm SO₂ leads to a seperation of the melt in a K₂S₂O₇ part close to the metal surface and a Ca‐rich part on top in contact with the gas atmosphere. Compared to the N₂‐5 vol. % O₂ atmosphere accelerated corrosion was observed. In the K₂S₂O₇ part of the melt dissolved iron and nickel are identified, whereas in the Ca‐rich part iron‐ and nickel‐oxide precipitates are formed. Underneath the solidified salt, thin layers of sulfides are detected. In the N₂‐5 vol.% O₂‐1000 vppm HCl containing gas, the corrosive attack is also accelerated compared to the N₂‐5 vol.% O₂‐atmosphere. Much more oxide precipitates are found in the melt on every sample and the inward growth of the zinc‐free chromium‐rich oxides is significantly enhanced. Underneath the inward growing oxide small amounts of metal‐chlorides are detected. Compared to the SO₂ containing gas, the corrosive attack is enhanced for the iron‐based materials, but retarded for the nickel‐based alloys. In the 1000 vppm HCl‐250 vppm SO₂ containing gas, the corrosive attack is similar to the atmosphere containing only 1000 vppm HCl. In addition, sulfides are formed next to chlorides at the metal/scale interface.