Serious corrosion was experienced on 18 Cr-Si-Al steel tubes used in the high temperature zone of an oil-firing furnace. In order to investigate the parameters for resulting corrosion, several corrosion tests were carried on thirteen types of steels including 18 Cr-Si-Al immersed in the fused salts of V₂O₅, K₂SO₄ and Na₂SO₄ at various temperatures. Chromium steels containing Al were found resistant to the vanadium-attack but badly corroded by a K₂SO₄ and Na₂SO₄ mixture with a small addition of V₂O₅. K₃Al⋅(SO₄)₃ was thought to be a possible factor for the corrosion because its molten phase resulting from the high temperature reaction provoked by K₂SO₄ and Al₂O₃ seemed to be able to exist in the temperature range of 700-900°C in which the corrosion of the tubes occurred.