Oxide Formation and Degradation during Erosion-Corrosion of Iron and Nickel Based Alloys
Erosion-corrosion is, in many technical applications, a serious material degradation process caused by dynamic particle/fluid mixtures. Pipes transporting particle containing liquids, jet engines, gas turbines, and fluidised bed combustion (FBC) power plants are examples of systems which are considered to be especially susceptible. The aim of this thesis is to investigate the mechanisms of erosion-corrosion in an environment that relates to FBC power plants.
Five steels (Fe2.25Cr1Mo, Fe9Cr1Mo, AISI 304, Esshete 1250 and 353 MA) and one Ni based alloy (Inconel 625) were studied in this work. Two different types of erosion-corrosion test rigs were used for the exposures; one nozzle type test rig and one fluidised bed on laboratory scale designed to allow a wide variety of gas atmospheres, bed materials and fluidising conditions. The exposures were performed at low particle velocities and at temperatures ranging from room temperature up to 700 °C. The exposure time varied from 24 h up to 3 weeks. Air was primarily used as exposing atmosphere, but in a minor study 50 ppm HCl or 50 ppm SO2 was added. The erosion-corrosion mechanisms at the different exposure conditions were determined from the surface chemistry and morphology. The degraded surfaces were mechanically characterised by stylus profilometry. Oxide thicknesses and compositions were measured by Auger electron spectroscopy (AES) depth profiling. Complementary information was received from SEM, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD).
At 350 °C all the alloys investigated exhibit a minimum in wastage rate, due to the formation of a composite scale with a high erosion resistance. The scale is composed of fragments from the erodent material, metal oxides and bulk metal. The degradation mechanism is erosion of both the composite scale and the bulk metal. At 550 °C an oxide scale is present, which grows at a higher rate than the corresponding scale formed during pure oxidation. It was shown that formation of cracks and other defects allowing molecular oxygen access to the oxide/metal interface was the major cause of the enhanced oxidation rate. The degradation mechanism is chipping of small oxide fragments. At 700 °C less crack formation takes place, but it is still sufficiently comprehensive to enhance the oxidation rate markedly. The degradation mechanism is extensive oxidation at the bottom of the cracks resulting in flaking of the overlaying oxide. The presence of SO2 in the atmosphere results in a decreased wastage rate and altered degradation mechanism as reflected from the wastage patterns recorded on the exposed tubes.