Wen C. Say

Surface characterization of cast Ti-6Al-4V in hydrofluoric-nitric pickling solutions

Abstract Hydrofluoric acid offers a fast etching rate for removing 200 μm-thick alpha-cases from the surfaces of cast Ti-6Al-4V. The acid picks up hydrogen gas into substrate of the cast, which can be limited by introducing an oxidizing acid. This study proposes the optimal ratios of hydrofluoric/nitric acid as the pickling solutions. The cast Ti-6Al-4V in these solutions is monitored with chemical etching and electrochemical polarization. The mechanisms of etching procedures are initiated by generating oxygen vacancies that lead to several products from a series of quasi-chemical steps. The final product of the reactions may deduce as the form of H 2 TiF 6 that is peeled out by the evolution of produced gases. Polarized curves are performed to illustrate passive behaviors of the alpha-case removed Ti-6Al-4V. The amounts of charge transference increase with increasing hydrofluoric acid concentrations. The newly formed passive film was estimated to be 20 nm in thickness with non-stoichiometric arrangements in the 4% HF electrolyte.

Gaseous corrosion mechanisms of silicon carbides in Na2SO4 and V2O5 environments

Hot-pressed polycrystalline SiC (HPSiC) and single crystal SiC (SCSiC) were exposed to Na2SO4 and V2O5 vapours at 1000 °C. Vapours were carried by the argon gas over the specimens where corrosion studies were conducted. Mixed-kinetics mechanisms for HPSiC under gaseous environments have been developed in the form of corrosion weight loss per unit area with time. The decomposed gases diffusing through the SiO2 film and the reaction products at the substrate surfaces are the two major steps that contribute to the overall reaction for this material. SCSiC was observed to have severe corrosion attacks on its surfaces and corrosion pits with honeycomb shapes were left.