Magnesium alloys have high strength/weight ratios, but poor corrosion resistance affects the viability of increased magnesium usage in aerospace and other applications. This review approaches the p...

Corrosion of magnesium

This review aims to provide a foundation for the safe and effective use of magnesium (Mg) alloys, including practical guidelines for the service use of Mg alloys in the atmosphere and/or in contact with aqueous solutions. This is to provide support for the rapidly increasing use of Mg in industrial applications, particularly in the automobile industry. These guidelines should be firmly based on a critical analysis of our knowledge of SCC based on (1) service experience, (2) laboratory testing and (3) understanding of the mechanism of SCC, as well as based on an understanding of the Mg corrosion mechanism.

A Critical Review of the Stress Corrosion Cracking (SCC) of Magnesium Alloys

Environmentally-induced cracking of magnesium

Influence of solution treatment on the corrosion fatigue behavior of an as-forged Mg-Zn-Y-Zr alloy

Initiation and early propagation of pitting and filiform corrosion on bare alloy AZ91 (9% Al, 1% Zn) are investigated by natural immersion corrosion tests, electrochemical measurements and microanalytical studies. Initiation sites are few. Corrosion spreads from these sites first in the form of filiform corrosion for a limited period of time and pitting which later develops into a cellular type of etching. The important factors affecting filiform corrosion are temperature, material structure and degree of polarization at the anodic sites. Filiform attack on AZ91, unlike the classical mechanisms of filiform corrosion on coated metals, is driven by hydrogen evolution reaction on the cathodic sites of the surface, occurs under significant anodic control, propagates at a high, constant speed independent of degree of polarization along preferential paths determined by compositional and crystallographic factors, and is a temporary phenomenon under open circuit conditions. Pitting corrosion is more predominant with decreasing anodic polarization.



Korrosionsmorphologie der Magnesiumlegierung AZ 91



Die Initiierung und die Fortpflanzung von Lochkorrosion und fadenformiger Korrosion wurde an der unbeschichteten Legierung AZ91 (9% Al, 1% Zn) mittels Tauchversuchen, elektrochemischen Messungen und mikroanalytischen Methoden untersucht. Es wurde eine Vielzahl von Initiierungsbereichen festgestellt. Von diesen Punkten breitet sich die Korrosion fur einen begrenzten Zeitraum in Form fadenformiger Angriffe aus, der spater in Lochfras mit zellenartiger, Erscheinungsform ubergeht.



Die wichtigsten Parameter, die die fadenformige Korrosion beeinflussen, sind Temperatur, Werkstoffstruktur und Grad der Polarisation an den anodischen Bereichen. Der fadenformige Angriff an der Legierung AZ91 wird abweichend von dem klassischen Mechanismus, der fur beschichtete Metalle gilt, durch die Wasserstoffentwicklung an den kathodischen Bereichen der Oberflache angetrieben. Der Vorgang ist anodisch kontrolliert und lauft mit hoher, konstanter Geschwindigkeit ab, unabhangig vom Grad der Polarisation entlang bevorzugter, durch die Zusammensetzung sowie kristallographischer Faktoren bestimmter Pfade. Es handelt sich um ein temporares Phanomen ohne Einwirkung von auseren elektrischen Stromen. Mit abnehmender anodischer Polarisation wird Lochkorrosion ausgepragter.

Corrosion morphologies on magnesium alloy AZ 91

Transgranular stress corrosion cracking of notched, solution treated and quenched Mg-8.6 wt % Al alloy rods tested in both salt solution and distilled water environments at 20 C (68 F) has been studied by direct electron diffraction techniques conducted on actual stress corrosion fracture surfaces, optical fractography, scanning electron fractography, and replica electron fractography. The studies revealed that considerable deformation twinning accompanied the formation of a stress corrosion fracture surface and that opposite faces of a stress corrosion crack contained matching faceted (stepped) topography. The detailed faceted topographical patterns were consistent with their origin being associated with underlying deformation twins formed prior to the passage of the crack tip and, frequently, deformation twins were observed adjacent and parallel to the stress corrosion fracture surface. All observations were consistent with a stress corrosion crack propagation process involving the formation of...

Greg Oryall

Papers

Transgranular Stress Corrosion Cracking of Solution Treated and Qeenched Mg-8.6 Al Alloy