Influence of casting porosity on the corrosion behaviour of Mg0.1Si

•Casting porosity caused significant corrosion rates.•The pores activated significant corrosion over the whole specimen surface.•Important was micro-galvanic acceleration of the corrosion by Fe-rich particles.•Each measured value of apparent valence for Mg was less than 2.•Electrochemical measurements of the corrosion rate of Mg must be used with caution. The influence of casting porosity on the corrosion behaviour of Mg0.1Si was investigated for immersion in 3.5% NaCl solution saturated with Mg(OH)2. The corrosion behaviour was characterised using (i) hydrogen evolution, (ii) weight loss, (iii) cathodic polarisation curves, (iv) electrochemical impedance spectroscopy (EIS), and (v) an examination of the corroded surfaces. Specimens with porosity had higher corrosion rates attributed to the corrosion associated with the pores activating significant corrosion over the whole specimen surface, wherein important aspects were (i) the breakdown of a partly protective surface film, and (ii) micro-galvanic acceleration of the corrosion by Fe-rich particles.