Fig.1 SEM image on polished alloy surface a and elemental distributions of Ca, Mg and O on the eutectic micro-constituent b (L1, L2 and L3 refer to location 1, location 2 at eutectic micro-constituent and location 3 at matrix sites, respectively; CP means backscattered electron image)

Fig.2 OCP of Mg-1.0Ca alloy measured during 6 h of exposure in 0.9 wt% sodium chloride solution

Fig.3 Nyquist a and Bode b plots of Mg-1.0Ca alloy in 0.9 wt% sodium chloride solution for 6 h and equivalent circuit used for the spectra fitting c

Fig.4 In situ optical images of the sample surface after 0 min a, 15 min b, 30 min c, 1 h d, 2 h e, 4 h f of exposure in 0.9 wt% sodium chloride solution

Fig.5 Sequential 3D potential images (4 mm × 4 mm) on the surface of Mg-1.0Ca alloy in 0.9 wt% sodium chloride solution at OCP, obtained after 10 min a, 1 h b, 2 h c, 3 h d, 4 h e and 6 h f exposure

Fig.6 SEM micrographs of the sample surface after the SRET measurement a, high-magnification images of the area with an active spot before b and after c the removal of corrosion products by ultrasound and SEM image of a corroded defect on the Mg-1.0Ca alloy surface d

Fig.7 SEM image a and elemental distribution b of a large active spot on the Mg-1.0Ca alloy surface after the SRET measurement (CP means backscattered electron image)

Fig.8 SEM images and EDX linear analysis before a and after b etching experiment on the eutectic micro-constituent

Fig.9 Schematic diagrams of corrosion process for the Mg-1.0Ca alloy in 0.9 wt% sodium chloride solution