Corrosion protection ability of Al2O3 coatings deposited with ion beam assisted deposition

Insulating ceramic films, which have the property of being hard and corrosion resistant, are good candidates for corrosion protection coatings. Al2O3 has the additional advantage that it is transparent. Therefore it is used as protective coating in many fields, such as microelectronics and laser optics. Ion beam assisted deposition (IBAD) provides a good means for depositing stoichiometric Al2O3 coatings, which have a very good adhesion, on any bind of vacuum compatible materials. Earlier studies already showed that IBAD Al2O3 coatings act as corrosion protection coatings against pitting of aluminium. The present paper deals with its protection ability against uniform corrosion of mild steel. The Al2O3 coatings were prepared with ion beam assisted deposition using three different deposition modes: (1) evaporation of Al in an oxygen atmosphere under bombardment with Ar ions; (2) evaporation of Al in an oxygen atmosphere under bombardment with O ions; (3) evaporation of Al2O3 under bombardment with Ar ions. In all cases appropriate combinations of the process parameters, namely the ion energy, the ion to atom arrival ratio IIA, and the backfilling gas pressure, were chosen in order to deposit stoichiometric Al2O3. The electrochemical and corrosion behaviour were determined by current/potential measurements under conditions for uniform corrosion attack. The results showed that the Al2O3 coatings considerably increase the resistance of steel against uniform corrosion. In the examined range the modes of deposition do not significantly influence the corrosion protection effect. However, there is a distinct influence of the ion energy and the IIA ratio on the protective power. It can be concluded, that, up to a certain level, an increase of the power input per condensing atom during the growth of the Al2O3 films leads to an increased protection ability.