Localized Corrosion of Magnesium Alloys in NaCl Solutions Explored by Scanning Electrochemical Microscopy in Feedback Mode

• Initial surface film of Mg alloy is an electrical insulator by approach curve.
• Changes of small active spots are detected by SECM in real time.
• Influence of film composition is studied by mapping and linear scan.
• Higher Cl−concentration, more rapid initiation and quick growth of active spot.
• Ce promotes the formation of more protective surface film on Mg alloy.

Corrosion of magnesium is initiated from film-free region where the pit corrosion is the main corrosion form. The incubation and growth of active spots in film-free region is affected by the concentration of Cl−and the composition of surface film. Application of scanning electrochemical microscopy (SECM) in feedback mode for corrosion study of AM60 and AMCe1 (AM60 with additional Ce) magnesium alloy in different NaCl solutions is presented. SECM mapping mode was also used to monitor the electrochemical processes on AM60 and AMCe1 alloys immersed in NaCl solutions. The results indicate that the electrochemical behavior of magnesium alloy is different depending on chloride ion concentration and its surface film. Enhanced localized corrosion of magnesium alloy with chloride ions concentration shows that higher concentrated chloride ions results in the rapid initiation and quick growth of active spots, facilitates breakdown and high hydration of surface film. Meanwhile, scratch tests were performed on magnesium alloy, known to develop an artificial defect to monitor variation of electrochemical activity. In the proximity of the local defect, the accelerated electrochemistry activity originating from the high-hydration of surface film is observed when chloride ions are concentrated in the environment. Furthermore, the addition of rare earth element Ce is advantage of reducing the tendency of incubation and growth of active spots due to its more complete and protective surface film.