AC impedance spectroscopy in characterizing time-dependent corrosion of AZ91 and AM50 magnesium alloys - Characterization with respect to their microstructures

The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion poten tial (E-corr) using electrochemical impedance spectroscopy. Transmission el ectron microscopy was used to analyze the corrosion product layer, and phas e shifting interferometric microscopy was carried out to characterize the r eactivity of intermetallic particles. Due to its microstructure, the AM50 a lloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutect ic areas. For the AZ91 alloys, the dissolution of the alpha -eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, wh en a threshold was reached in the level of Al2O3 in the magnesium oxide (or hydroxide) layer, a change of phenomenology occurred in the impedance diag rams. In addition, electrochemical results revealed that an increase of sil icon concentration for the AZ91 alloys decreased the corrosion resistance. This was attributed to an increase of the number of Mg2Si particles, accele rating the dissolution of eutectic areas. (C) 2001 The Electrochemical Soci ety.