| The hot corrosion behavior of preoxidized nickel coated with Na₂SO₄ has been studied at 1014℃, 932℃ and 853℃ in dry air. It has been observed that hot corrosion rates are remarkably retarded by means of the protective pre-oxide but, after a certain time, accelerated oxidation occurs severely because of local dissolution or mechanical fracturing by thermal stress of the protective oxide layer. The oxide layers which were formed during an accelerated oxidation stage were porous and lamellar type and sulfide phase was observed at just outer side of the matrix and grain boundaries. Based upon the above results, the hot corrosion mechanism of preoxidized nickel by Na₂SO₄ is considered as follows. In an initial stage, protective oxide layers are locally dissolved by oxide ion in fused Na₂SO₄ or mechanically fractured due to thermal stress. Fused Na₂SO₄ penetrates through the porous outer pre-oxide and forms liquid sulfide reacting with nickel matrix. In the later stage, liquid sulfide easily penetrates along the grain boundaries due to its low interfacial energy with nickel, thus grain boundary isolation occurs. |
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