Cryogenic machining induced corrosion resistance of magnesium alloy AZ91

Magnesium (Mg) alloys are popular in the aerospace and automotive sector owing to their light-weight aspects. Amongst various Mg alloys, AZ91 alloy has been trending and needs to be completely explored. The corrosion resistance of AZ91 alloy has always been a question, hence needs further investigation. The impact of face milling operations under cryogenic conditions on the corrosion properties of AZ91 alloy is studied in the current work. Using electrochemical polarization testing technique with a 3.5 % NaCl solution, the optimal surface finish of the alloy produced by cryogenic assisted face milling has been contrasted with dry milling. The polarization method compares both specimens after testing them for 0.5 h. Cryogenic machining considerably minimized the roughness by upto 28 % with values ranging between 0.0525 µm and 0.1169 µm when compared to the 0.1259 µm and 0.0691 µm from dry condition. The comparatively low roughness surface from cryogenic condition demonstrated a relatively low corrosion rate (1.1992 mm/year in cryogenic; 1.2891 mm/year in dry), indicating that the corrosion rates are directly related to the surface roughness. The superior corrosion resistance of the cryogenic milled surface over the dry milled surface was supported by the polarization curves. Additionally, the elemental analysis test on the surfaces after the polarization test revealed that the cryogenic milled surface had less oxygen (44.28 wt% and 7.29 wt%) than the dry milled surface (47.13 wt% and 10.01 wt%), proving the superior corrosion resistance of cryogenic milled surface.