Ion-beam-assisted deposition of aluminium and aluminium alloy coatings for corrosion protection

Ion-beam-assisted deposition (IBAD) was used for depositing pure aluminium coatings on low carbon steel in order to study the corrosion properties as a function of the energy of the incident argon ions, the ion-to-atom arrival ratio and the angle of ion incidence. With theoretical considerations about the localized corrosion behaviour of substrate-coating systems it is possible to evaluate the porosity, the relative number of coating defects and their mean diameter. Their dependence on the process parameters gives an insight into the initial stages of film growth and shows that the energy deposition in the near-surface region is the most important factor for modifying coating defects and therefore the corrosion properties of the steel-aluminium system. The modification potential of each parameter of IBAD is studied in comparison to pure vapour deposition for the corrosion in acetate buffer. In chloride-containing aqueous solutions the depassivation-behaviour of aluminium can be improved by use of alloying elements. Aluminium and magnesium were deposited on inert glass substrates; the magnesium concentration and the bombarding energy were varied. The resulting alloys have equilibrium potentials less noble than pure aluminium and increased passive regions. The passive region decreases with increasing magnesium concentration. From glancing-angle X-ray diffraction measurements it can be seen that structural changes influenced by the ion beam go parallel with changes in the localized corrosion behaviour.