Oxidation Kinetics of Cast Alumina-Forming Austenitic Alloys in Steam

摘要

Alloy tubes used in petrochemical processing reactor systems are often subjected to oxidizing conditions in high temperature steam such as during de-coking cycles. The ability of the alloy to form an adherent and continuous oxide layer is critical to ensure the material resists coking and other high temperature attack during operation. A new class of heat resistant austenitic cast alloys are being developed that are designed to form protective oxides of alumina. Experimental compositions based on a 25Cr-35Ni base composition with aluminum additions ranging from 2.6 to 3.9 wt% were fabricated via centrifugal casting. An environment of pure steam was constructed in which samples were exposed at temperatures between 800 and 950°C for 1-30 hours. Oxide layers were characterized by surface and cross sectional Scanning Electron Microscopy (SEM), Raman spectroscopy, and Energy-Dispersive X-Ray Spectroscopy) EDS. Results via Raman and EDS show an aluminum oxide (alumina) layer covering the entire surface of all oxidized samples. Average thicknesses of alumina formed were used to generate experimental prediction curves, and showed that average alumina thickness increased with increases in both time, temperature and aluminum composition.