Microstructural, electrochemical and immersion based corrosion analysis in milling induced magnesium alloy AZ91

Present work discusses the effect of face milling operations on the corrosion characteristics of AZ91 alloy. A full factorial L27 design of experiment is prepared and face milling is performed to identify the optimal conditions with lowest roughness. The optimal surface of the alloy obtained from face milling has been compared with the slightly polished surface of as-cast alloy using weight loss and electrochemical polarization testing methods using 3.5% NaCl solution. In weight loss method, the alloy surfaces are solution treated for various time intervals ranging from 0.5 to 72 hours, to study the effect of surface finish obtained from face milling in comparison to polishing (as-cast). Whereas, in polarization method, the tests are conducted for only 0.5 hour on both specimens and compared. The corrosion rates are found to be in direct relation to the surface roughness, as the low roughness surface from face milling showed comparatively low corrosion rate. The polarization curves justified the better corrosion resistance of milled surface when compared to polished surface. Further, FESEM micrograph comparison between the before and after polarization test surfaces showed the severity of corrosion attack in both tested surfaces. Furthermore, the elemental analysis using EDAX test on the before and after polarization test surfaces showed the presence of less oxygen in the milled surface compared to polished surface, which established the better corrosion resistance of milled surfaces which has low surface roughness.