Influence of Sodium Hydroxide on the Austenitic 330Cb Alloy Oxidation at High Temperature

International audienceSodium hydroxide is found on metallic conveyor belts as residue of the degreasing process of metallic screws put inside heat treatment furnaces at 900 °C. The 330Cb (Fe-34Ni-23Cr-1Nb-1.55Si) alloy is used to build these conveyor belts. With time, the amount of sodium hydroxide increases on the 330Cb steel surface. The aim of this work is to show the influence of the NaOH amount on the 330Cb oxidation during 48 h, at 900 °C. Results show that, on this austenitic steel, a protective silica scale formation is promoted by low oxygen containing gaseous environments and the high alloy silicon content. With low NaOH deposits the oxide layer is adherent and a SiO 2 cristobalite subscale is formed. It is also demonstrated that sodium combines with niobium to form NaNbO 3. When the NaOH amount exceeds 0.265 mg.cm-2 on the alloy surface, severe oxidation is observed leading to a fast growing FeCr 2 O 4 oxide scale