Towards Unravelling the Source of Cathode-Activated Corrosion Filaments Formed on Corroding Mg Alloy Surfaces

Reported scanning vibrating probe (SVP) measurements have shown that the localized corrosion filaments formed on dissolving Mg and AZ31B (Mg-3Al-1Zn-0.3Mn) become activated cathodes following their formation. These cathodicly-activated filaments galvanically couple with intensely anodic regions at the heads of the corrosion filaments which subsequently drives the lateral propagation of the corrosion filament across the exposed surface. The mechanism by which the cathode (H₂ evolution reaction) is enhanced on corroded Mg or Mg alloys remains elusive. Proposed mechanisms include: (i) enrichment of noble metal constituents such as Fe impurity particles (for pure Mg) or Al-Mn intermetallic particles (for AZ31B) within the corroded filaments, (ii) the release and subsequent dissolution of metallic Mg “chunks” and (iii) formation of a significantly roughened surface film. Our recently reported TEM examination of the corrosion filaments formed on AZ31B has challenged the role played by the enrichment of Al-Mn intermetallic particles in cathodic activation whilst raising the possibility that the formation of a noble Zn-enriched layer (relative to Mg) at the alloy surface may be playing a key role in cathode activation.

To interrogate the proposed noble Zn-enriched surface layer hypothesis for cathodic activation in more detail, a study of the localized corrosion of a Zn-free AM30 (Mg-3Al-0.4Mn) surface, complete with site-specific analyses of the surface films that formed, was conducted. The localized corrosion behavior was characterized by making conventional electrochemical polarization measurements along with SVP measurements in a near-neutral 0.05 M NaCl solution. Focused ion beam (FIB)-prepared thin-foil cross-sections of the surface films formed after exposure were examined using TEM and associated techniques. A cryogenically-cooled stage (95 K) was used to minimize electron beam-induced damage to the foils during electron beam irradiation. The films were examined using bright field (BF) imaging along with selected area diffraction (SAD). The films were also characterized via the scanning TEM (STEM) mode, with a high angle annular dark field (HAADF) detector utilized for imaging and energy dispersive spectroscopy (EDS) to determine the composition of various microstructural features. Microstructural features of the underlying AM30 metal were also characterized using the same thin-foil cross-sections. 

The SVP measurements revealed that localized corrosion was indeed of the filament-like mode and the surface regions consumed by the propagation of the corrosion filaments acted as local cathodes. It was also revealed that the cathodic current density was diminished above a defined region of a filament with increasing exposure time (Figure 1a). Although the surface coverage of the cathodic corrosion filaments progressively increased with exposure time, the proportionately diminishing cathodic current density from a defined region of filaments resulted in relatively constant integrated cathode and anode currents with respect to exposure time. This behavior correlated well with the open-circuit potential, corrosion current density and cathode current density for,each alloy as measured by potentiodynamic polarization after 1 h and 24 h.

It was observed that the freshly formed corrosion filaments were primarily MgO, whereas the aged corrosion filaments became partially hydrated; containing a detectable proportion of Mg(OH)₂. The Al-Mn intermetallic particles, identified as Al₈Mn₅, were present in both the intact film and the corrosion filaments: thus challenging the role played by these intermetallic particles as “primary” cathode activation enablers. Al-enrichment was detected at the aged corrosion filament/metal interface, but not at the freshly-formed corrosion filament/metal interface: thus challenging the role played by noble solute enrichment as “primary” cathode activation enablers. In view of our AZ31B results, the noble Zn-enriched layer is proposed to act as a “secondary” cathodic activation enabler, implying that Zn alloying does indeed have a detrimental effect on localized filiform-like corrosion susceptibility of Mg-Al-Zn alloys. As for the “primary” enabler of the cathodic activation, it is proposed that it is the film itself with the controlling chemical or physical feature yet to be unambiguously identified.