The salt-induced corrosion behaviour of FeCr alloys at elevated temperatures—II. Alloys rich in chromium

The oxidation behaviour of two Na₂SO₄-coated, chromia-forming, iron-based alloys at 900°C has been studied thermogravimetrically, and the reaction products examined in detail by metallographic and E.P.M.A. techniques. Na₃SO₄ coatings markedly enhance the oxidation rates of both alloys and result in the formation of thick, compact, stratified scales. On the basis of subsequent experiments designed to characterize the singular roles of sodium oxide and sulphur, and in the absence of scale fluxing, it is postulated that the formation of sulphides in the alloy substrate and the mechanical failure of scale are responsible for the enhanced oxidation. Sodium chromate, a feature of the hot corrosion reactions of Na₂SO₄-coated chromia-forming, Ni- and Co-based binary alloys, is shown not to be a by-product of the corrosion reaction of equivalent iron-based alloys. Instead salt/scale reactions result in the formation of sodium-iron oxide, which is capable of assisting in the corrosion reaction, albeit in a minor way. The role of NaCl additions during Na₂SO₄ induced corrosion is also examined.