Pitting corrosion of aluminium coated with amorphous carbon films by argonion 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 inhi bited. Amorphous carbon films (a-C) are particularly suitable as they are c hemically resistant and show a low microporosity. However, owing to the tem perature sensitivity of aluminium alloys, the films have to be deposited at low process temperature. Low process temperature often leads to coatings w hich are porous and not well adherend. The situation may be improved when t he films are irradiated with energetic ions. Amorphous carbon (a-C) films w ere deposited by electron beam evaporation from graphite under concurrent b ombardment 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 t he corrosion behaviour, polarisation measurements were carried out in neutr al 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 r oughness beyond the film thickness no corrosion protection could be achieve d. (C) 2001 Elsevier Science B.V. All rights reserved.