Corrosion behaviour of magnetron-sputtered Al1−x–Mnx coatings in neutral saline solution

Al–Mn coatings with different contents (0–41 at.%) were deposited on glass slides by magnetron co-sputtering. These coatings were characterised, before and after immersion tests, by X-ray diffraction, scanning electron microscopy (SEM) and electron probe microanalysis. The electrochemical measurements in a neutral saline solution showed that the pitting corrosion resistance of Al–Mn coatings increased with increasing Mn content as the pitting corrosion potentials are progressively shifted towards positive values. The immersion tests reveal that for all Mn contents, Al–Mn coatings keep a sacrificial character compared with steel. For Mn content above 26 at.%, XRD analysis showed the formation of an amorphous phase. This structure would be responsible for the high increase of the hardness of Al–Mn coatings and of the stabilisation of their open circuit potentials during the 48 h immersion tests.

Research highlights
► Microstructure evolution occurs with incorporation of manganese in PVD Al coatings
► Decrease of grain size is observed with Mn incorporation
► Microhardness gradually increases with Mn incorporation
► Enhancement of the pitting corrosion with Mn incorporation
► Pitting potential reaches a steady-state value independent of Mn incorporation.