Effects of Si, Mn and Rare Earth Elements on the Tenacity of High Temperature Oxide-Scale of 20Cr/30Ni/Fe Alloys

20Cr/30Ni/Fe alloys with various amounts of Si, Mn and rare earth elements were oxidized for twenty hours at 1200°C in air. The tenacity of the surface scale was estimated and the internal oxidation was investigated by means of the electron-probe microanalyzer. The internal oxide, SiO2, of the alloy without rare earth is formed irregularly and at random, while on the rare earth-containing alloy it is formed uniformly at the scale/matrix interface and extends into the grain boundaries of matrix. Mn lowers the tenacity of surface oxide scale to the matrix, but the influence of Si is different if the rare earth is present; the tenacity of the surface scale of rare earth-containing alloy increases, while that of the alloy containing no rare earth decreases, with increasing Si content. The tenacity of the scale is dependent on the mode of internal oxidation of Si. It is concluded, as reported in our earlier paper on 80Ni/20Cr alloys, that rare earth addition improves the oxidation resistance through indirect effects, i.e. by accelerating internal oxidation of Si and by making the surface chromium oxide more protective and tenacious.