The evolution of titanium oxidation at elevated temperature and its oxide scale morphology

The purpose of this study was to experimentally quantify the multi-dimensional growth characteristics of the oxide scale formed on commercially pure titanium at 700°C in a flowing air environment. The geometries considered herein had characteristic dimensions that were appropriately sized to match the thickness of the oxide scale and were fabricated into shapes of solid and hollow cylinders and external and internal wedges. Scanning electron microscopy (SEM) image analysis was used to measure the oxide layer thickness and the Pilling-Bedworth ratio (PBR) as a function of time. An effective diffusion coefficient was determined from one-dimensional planar oxide thickness data and experimentally obtained PBR values served as the necessary input to a solid state diffusion model, which was modified to account for the volumetric expansion of the oxide.; Oxidation of the solid cylinder and external wedge geometries were shown to develop a scale morphology similar to that observed on a flat specimen. The oxide had two notable features: (1) at the air-oxide interface, the oxide appeared to be compact and its thickness grew with time and (2) from the metal-oxide interface up to the compact scale, the oxide was found to have a porous-layered arrangement with the pore size being a function of distance from the metal-oxide interface. Conversely, the oxide scale growth on the hollow cylinders and external wedges, while still layered, appeared to be much less porous and had considerably less cracking and spalling damage.; The modified solid-state diffusion model and experimentally obtained values of the diffusion coefficient and PBR were used to demonstrate the competing influences of oxide expansion and curvature effects. In addition, the predictive capability of the model, for the case of a solid cylinder, was shown to under predict experimental results, whereas scale growth on the inner surface of a hollow cylinder was over predicted. The differences are primarily attributed to an effective diffusion coefficient that vanes with scale morphology, that is, with the level of porosity.