Qualification of TA6V alloy cleaning processes using supercritical CO₂ cleaning, from coupled SEM, XPS, and TPD analyses

Traditional industrial cleaning processes of titanium and titanium alloys use solvents which are potentially toxic and environmentally harmful. Carbon dioxide, which is nontoxic, nonflammable, recyclable, and available on a large scale, is a potential substitute for these solvents. Indeed, the zero surface tension of supercritical CO₂ (SC-CO₂) provides a significant advantage over liquid and aqueous solutions, and facilitates the access to small structures, while a high diffusion coefficient allows a fast rate of mass transfer. Moreover, its viscosity and density can be adjusted by tuning pressure and temperature. In this study, the performances of solvent-assisted SC-CO₂ process are compared to traditional TA6V alloy cleaning processes. The efficiency of residual oil removal from a TA6V surface has been investigated by several techniques such as x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). While XPS gives a measure of the so-called relative atomic surface concentration of carbon, which is commonly used to characterize cleanliness of the surface in the nanometer range, both EPMA and SEM provide elementary analysis at μm³ range and are often associated with characterizing morphologies of protuberances. Temperature programmed desorption coupled to a quadrupole mass spectrometer has also been used to obtain qualitative information on the nature and the relative amounts of the species degassed from the polluted and cleaned material surfaces.