Improving the corrosion protection of aluminium alloys using reactive magnetron sputtering

Most high strength aluminium alloys used in the aircraft industries are susceptible to corrosion. Up to now hexavalent chromium is the conventional corrosion inhibitor. Because chromium in hexavalent state is carcinogenic, it is necessary to develop effective alternative inhibitor systems. We investigated magnetron sputtered substoichiometric and stoichiometric aluminium nitride (AlNx with x ⩽ 1) coatings for corrosion protection of aluminium alloys 2024-T3, 6061-T4 and 7075-T6. The corrosion behaviour of the treated surfaces was tested by anodic polarization scanning and salt spray testing. From the polarization curves it can be concluded that magnetron sputter coating with AlNx leads to a higher pitting potential of the three aluminium alloys investigated. The salt spray tested samples also confirm the protective effect of the coatings. In addition we found that AlNx layers with high nitrogen content lead to a stronger shifting in pitting potential than those with low nitrogen content. Anyway, the results of the salt spray testing show that particularly nitrogen-rich AlNx layers are less stable towards NaCl electrolyte.