Inhibited aluminization of an ODS FeCr alloy

Aluminide coatings are of interest for fusion energy applications both for compatibility with liquid Pb–Li and to form an alumina layer that acts as a tritium permeation barrier. Oxide dispersion strengthened (ODS) ferritic steels are a structural material candidate for commercial reactor concepts expected to operate above 600 °C. Aluminizing was conducted in a laboratory scale chemical vapor deposition reactor using accepted conditions for coating Fe- and Ni-base alloys. However, the measured mass gains on the current batch of ODS Fe–14Cr were extremely low compared to other conventional and ODS alloys. After aluminizing at two different Al activities at 900 °C and at 1100 °C, characterization showed that the ODS Fe–14Cr specimens formed a dense, primarily AlN layer that prevented Al uptake. This alloy batch contained a higher (> 5000 ppma) N content than the other alloys coated and this is the most likely reason for the inhibited aluminization. Other factors such as the high O content, small (~ 140 nm) grain size and Y–Ti oxide nano-clusters in ODS Fe–14Cr also could have contributed to the observed behavior. Examples of typical aluminide coatings formed on conventional and ODS Fe- and Ni-base alloys are shown for comparison.