PNNL

Abstract

The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy has been shown to depend on the precipitation of the Mg rich phase, Al3Mg2, but not the enrichment of elemental Mg at grain boundaries to concentrations up to 3 times that of the alloy. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal treatment times, using Auger Electron Spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by Analytical Electron Microscopy (AEM) of thin foils. The progess of the Al3Mg2 precipitation was followed by AEM and Scanning Electron Microscopy (SEM), for the same thermal treatment times. The lack of a Mg segregation effect was demonstrated by both X-Ray Photoelectron Spectroscopy (XPS) analysis of Mg implanted Al following in situ electrochemical tests (1) and SCC testing while the dominance of Al3Mg2 precipitation was demonstrated by electrochemical analysis and SCC testing. Crack growth tests of alloy AA5083 conducted at potentials cathodic and slightly anodic potentials with no increase at cathodic potentials. This is consistent with reported hydrogen permeation results for Al which suggests a hydrogen contribution to crack growth.