Investigation of ion assisted palladium treatments for improved corrosion resistance of titanium foil in the electron beam dry scrubber process

Titanium foil windows have been employed in the dry scrubbing of flue gases by electron beam irradiation, with the window lifetime limited by corrosion. As part of a programme aimed at improving the lifetime of windows, the corrosion resistance of titanium foil has been assessed following palladium surface treatments applied by vacuum evaporation, ion beam mixing, ion beam assisted deposition and plasma source ion assisted deposition, with production of palladium layers up to approximately 400 nm thick. For evaluation of corrosion behaviours, immersion tests and potential–time and potentio-dynamic polarization measurements were carried out, mainly in 0.1 M H2SO4 and 0.1 M HNO3 environments at 80°C, which approximates the possible conditions in a plant. The foil surfaces were examined by scanning electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis to determine the nature of surface degradation and changes in surface compositions. The results indicated major improvements in the corrosion resistance of the titanium foil from all palladium treatments, with surfaces undergoing relatively minor or negligible corrosion following immersion testing for up to 2000 h. However, the coatings often suffered loss of adhesion with extended periods of immersion in H2SO4, either locally or generally, possibly associated with the presence of a thin oxide layer at the interface between the coating and the substrate. The optimum performance was found for coatings produced by ion beam assisted deposition and plasma source ion assisted deposition. The coated foil revealed significantly increased open-circuit potentials compared with untreated titanium, which assists protection of the titanium foil when the coating is detached locally from the substrate.