In-situ Study of Corrosion for AZ91 Magnesium Alloy

There exist plenty of atmospheric corrosion research results, however, limited data on the study of early stage atmospheric corrosion.  Magnesium is very sensitive to corrosive environment and becomes excellent model material to understand the mechanism for early stage atmospheric corrosion. 
The cyclic wet-dry tests were conducted to in-situ study the early stage corrosion of AZ91 magnesium alloy using environmental scanning electron microscopy (ESEM). The phenomenological results of the formation, spreading of micro-droplets and transformation of micro-droplets into a thin water film on pure magnesium and as-cast AZ91 magnesium alloy during cyclic wet-dry condition confirmed that the addition of Al changed the hydrophilicity of the alloy, which is one of the reasons that AZ91 alloy had better corrosion resistance.  The grain boundary and b phase for AZ91 magnesium alloy had no influence on the formation and spreading of micro-droplets.  The corrosion product stimulated the formation and spreading of micro-droplets due to the special structure of the corrosion product. The corrosion rate changed with the wetting time during the initial corrosion process and the maximum corrosion rate appeared at the wetting time of 20min per cycle because of the transition between water droplets and water film.  The pre-charging of hydrogen caused crack initiated at b phase, and with the increase of wetting time the crack propagated, implying that hydrogen produced by corrosion reaction participated in the process.