The cathodic behaviour of Al–Mn precipitates during atmospheric and saline aqueous corrosion of a sand-cast AM50 alloy

•Site-specific analytical electron microscopy was performed on corroded AM50.•In air-exposed alloy, Al–Mn particles show enrichment in Al and O at the surface.•In immersed AM50, Al–Mn particles develop domes of corrosion products, Mg(OH)2/MgO.•Immersed intermetallics show Al depletion at the surface, indicating high local pH.•Al depletion leads to surface delamination and ultimately formation of Mn3O4.

The behaviour of Al–Mn precipitates during atmospheric and aqueous corrosion of an AM50 Mg alloy was investigated using site-specific analytical electron microscopy. After air-exposure, localized attack was observed close to Al–Mn precipitates, with the top layer of the intermetallic enriched in Al and O. During immersed corrosion, these precipitates developed protruding domes of corrosion products, with crystalline Mg(OH)2 on top and an inner layer of crystalline MgO. After prolonged immersion, these precipitates showed evidence of preferential Al dissolution, ultimately developing a fragmented interlayer of Mn3O4. This phase transformation is linked to the enhanced hydrogen evolution rates adjacent to these precipitates.