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

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. - Abstract: 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.