The localised corrosion behaviour of ZE20 magnesium alloy (composition Mg-2.4Zn-0.2Ce) is studied by a scanning vibrating electrode technique (SVET) in immersion conditions. Such alloys have improved ductility compared to commercial alloys due to the random structure produced by Ce additions, making them suitable for lightweight automotive parts. The alloy has been developed such that ductility and strength are optimised (27% elongation, 135 MPa yield strength; 225 MPa ultimate tensile strength) for such applications where ‘crashworthiness’ and formability are key. The corrosion resistance of magnesium alloys is still a significant challenge and understanding the mechanism by which corrosion occurs is fundamental to advancing performance. In the current study, SVET is used to measure the time-resolved local current density distributions in situ over freely corroding sample surfaces. The localised corrosion rates of ZE20 are studied in terms chloride concentration and pH. Figure 1 shows representative current density maps of the alloy freely corroding in immersion conditions.

Figure 1 Surface plot showing the distribution of normal current density (j_z) above a ZE20 magnesium alloy sample at times (a) 5 h and (b) 6.5 h following immersion in aerated 2 M aqueous NaCl at pH 7.