Effect of surface modification of Ti–6Al–4V by laser and duplex treatment on selective dissolution of aluminium

Abstract

The corrosion properties of laser surface melted (LSM) Ti–6Al–4V and duplex treated Ti–6Al–4V samples were investigated in 0·05M H2SO4/0·05M NaCl solution. Duplex treatment was found to increase the corrosion potential and the corrosion current of the alloy. Semipassive regions were evident in the polarisation curves for both types of treated surface. The current in the semipassive region increased at a more rapid rate in the case of the duplex treated surface. The potential current diagrams were found to be generally noisy for both types of surface, indicating an unstable surface which undergoes continuing active dissolution and repassivation. Inductively coupled plasma (ICP) analysis of metal ions in solution was carried out following electrolysis at controlled potentials. Generally, the percentage of aluminium ions in solution was found to be high in comparison with titanium and vanadium, reaching ∼94% after polarisation in the active region at 0·4 V (cf. Ag/AgCl) for 2 h. Duplex treatment, which involved physical vapour deposition coating with TiN after laser surface melting, lowered the percentage of aluminium leached into solution to 55% after polarisation at 1·0 and 2·7 V. SEM showed that some local, shallow pitting had occurred on both the LSM and duplex treated surfaces. Energy dispersive spectroscopy showed that the aluminium content in the surface region of the LSM material after polarisation at 0·4–2·7 V (cf. Ag/AgCl) for 2 h was lower than that for the original material, having decreased from 6% in the original alloy to 0·18–0·21%. The percentage of aluminium in the surface of the duplex treated material following polarisation at the same potentials for 2 h was also low, in all cases being in the range 0·2–0.3%.