EFFECT OF SURFACE-APPLIED REACTIVE ELEMENTS ON THE EARLY-STAGE OXIDATION OF FE-18CR-5AL AND FE-18CR-5AL-1HF ALLOYS

The effect of lanthanum implantation on the early stage oxidation of Fe-18wt%Cr-5wt%Al and Fe-18wt%Cr-5wt%Al-1wt%Hf alloys at 1100-degrees-C has been investigated. It was found that the reactive element enriched surfaces oxidized almost two times faster initially, and the scale that formed had a higher iron content. An extremely wavy scale/alloy interface developed in the early stage due to the accelerated growth and the change in scale composition. More scale separation was observed on the implanted surface of the Fe-Cr-Al alloy. Once separated, the alumina scale was free to grow laterally, and large growth buckles developed with oxidation time. The Hf-containing alloy did not show any scale separation regardless of the accelerated initial growth from lanthanum implantation. Sulfur was found everywhere at the scale/alloy interface on the Fe-Cr-Al alloy, but not on the Hf-containing alloy. These results suggest that for this alloy system, scale adhesion was dominated by a strong scale/alloy interface, and weakening of the interface may be caused by interfacial sulfur segregation.