High Temperature Oxidation Behaviour of High Al Content Ferritic and Austenitic Stainless Steels With and Without Rare-Earth Element Addition

The oxidation behaviour of austenitic and ferritic alloys containing 4% Al and rare-earth element addition of (La + Ce) has been investigated, and comparisons made to an austenitic alloy with no such addition. The alloys were all found to exhibit good oxidation resistance; although, such resistance was highest when the alloy contained rare-earth elements. The addition led to a reduction in the amount of scale spalling. The scales formed after 10 and 100 h at 1,000 degrees C were examined using transmission electron microscopy and found to have bi-layered microstructures. The dislocation density and an amount of distortion in the scale were found to differ, depending on the absence or presence of (La + Ce) in the metal. It was observed that the outer-to-inner layer thickness ratio changed with time and the rare-earth element addition promoted growth of the inner layer relative to the outer layer. Analysis of the scale compositions demonstrated an apparent synergistic relationship between the effects of the rare-earth element addition and the degree to which iron is incorporated within the scales. The results are discussed in relation to the relative oxidation performance of the austenitic and ferritic alloys.