Corrosion behaviour of laser gas-nitrided Ti–6Al–4V alloy in nitric acid solution

Laser gas nitriding of the Ti–6Al–4V alloy under a pure nitrogen environment of 30 l/min gas flow rate, was carried out with 500 mJ laser beams, produced by a Nd:YAG-pulsed laser at 5 mm defocused distance, and 1.5 mm/s sample traverse velocities. The microstructure and corrosion behaviour of the nitrided samples were examined, using scanning electron microscopy, X-ray diffractometry (XRD), Tafel and cyclic polarization tests in 4 M HNO3 and immersion tests in 60% HNO3 solution. For comparison, untreated samples were tested under the same conditions. The microstructures consisted mainly of a thin continuous layer of TiN followed by nearly perpendicular dendrites, and below this, a mixture of small dendrites and large needles with a random orientation. While the untreated alloy showed a passive behaviour, the laser treated layer showed high electrochemical inertia and low corrosion currents. However, when the polarization potential was higher than threshold, or the period of immersion in high oxidizing acid solutions was sufficiently long, then they were attacked and removed with a corrosion rate higher than untreated Ti–6Al–4V alloy.