Should Gold Marker or TEM-ASTAR Characterization Be Used to Determine Oxide Growth Direction?
Résumé
Oxidation of chromia-forming alloys (pure chromium, nickel–chromium alloy and ferritic stainless steel alloy) at temperature ranging from 800 to 1050 °C was conducted on samples with or without gold markers applied prior to testing. Gold is often used as a marker in order to identify oxide growth direction. Its localization inside the oxide scale can demonstrate an inward or an outward growth. It is found that gold particles can be found either at the internal metal/oxide interface or inside the scale or again at the external oxide/gas interface on the same sample (on NiCr and stainless steel alloys). For pure chromium oxidation, gold is found inside the chromia scale, at the equiaxed-columnar interface, as expected from ASTAR-TEM characterization. However, photoelectrochemical characterization has shown that gold can modify the semiconducting chromia properties. Finally, gold is not inert in these several conditions and should not be considered in these present cases, but also more generally in most cases, as an appropriate marker. On the contrary, ASTAR maps may permit to access the oxide growth direction without interfering the studied system.