No abstract

No abstract

No abstract

The nature of the surface film formed spontaneously on Mg exposed to 1 M NaOH was investigated using scanning transmission electron microscopy, coupled with energy dispersive spectroscopy. Thin foils containing the surface film in cross-section were prepared by focused ion beam milling. The surface film formed is found to consist mainly of a crystalline MgO layer that has been hydrated to Mg(OH)2 to a variable degree. Although the film exhibits excellent corrosion resistance, it is not stable. The film tends to experience an irregular breakdown/repair process, which is characterized by large irregular potential drops (about 1 V) under open-circuit conditions. The breakdown/repair process is believed to involve the hydration-induced stress-rupture of the MgO film at discrete sites and the subsequent formation of a Mg(OH)2 self-healing corrosion product nodule.

The composition and structure of the film formed on Mg corroding in 0.01 M NaCl prior to any visible localized breakdown event was examined using a transmission electron microscopy (TEM). Focused ion beam (FIB) milling was used to prepare electron transparent lamellae of the film in cross-section. The TEM examination revealed that the film was comprised chiefly of a magnesium oxide (MgO) region adjacent to the Mg-film interface and a more porous mixed MgO and magnesium hydroxide [(Mg(OH) 2 ] region adjacent to the film/electrolyte interface. It is suggested that Mg corrosion involved both MgO film formation (anodic oxidation) and dissolution (chemical hydration) processes. EDS revealed that chlorine (Cl) was present throughout the film. This suggested that chloride ions (Cl − ) were incorporated into the film at potentials not associated with enhanced localized corrosion, likely as a precursor to breakdown.

The objective of this study was to clarify the effect of grain size on the corrosion rate across friction stir welded AZ31B joints when exposed in aqueous NaCl solutions. Grain size variations were achieved by preparing welded joints using AZ31B in different tempers. Single potentiodynamic polarization measurements made on the isolated base metal and stir zone in 0.6 M NaCl solution revealed no significant difference between the anodic kinetics, but revealed an apparent difference between the cathodic kinetics. The attempt to correlate extracted corrosion current density (i corr ) values to grain size (d −0.5 ) revealed that the slope coefficient was insignificantly different from zero, meaning that no meaningful correlation exists (albeit during the corrosion propagation stage). Scanning vibrating electrode technique (SVET) measurements across an intact AZ31B-O welded joint revealed that grain size had a more complex effect on the anodic and cathodic kinetics of the filiform-like corrosion exhibited in 0.86 M NaCl solution. Grain size had an effect on filament initiation as it consistently occurred on the surface of the base alloy (coarser-grained structure). In contrast, grain size had no effect on filament propagation as the anodic and cathodic kinetics were unaffected when moving from the base metal (coarser-grained surface) across the stir zone (finer-grained surface).

No abstract

The nature of the surface film formed on the α-Mg matrix phase of AZ80 in pure H2O was investigated using scanning transmission electron microscopy–energy-dispersive spectroscopy (STEM-EDS). Thin foil cross sections of the film/substrate interface were prepared using focused ion beam (FIB) milling. The film formed consists of a diffuse bilayer structure. A relatively thick, more-hydrated outer layer is found to form on a thinner, less-hydrated inner layer. Al is found to be enriched in both layers relative to the substrate, but is enriched to a higher level within the outer layer. No evidence of an inner aluminum oxide (Al2O3)-rich skeletal layer is observed. The EDS analyses suggest that the more-hydrated outer layer is comprised of double Mg-Al hydroxide-type compound, which is formed as a corrosion product.