Filiform-Like Corrosion Mechanism on Magnesium-Aluminum and Magnesium-Aluminum-Zinc Alloys

Abstract

The filiform-like corrosion of Magnesium (Mg) alloys AZ31B and AM30 was investigated with electrochemical and microanalytical techniques. Potentiodynamic polarization testing and scanning vibrating electrode technique (SVET) measurements confirmed the “differential electrocatalytic” mechanism previously reported for filiform and filiform-like corrosion on pure Mg and AZ31B. Transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) revealed that the MgO corrosion filaments on both alloys were likely a product of the direct reaction of Mg and water (H2O), responsible for the rapid hydrogen (H2) evolution observed at the propagating corrosion fronts. TEM analysis also revealed through-thickness cracks and noble intermetallic particles within the corrosion filaments and noble metal enrichment at the corrosion filament/metal interfaces, which were proposed to play significant roles in the cathodic activation of the corrosion filaments. The higher susceptibility of the AZ31B alloy to cathodic activation versus AM30 suggested that Zinc (Zn) has a detrimental effect on the resistance of Magnesium-Aluminum-Zinc (Mg-Al-Zn) alloys to filiform and filiform-like corrosion.