Physical and Electrochemical Evidence for the Role of a Mg Hydride Species in Mg Alloy Corrosion

The role of MgH₂ in the process of filiform-like corrosion on Mg alloys has been investigated. MgH₂ was grown galvanostatically on the Mg ZEK100 alloy at a current density of −5 mA/cm² for various times in 0.1 M NaOH and the subsequent electrochemical and corrosion behavior compared to that of as-polished specimens. Polarization curves demonstrated cathodic activity under anodic polarization (i.e., the “negative difference effect”) while x-ray diffraction confirmed the presence of MgH₂. Dynamic secondary ion mass spectrometry measurements show strong evidence for the presence of MgH₂ in aged sections of corrosion filaments, in the leading edge of corrosion filaments, and in the Mg matrix ahead of the leading edge of corrosion filaments in alloys corroded at E_corr. The presence of these hydrides is linked to the filiform-like corrosion morphology observed on Mg alloys via in situ monitoring. These hydrides are responsible for the filiform-like corrosion by acting as temporary metastable catalysts for hydrogen evolution prior to their decomposition to Mg(OH)₂.