AbstractThe corrosion behaviour of as-cast and solution-annealed Mg-0.5Zn-0.2Ca alloy containing 150 ppm Si was investigated in 0.9% NaCl solution. The main secondary phases in the as-cast state are isolated MgCaSi and a coexisting intermetallic particle which consists of Mg2Ca and Ca2Mg6Zn3. A quasi-in situ approach is applied to investigate the corrosion initiation and development. It is proved that Mg2Ca phase is anodic and preferentially corrodes within the first hour of immersion. Whereas, Ca2Mg6Zn3 and MgCaSi phases continuously act as cathodes until 24 h. After solution annealing, the more homogeneous microstructure and reduced galvanic corrosion result in a higher corrosion resistance.