The Atmospheric Corrosion of Magnesium Alloys : Influence of Microstructure and Environments

Abstract: The low density and high specific strength of magnesium alloys have created a great deal of interest in the use of these alloys in the automotive and aerospace industries and in portable electronics. All of these industries deal with applications in which weight is extremely important. However, an obstacle to overcome when using magnesium alloys in engineering applications are their unsatisfactory corrosion properties. This thesis is devoted to the atmospheric corrosion of the two magnesium alloys AZ91D and AM50, in particular the ways the microstructure and exposure parameters of these alloys influence their corrosion behaviour. The work includes both laboratory and field studies. The results obtained show that the microstructure is of vital importance for the corrosion behaviour under atmospheric conditions.The microstructure of magnesium-aluminium alloys contains different intermetallic phases, e.g. Al8Mn5 and β-Mg17Al12. The local nobility of these intermetallic phases was measured on a submicron level in an atmospheric environment. It was shown that particles of the Al-Mn type exhibit the highest Volta potential among the microstructure constituents of the AZ91D magnesium alloy. Further, it was shown that the Volta potential was highly dependent on the aluminium content of the magnesiumaluminium phases in the surface layer.When thin electrolyte layers are present, CO2 diffuses readily to the surface forming magnesium carbonate, hydromagnesite. The CO2 lowers the pH in areas on the surface that are alkaline due to the cathodic reaction. This stabilises the aluminium-containing surface film, the result being increased corrosion protection of phases rich in aluminium. Both in the laboratory and under field conditions the corrosion attack was initiated in large α-phase grains, which is explained by the lower aluminium content in these grains. The thin electrolyte film, which is formed under atmospheric conditions, decreases the possibility of galvanic coupling of alloy constituents located at larger distances from each other. Thus the cathodic process is in most cases located in the eutectic α-/β phase close to the α-phases, instead of in intermetallic Al-Mn particles, even though the driving force for the initiation of the corrosion attack in Al-Mn particles should be high, due to their high nobility.