Corrosion Behavior of Friction Stir Welded Hpdc AM60B Lap Joints
Electrochemical polarization and scanning vibrating probe (SVP) measurements were made to establish links between the localized corrosion susceptibility observed across the FSW joint and the resultant variations in microstructure, as revealed by scanning electron microscopy (SEM). Microstructural parameters of interest included microhardness, grain size and the size and spacial distribution of Al-Mn intermetallic particles and the secondary Mg17Al12(β) phase. The corrosive environment used was a near-neutral 5 wt.% NaCl solution. Complementary surface analyses by X-ray photoelectron spectroscopy (XPS) of the starting surface films were made to delineate the role played by the native oxide film on the subsequent localized corrosion susceptibility. The findings based on the bulk immersion testing were subsequently validated by continuous salt fog atmospheric corrosion testing.
Of the microstructural parameters studied, only the grain size and spacial distribution of the secondary Mg17Al12 (β) phase were significantly affected across the friction stir welded joint. The stir zone exhibited a finer grain size relative to the unaffected base material, but did not exhibit a secondary Mg17Al12 (β) phase network. The combined set of electrochemical measurements (open-circuit potential, potentiodynamic polarization and SVP) consistently revealed that the stir zone was more susceptible to localized corrosion than the unaffected base material. SVP measurements on both the original topography and a flat-polished surface demonstrated that the stir zone striations had little effect on this tendency (see Figure 1). The findings thus far point to a combined alloy chemistry and structure link (centering on the fate of the secondary Mg17Al12 (β) phase network) between the localized corrosion susceptibility observed across the friction stir welded joint and the resultant variations in microstructure.