Cathodic Corrosion of Magnesium Alloy AM50 in Deicing Salt Solutions During “Cathodic Protection”

A widely used method to protect steel from corrosion is the cathodic protection by galvanic coupling to a less noble metal (e.g., Zn on Steel) or by applying an external negative potential. Because magnesium is already one of the most reactive technical metals, only the latter approach may be feasible. The question arises if cathodic protection by an external applied potential is a suitable method to protect magnesium alloys from corrosion as, because of the very negative standard potential of Mg, the potential regime of cathodic protection would be situated far from the stability region of water and strong hydrogen evolution will take place. Furthermore, the cathodic polarization provokes the formation of deposition products resulting from the strong alkalization, in theory without the parallel effect of Mg corrosion, so that deposition products could be better qualified regarding their protective properties in electrochemistry. Deposition products are especially expected from the deicing salt additives CaCl₂ and MgCl₂ because of their tendency to form much more stable chemical components in contrast to Na⁺ from NaCl. However, a cathodic polarization of AM50 led to nonprotective surface conditions and the presence of localized corrosion was visible. The high local pH values during sustained cathodic polarization lead to destabilizing conditions of alloying elements such as Al or Zn. This may lead to dealloying and selective dissolution of the second phase intermetallic particles, causing formation of corrosion initiation sites while the Mg matrix is stable. This is the most probable reason to explain the significantly more activated surfaces after sustained cathodic polarization on AM50.