Pitting corrosion of aluminium coated with amorphous carbon films by argon ion beam assisted deposition at low process temperature

It is well known that aluminium alloys are prone to pitting corrosion when they are exposed to a wet environment in the presence of aggressive anions such as chloride. When a dense inert coating is applied pitting may be inhibited. Amorphous carbon films (a-C) are particularly suitable as they are chemically resistant and show a low microporosity. However, owing to the temperature sensitivity of aluminium alloys, the films have to be deposited at low process temperature. Low process temperature often leads to coatings which are porous and not well adherend. The situation may be improved when the films are irradiated with energetic ions. Amorphous carbon (a-C) films were deposited by electron beam evaporation from graphite under concurrent bombardment with 10 keV Ar+ ions at process temperature below 150 degreesC. Both mirror-polished and ground aluminium samples were used as substrates. Films of different thickness up to 0.8 mum were deposited. For evaluating the corrosion behaviour, polarisation measurements were carried out in neutral aqueous solution of sodium chloride. The results show that for polished surfaces the films reduce the pitting corrosion susceptibility of aluminium considerably when they are thick enough, while for ground samples with a roughness beyond the film thickness no corrosion protection could be achieved. (C) 2001 Elsevier Science B.V. All rights reserved.