The effects of reactive elements on the oxidation of alumina-forming iron-chromium-aluminium alloys at high temperature

A study is being undertaken into the influence of sulphur and reactive elements on the growth and adhesion of alumina scales on iron-chromium-base alloys during thermal cycling in air from 1300 degrees C. A tow-sulphur-(< 10 ppm) containing alloy has shown considerable improvements in scale adhesion compared with an alloy containing a higher concentration of sulphur. Additions of reactive elements to the low sulphur-containing alloy had little further effect on scale adhesion, although second-phase precipitates facilitated preferential inward growth of the scale and the development of a very irregular scale/alloy interface. All the alloys showed evidence for stress development during scale growth, but this was often relieved by flow of the weak alloy. Although not extensive, all the low sulphur-containing alloys exhibited some scale failure, involving both a decohesion, buckling and tensile cracking route and a shear cracking and wedging route.