THE EFFECT OF REACTIVE ELEMENT COATINGS ON THE OXIDATION BEHAVIOR OF PURE CR AND HIGH CR CONTENT ALLOYS AT 900-DEGREES-C

The influence of various reactive element (RE) oxide coatings (Y2O3, CeO2, La2O3, CaO and HfO2) on the oxidation behaviour of pure Cr, Fe26Cr and Ni25Cr at 900-degrees-C in oxygen at 5 x 10(-3) torr has been investigated using the O-18/SIMS technique. Polished samples were reactively sputter-coated with 4 nm of the RE oxide and subsequently oxidized sequentially in O-16(2) and then in O-18(2). The effectiveness of each RE on the extent of oxidation rate reduction varied with the element used. SIMS sputter profiles showed that the maximum in the RE profile moved away from the alloy-oxide interface during the early stages of oxidation and after a certain time remained within the oxide scale, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings O-18 was found to have diffused through the oxide to the alloy-oxide interface during oxidation the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxide growth mechanism to predominantly anion diffusion.